NEMA Publication OS 3-2002
Selection and Installation Guidelines for
Electrical Outlet Boxes
®
© 2002 by National Electrical Manufacturers Association
Page 2 NEMA OS 3-2002
NOTICE AND DISCLAIMER
The information in this publication was considered technically sound by the consensus of persons
engaged in the development and approval of the document at the time it was developed. Consensus
does not necessarily mean that there is unanimous agreement among every person participating in the
development of this document.
The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which
the document contained herein is one, are developed through a voluntary consensus standards
development process. This process brings together volunteers and/or seeks out the views of persons
who have an interest in the topic covered by this publication. While NEMA administers the process and
establishes rules to promote fairness in the development of consensus, it does not write the document
and it does not independently test, evaluate, or verify the accuracy or completeness of any information or
the soundness of any judgments contained in its standards and guideline publications.
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exercise of reasonable care in any given circumstances. Information and other standards on the topic
covered by this publication may be available from other sources, which the user may wish to consult for
additional views or information not covered by this publication.
NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this
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purposes. Any certification or other statement of compliance with any health or safety–related information
in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker
of the statement.
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NEMA OS 3-2002 Page i
Table of Contents
Foreword.................................................................................................................................. iii
Introduction ..............................................................................................................................1
Product Standards and Installation Codes ...........................................................................2
Coordination of Trades ...........................................................................................................2
Scope ........................................................................................................................................2
Section 1 PRODUCT SELECTION............................................................................................................3
General......................................................................................................................................3
Select the Outlet Box for the Type of Wiring System Used ....................................6
Connection to the Wiring System.....................................................................6
Wiring Systems Employing Tubing or Conduit.................................................6
Wiring Systems Employing Metal Jacketed or Nonmetallic-Sheathed Cable.......6
Grounding ........................................................................................................7
Select the Box Based on Contents and Function After Installation ......................7
Internal Volume Capacity of Boxes..................................................................7
Junction and Pull Boxes...................................................................................7
Outlet and Device Boxes–Mounting of Wiring Devices ...................................8
Outlet and Device Boxes–Support of Fixtures/Luminaires, Ceiling-Suspended
Fans, and Other Products................................................................................8
Outlet and Device Boxes with Fire Resistance Ratings (Hours)......................9
Boxes for High and Low Voltage Wiring Circuits (Class 1, Class 2, and Class
3 Circuits) .........................................................................................................9
Select the Outlet Box for the Intended Location of Installation ...........................10
General ..........................................................................................................10
Boxes for Installation in or on Walls or Ceilings.............................................10
Boxes for Installation in Floors.......................................................................12
Boxes for Other Locations .............................................................................13
Outlet and Device Boxes for Use in Fire Resistance-Rated Construction.....14
Special Use Boxes.........................................................................................15
Covers and Cover Plates for Dry Locations...................................................15
Extension Rings and Box Extenders..............................................................16
FIGURES
Figure 1-1 – TYPICAL BOX MOUNTING POSITIONS .............................................17
Figure 1-2 – TYPICAL BOX MOUNTING STYLES...................................................19
Figure 1-3 – SEPARATELY SUPPLIED BOX SUPPORTS ......................................20
Section 2 PRODUCT INSTALLATION ....................................................................................................23
Introduction ...............................................................................................................23
Storage and Preparation for Installation ........................................................23
Coordinating Trades (After Boxes are Mounted) ...........................................23
Fire Resistance-Rated Construction.......................................................................23
Metallic Boxes................................................................................................23
Nonmetallic Boxes .........................................................................................24
Mounting and Securing of Electrical Boxes...........................................................24
General ..........................................................................................................24
Surface Mounting on Walls or Ceilings..........................................................24
Boxes Supported by Raceways .....................................................................25
Flush Mounting in Walls and Ceilings ............................................................25
Pendant Mounting ..........................................................................................26
Page ii NEMA OS 3-2002
Pedestal Mounting .........................................................................................26
Masonry Wall Mounting .................................................................................26
Floor Box Mounting........................................................................................26
Poke-Through Floor Fitting Mounting ............................................................27
Other Boxes ...................................................................................................27
Connection to Wiring Systems................................................................................28
General ..........................................................................................................28
Connection to Conduit Entries .......................................................................28
Connection to Cable Entries ..........................................................................29
Connection to High and Low Voltage Wiring Circuits ....................................30
Completing the Installation......................................................................................30
Introduction ....................................................................................................30
Grounding and Bonding .................................................................................31
Installation of Wiring Devices.........................................................................32
Flush Device Boxes: Coordination with Finished Wall Surface Installation.......34
Installation of the Cover and/or Cover Plate.......................................................34
Verification of Installation ........................................................................................34
FIGURES
Figure 2-1 – SQUARE BOXES IN 4” x 4” WALL OPENING....................................35
Figure 2-2 – TYPICAL THREADED CONDUIT ENTRIES ........................................35
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NEMA OS 3-2002 Page iii © National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Foreword
These selection and installation guidelines offer practical information on correct product selection and
industry recommended practices for the installation of electrical outlet boxes in accordance with the
National Electrical Code®.
These guidelines have been developed by the NEMA Outlet and Switch Box Section, which periodically
reviews them for any revisions necessary to address changing conditions, product listing and installation
requirements, and technical progress. Comments for proposed revisions are welcomed and should be
submitted to:
Vice President, Engineering
National Electrical Manufacturers Association
1300 North 17th Street, Suite 1847
Rosslyn, VA 22209
At the time of approval, the Outlet and Switch Box Section of the National Electrical Manufacturers
Association had the following members:
Allied Moulded Products—Bryan, OH
Arlington Industries, Inc.—Scranton, PA
Bowers, A Subsidiary of Thomas & Betts—Memphis, TN
Bridgeport Fittings, Inc.—Bridgeport, CT
CANTEX, Inc.—Mineral Wells, TX
Cooper B-Line—Highland, IL
Crouse-Hinds—Syracuse, NY
EGS Electrical Group—Skokie, IL
Erico, Inc.—Solon, OH
Kellems Division, Hubbell Incorporated—Stonington, CT
Lamson & Sessions, Carlon—Cleveland, OH
Pass & Seymour/Legrand—Syracuse, NY
RACO—South Bend, IN
Steel City—Memphis, TN
Thomas & Betts Corporation—Memphis, TN
Wiremold Company, Walker Systems—Williamstown, WV
Wiegmann, Hubbell Electrical Products—Freeburg, IL
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NEMA OS 3-2002 Page 1
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Introduction
There may be no other component of our
electrical distribution system that serves more
purposes than the electrical outlet box.
When installed with a cover, the box
1. provides connection to a raceway system and
a means to access wiring;
2. houses and provides a degree of mechanical
protection for conductor splices, and wiring
devices such as switches and receptacles;
and
3. limits access to live parts; and where
applicable, bonds electrically conductive
components to the equipment grounding
conductor.
When designed and specifically identified for the
purpose, a box and other components form an
enclosure system which
1. provides degrees of protection from external
environmental conditions (rain, dust, scrub
water, etc);
2. preserves the integrity of fire resistance rated
walls, ceilings, floors, partitions, or furniture in
which it is installed; and/or
3. provides independent support for attached
fixtures such as luminaires, ceiling
suspended fans, and safety and security
devices.
For the most part, boxes within the scope of
these guidelines fall into just a few general
classifications. Each classification has been
adapted over the years to meet market needs.
These guidelines are written by the NEMA Outlet
and Switch Box Section (5-OS) to provide
qualified designers, installers, and inspectors
with an industry perspective of the appropriate
selection and installation criteria based on the
designed, intended use of the product. The
member companies of the NEMA 5-OS Section
promote selection and installation of listed boxes
and covers, and listed raceways and fittings.
Listing of electrical system components qualifies
them to minimum performance requirements
and provides for ongoing conformity
surveillance. Listed boxes and covers can be
recognized by the trademark of the qualified
electrical testing laboratory on the part or its
smallest unit container.
It is our objective to develop a closer liaison with
the installers of our products and the
professional electrical inspector. Through this
liaison, we intend to provide uniform education
and understanding as to the intended use and
application of our products. Through education,
we hope to develop an alliance, founded in trust
that will ensure that the concerns and challenges
we each face are addressed.
Compatibility of Boxes with Metallic and
Nonmetallic Wiring Systems–Grounding and
Bonding
Metal and nonmetallic outlet boxes are typically
designed for use in electrical distribution
systems rated 600 volts or less.
Metal systems and raceways that are recognized
as equipment grounding conductors provide
protection by grounding and bonding the entire
system. All the components of the system
(boxes, fittings, devices, fixtures, raceways, etc.)
with proper installation are designed to maintain
continuous grounding and bonding throughout
the system. Products must be installed so that
the removal of any device, fixture, or box cover
does not interrupt the downstream grounding or
bonding path.
Nonmetallic systems protect the installer and
end users by providing isolation (insulation)
throughout the wiring system. All the
components (boxes, fittings, devices, fixtures,
raceways, etc.) with proper installation are
designed to maintain this isolation (insulation).
When nonmetallic systems are installed:
a) there are no ungrounded exposed metal
parts that may be likely to become energized
in case of a ground fault;
b) ground wires are installed inside the
raceways and throughout the system; and
c) ground wires are joined so that removal of
a device or fixture does not interrupt the
downstream ground path.
Generally, this is done by joining the incoming
and outgoing ground wires with a third ground
wire that then attaches to the device or fixture.
Page 2 NEMA OS 3-2002
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Thus, the removal of the device or fixture does
not affect the downstream ground path.
Each system is designed to distribute electrical
power while protecting the installer and end
users. Metallic and nonmetallic systems employ
different means to provide protection.
Therefore, for safety reasons, these two systems
should not be randomly mixed without due
consideration.
Where it is necessary and desirable to mix
metallic and nonmetallic components in a wiring
system, special products, applications, and
procedures can be used to ensure the desired
level of safety in accordance with the National
Electrical Code®.
Product Standards and Installation Codes
Boxes and covers within the scope of these
guidelines are designed for installation in
compliance with the National Electrical Code®
(NEC®) NFPA 70. The broadest range of general
purpose designs are included in ANSI/NEMA OS
1, Sheet-Steel Outlet Boxes, Device Boxes,
Covers, and Box Supports; and ANSI/NEMA OS
2, Nonmetallic Outlet Boxes, Device Boxes,
Covers, and Box Supports. NEMA FB 1,
Fittings, Cast Metal Boxes, and Conduit Bodies
for Conduit, Tubing, and Cable Assemblies,
contains those boxes described in its title, and
ANSI/NEMA WD 6, Wiring Devices–Dimensional
Requirements, includes flush-device cover
plates. Conformity to these standards provides
assurance of the compatibility and
interchangeability of box designs with covers,
raceways, wiring devices, and other components
of the electrical distribution system whose
performance may be dependent on the box
design.
Listed boxes are typically evaluated to the safety
and performance requirements of ANSI/UL
514A, Metallic Outlet Boxes or ANSI/UL 514C,
Nonmetallic Outlet Boxes, Flush-Device Boxes,
and Covers. The safety and performance
requirements for flush device cover plates are
covered in ANSI/UL 514D, Covers for Flush-
Mounted Wiring Devices. Boxes intended to
provide degrees of protection from external
environmental conditions may also be subject to
the requirements found in ANSI/NEMA 250,
Enclosures for Electrical Equipment (1000 Volts
Maximum) or ANSI/UL 50, Enclosures for
Electrical Equipment. Nonmetallic boxes
intended to be installed in fire resistant rated
walls, ceilings, floors, partitions, or furniture must
also be separately evaluated to establish a rating
classification in accordance with ANSI/UL 263
and ASTM E 119.
Given that all listed boxes have met the same
minimum safety and performance requirements,
selection of the right box for the application is
the single most important factor for safe,
effective, and permanent installations. Design
preference completes the selection process, as
there are many optional features that distinguish
alternative brands. Beyond selection, almost
every other variable comes down to good
workmanship, something every craftsman takes
pride in and that is fundamentally required by the
NEC.
NOTE: All references to the NEC® are to the 2002
edition. References to the 1999 NEC® are included in
brackets.
Coordination of Trades
With regard to workmanship, the quality of a box
installation can be seriously compromised when
an effort is not made to coordinate the
installation of electrical outlet boxes with other
construction trades. In particular, installation
problems have been evidenced where there was
insufficient coordination with the trades that
apply finished surfaces (drywall) and poured
concrete (concrete boxes and floor boxes).
Every effort should be made to coordinate with
these and other trades who, through lack of
careful consideration may compromise the
quality of an installation.
Scope
This publication covers guidelines for selection
and installation of general purpose and specialty
metallic, nonmetallic, and composite outlet
boxes such as device boxes, fixture boxes, and
junction boxes; extension rings; covers; and
flush device cover plates. The boxes addressed
in these guidelines are intended for use in
ordinary, non-hazardous locations. With the
exception of masonry boxes and some multi-
gang boxes and floor boxes, the boxes
addressed in these guidelines are limited to
1650 cm³ (100 in³) internal volume.
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NEMA OS 3-2002 Page 3
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Section 1
Product Selection
General
The three key elements to the selection of an
electrical outlet box are:
� Type of wiring system
� Contents and function after installation
� Location and type of construction
Page 4
© National Elec
The following chart illustrates the critical underlying elements that should be considered when making the
appropriate outlet box selection. The sections that follow are intended to furt ide the user in making
these choices.
D
Electrical
Boxes
Nonmetallic-
Sheathed Cable
Conduit/
Tubing
al-sheathed
cables
Wiring System Type (Section )
Contents and Function (Section )
Device Outlet Special Purpose
Junction/Splice
Pull
Product Support
Single or Multiple
Devices
Junction/Pull
Dry Wet/Damp
Wall
Ceiling
Floor
Wall
Ceiling
Other
Flush mounted
Surface mounte
Flush mounted
Surface mounted
Flush mounted
Pendant mounted
Flush mounted
Surface mounted
Plenum
Flush mounted
Surface mounted
� �
Location of Installation (Section )
Surface mounted
Poke-through
Floor Fixture support, fa
n support
d
� �
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Pedesta
Swimmi
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Flush m
Surface
Poke-th
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NEMA OS 3-2002
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Flush mounted
(In walls, ceilings or floors)
�
Hollow
Construction
Solid
Construction
Concrete
Wood
Framing
Metal
Framing
Hollow
Masonry
New
Work
Old
Work
Page 5
Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Page 6 NEMA OS 3-2002
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Select the Outlet Box for the
Type of Wiring System Used
Local electrical and building codes
determine the acceptable wiring methods for the
building structure.
Connection to the Wiring System
The outlet box is an integral part of the
wiring system, and as such, the box selected
must be provided with a means to connect to the
wiring system.
Mechanical connection—Every outlet box
must have a means for entry of conduit, tubing,
or cable. In addition, outlet boxes must provide
for secure mechanical attachment of the conduit,
tubing or cable at the point of entry. SINGLE-
GANG nonmetallic outlet boxes intended for use
only with Nonmetallic Sheathed Cable (NM),
where the cable can be secured within 200 mm
(8 inches) of the box, need not have a direct
mechanical attachment means.
Wiring Systems Employing Tubing or
Conduit
Conduit Entries: Conduit entries in an outlet
box may be either threaded or unthreaded. It is
important to select boxes having the correct
number and size of conduit entries for the
installation. Boxes such as junction and pull
boxes supplied without conduit entries are
intended to have such entries made in the field
in accordance with manufacturer’s instructions.
Threaded entries in outlet boxes for
threaded conduit or threaded fittings are
permitted to have taper or straight threads
conforming to the specifications in NEMA FB 1.
Unthreaded entries in outlet boxes for
conduit or tubing include knockouts, or sockets
(hubs).
Knockouts are removable to provide
openings in metallic and nonmetallic outlet
boxes that allow for a secure mechanical
attachment of conduit and cables with the use of
appropriate fittings.
Unthreaded sockets (hubs) are provided in
some cast metal box designs. Set screws are
assembled in such sockets to securely attach
Rigid Metal Conduit, Intermediate Metal Conduit
(IMC) or Electrical Metallic Tubing (EMT) to the
box.
In the case of nonmetallic outlet boxes, such
entries may include sockets (integrally molded
hubs) for attachment of Rigid Nonmetallic
Conduit (RNC) or Electrical Nonmetallic Tubing
(ENT). Some sockets may contain knockouts.
Connection of RNC or ENT to sockets is made
using the appropriate solvent cement. These
boxes are typically designed for one size of
conduit or tubing. An appropriate sized reducer
bushing is to be used where a smaller trade size
conduit or tubing is to be installed.
Wiring Systems Employing Metal
Jacketed or Nonmetallic-Sheathed Cable
Cable Entries: Cable entries in an outlet box
are provided in several forms.
Threaded entries or unthreaded entries such
as knockouts described for conduit entries
above, also serve for the attachment of cables
by means of specific cable fittings.
Cable entries that permit entry of the cable
into metal outlet boxes without use of a separate
fitting are often called pryouts. Pryouts allow
easy access to cable entries in metal outlet
boxes intended to accommodate electrical
cables having an outside diameter of up to
mm (5/8 inch). A metal box with pryouts is
always supplied with cable securement means.
Typically they are in the form of a clamp installed
in the box, but can be in the form of a cable
fitting that is suitable for the application. These
boxes have markings on the clamps and carton
labels that tell the user the types and sizes of
cables for which the product is suitable.
Cable entries in nonmetallic outlet boxes are
designed for use with nonmetallic-sheathed
cable or flexible nonmetallic tubing.
Non-circular cable entries are provided in
nonmetallic outlet boxes intended for use with
Nonmetallic-Sheathed Cable. Where direct
mechanical attachment of the cable is required,
a box design that includes cable securement
clamps is to be selected. Boxes are available
that include removable and replaceable clamps,
or clamps that are integrally molded as part of
the box.
A cable securement clamp is not required for
a single-gang nonmetallic outlet box when the
cable can be secured within 200 mm (8 inches)
of the box.
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NEMA OS 3-2002 Page 7
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Grounding
Electrical connection—A box intended for
connection to metal raceway or metal jacketed
cable must provide a continuous electrical
grounding connection. A metallic outlet box
connected to a nonmetallic conduit, tubing, or
cable must have provision for connection of a
separate grounding conductor. A nonmetallic
outlet box may be connected to a metal
enclosed cable or metallic raceway, as long as a
bonding jumper is installed between the
raceways or cables.
Where it is necessary or desirable to use a
nonmetallic outlet box at any point in a metallic
raceway or wiring system, special precautions
are necessary. Often special grounding and
bonding accessories and techniques are
required to ensure the electrical connection of
the metallic raceway or wiring system remains
continuous (See “Compatibility of Outlet
Boxes…” section in the Introduction).
Select the Box Based on Contents
and Function After Installation
The intended purpose of an outlet box is the
second major criteria for selection. An outlet box
may be used as a JUNCTION or PULL BOX, or a
DEVICE BOX to house a wiring device such as a
switch or receptacle or communication circuit
accessory. An additional function of the outlet
box may be to provide means for supporting
equipment such as lighting fixtures (luminaires),
ceiling-suspended fans, safety and security
devices, etc. Although the primary function of
each of these types of boxes is described
separately below, each type when appropriately
equipped may provide additional functions.
Covers and/or cover plates are required to
complete the enclosure. The style of cover or
cover plate selected to complete the enclosure is
also determined by the contents and function, as
well as the location (See ) of the box. A cover
for a junction or pull box will have no pre-formed
openings other than for mounting screws,
whereas the cover or cover plate for a device
box must have appropriate openings to
accommodate the device. A canopy provided as
a part of a luminaire, ceiling-suspended fan or
other product that may be attached to a box,
also serves to complete the enclosure.
Internal Volume Capacity of Boxes
An outlet box must have sufficient internal
volume capacity. The electrical system design
will determine the size and number of
conductors, the type and number of wiring
devices, and the type of conduit or cable
securement. This information is essential in
order to select the appropriate outlet box design
having at least the minimum required internal
volume capacity, and size. Strong consideration
should be given to selecting a box having a
larger internal volume whenever the installation
requirements approach the box volume capacity.
Additional volume makes installation easier by
allowing for longer conductor ends and more
room for devices. Where used for high-
performance data conductors such as fiber-
optics, larger volume outlet boxes also ensure
adequate cable bend radius. When longer
conductor ends are available, future
maintenance of wiring devices is easier to
accomplish, and the additional volume in the box
will allow for the substitution of a larger wiring
device.
The internal volume of an outlet box,
historically measured in cubic inches (cu-in) but
now also referenced in cubic centimeters (cm³),
is an essential, easily understood, and almost
universal basis for reliable selection of an outlet
box. For many years the most common metallic
box designs have been standardized based on
nominal dimensions and internal volume
capacity in accordance with Table (A)
[370-16(a)] of the National Electrical Code® and
are not required to have their volume marking on
the box. Metallic boxes having dimensions
different from this table are marked with their
internal volume. All nonmetallic boxes are
marked with their internal volume. From
selection to verification of a completed
installation, use of internal volume of a box
based on the standardized set of deductions
presented in the National Electrical Code® in
Article 314 [370], has a proven safety record in
North American electrical distribution systems.
Junction and Pull Boxes
A JUNCTION BOX is used to join different runs
of raceway or cables, or both, and provides
space for the connection and branching of the
enclosed conductors. Most boxes can be used
Page 8 NEMA OS 3-2002
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
solely as junction boxes as long as they are used
with an appropriate cover.
A PULL BOX is a JUNCTION BOX having the
minimum dimensional requirements for wire
bending radius. A PULL BOX provides internal
access to the conductors in the raceway or
changes the direction of the raceway system.
A CONDUIT BODY may serve as a pull or
junction box.
Outlet and Device Boxes—Mounting of
Wiring Devices
A device box has a provision for attaching
and housing electrical switches, receptacles, or
similar wiring devices in accordance with NEMA
OS 1 and NEMA OS 2. Types of device boxes
include single device (SINGLE GANG), multiple
device (MULTIPLE GANG), and modular design
configurations (GANGABLE) that may be field
assembled, one to another, to accommodate
multiple devices. Clauses through
describe various boxes designed for
mounting of wiring devices.
Device Boxes for Flush Mounting
When installed as intended, these boxes
position the face of the securely mounted wiring
device flush to a structure's finished surface
covering. Some flush device boxes are designed
specifically for installation during the building
process (new work), while others have special
features that enable them to be flush mounted in
an existing finished surface (old work). See
Clause and Figure 1.
Device Boxes for Surface Mounting
Commonly referred to as “utility” boxes,
these boxes are specifically designed for
mounting onto a structure's finished surface.
Such surfaces may include appliances or
machinery. See Clause and Figure 1.
Outlet Boxes for Flush Mounting
Some outlet boxes, such as square boxes,
can be used in conjunction with raised covers
(commonly referred to as mud rings) for flush
mounting wiring devices. Selecting this type of
installation provides increased internal volume.
See Figure 1.
Outlet Boxes for Surface Mounting
Some outlet boxes, such as square boxes,
can be used in conjunction with appropriate
covers for surface mounting wiring devices.
When a raised-surface cover is selected, this
type of installation provides increased internal
volume (See Figure 1).
FS and FD Boxes
FS and FD boxes are designed for direct
attachment of a wiring device. “FS” refers to
shallow boxes (typically up to 1-1/4” deep) and
“FD” refers to deeper boxes (typically up to 1-
5/8” deep) for larger devices and increased
wiring capacity.
Outlet and Device Boxes—Support of
Fixtures/Luminaires, Ceiling Suspended
Fans, and Other Products
Outlet and Device Boxes Intended for
Support of Luminaires and Other Fixtures
For Products Weighing kg (6
lbs.) or Less
In or on walls, in old work or new work, all
outlet boxes including device boxes are capable
of supporting products, such as a
fixture/luminaire, smoke or gas detector or
similar product, weighing up to kg (6 lbs.). In
or on ceilings, an outlet box other than a device
box is capable of supporting any product
weighing up to kg (6 lbs.). A device box
mounted in or on a ceiling is capable of
supporting a product other than a luminaire
weighing up to kg (6 lbs.). No markings are
required to identify boxes for these uses.
For Products Weighing More Than
kg (6 lbs.) Up to 22 kg (49 lbs.) or Less
In or on walls, in old work or new work, outlet
boxes identified for such use, including device
boxes, are capable of supporting products
weighing between –22 kg (6–49 lbs.). Boxes
listed for support of products weighing between
–22 kg (6–49 lbs.) are marked to indicate the
maximum weight, the intended end use
application (wall, ceiling, or both) and any
specific installation techniques.
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NEMA OS 3-2002 Page 9
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
For Products Weighing Between
and 23 kg (6 to 50 lbs.)–New Work Only
Where structural framework is accessible for
mounting a box in or on a wall or ceiling, an
outlet box is capable of supporting a
fixture/luminaire or other products weighing 23
kg (50 lbs.) or less. The most common boxes
intended for this purpose are round, octagon, or
square. When listed for this purpose, the
smallest unit shipping carton containing such
boxes will be marked to indicate “For
Fixture/Luminaire Support” or similar wording.
Summary Table for Support of Luminaires/
Fixtures and Other Products
Maximum
Weight of
Luminaire/
Fixture or
Product
Wall Installation
Ceiling
Installation
kg (lbs.) Outlet
Box
Device
Box
Outlet
Box
Device
Box
0 to (0 to 6) YES YES YES YES
Other
than a
luminaire
+ to 22 (6+ to
49)
YESa YESa YESa
to 23 (6 to
50) New Work
Only
YESb YESb YESb
23+ (50+) New
Work Only
YESa
a Marking required on product: Maximum weight,
installation instructions, wall or ceiling
b Marking required on carton label: “Luminaire/Fixture
Support”
For Products Weighing Greater
Than 23 kg (50 lbs.) in or on Ceilings–New
Work Only
Where structural framework is accessible for
mounting a box in or on a ceiling, an outlet box
may be listed to support fixtures greater than 23
kg (50 lbs.). These boxes are required to be
marked with the maximum weight the box is
listed to support.
Articles 314 [370] and 410 [410] of the NEC®
provide details on the use of boxes to support
luminaires/lighting fixtures.
Safety Consideration: In old work installations, where
structural framing members are not accessible for
mounting the box, the installer must consider if the
mounting surface, such as drywall, will provide adequate
support for the box and the fixture/luminaire.
Outlet Boxes Intended to Support
Ceiling-Suspended Fans
Boxes and box-hanger assemblies that are
listed and marked “Fan Support” or other similar
wording are intended to support a ceiling-
suspended fan (with or without accessories such
as fixtures/luminaires) weighing 16 kg (35 lbs.)
or less. Boxes may be listed for supporting a fan
(with or without accessories such as
fixtures/luminaires) weighing up to 32 kg (70
lbs.), when marked “Acceptable for Fan Support
of 32 kg (70 lbs.)” or equivalent. Ceiling fan
boxes are to be supported from structural
framing members. These boxes are also
suitable for support of a fixture/luminaire when
additionally marked in accordance .
Where a separately supplied bracket such
as a bar hanger is to be used with a box marked
“Acceptable for fan support,” the separately
supplied bracket must also be marked to
indicate its acceptability for this use.
NEC® Section [422-18] provides
details on the use of boxes to support ceiling-
suspended fans.
Outlet and Device Boxes with Fire
Resistance Ratings (Hours)
Outlet and device boxes for flush mounting
in fire-resistance rated walls or ceilings are
addressed in Clause .
Boxes for High and Low Voltage
Wiring Circuits (Class 1, Class 2, and Class 3
Circuits)
Typical metallic and nonmetallic outlet and
switch boxes are designed for both high and low
voltage wiring circuits. These same boxes will
accommodate a wide variety of electric light,
power, data and communications circuits and
devices. For the purpose of these guidelines, we
will consider electric light and power circuits and
Class 1 circuits under the category of “high
voltage,” and communications and fiber optic
and Class 2 and 3 circuits as being “low voltage.”
NEC Article 725 [725] contains appropriate
wiring methods for Class 1, 2, and 3 circuits.
This Article also includes important restrictions
for boxes and enclosures that are intended to
house Class 2 or 3 circuit wiring with Class 1
circuit wiring.
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Additional low voltage wiring methods are
similarly addressed in NEC® Articles 770 [770]
and 800 [800].
Low Voltage Wiring Circuits (Class 2
and Class 3 Circuits)
Although the distinction between high and
low voltage is not clearly defined in the NEC,
Class 2 and Class 3 wiring circuits and devices
are not typically required to be installed in an
electrical outlet box unless the installation will be
subject to physical abuse in normal use as
required in NEC [725-7]. Simple
cover/device mounting frames are available to
conveniently complete these installations.
High and Low Voltage Wiring
Systems Installed Together
Where it is desirable or necessary for both a
high voltage wiring circuit (electric light, power,
Class 1) and a low voltage wiring circuit (Class 2
or 3) to be installed in the same box, special
consideration must be given to the restrictions
detailed in NEC® Sections [725-26] and
through [725-54]. Where
permitted for installation in the same outlet box,
the low voltage wiring must be separated from
the high voltage wiring. Outlet boxes of the
square and multi-gang type, and certain
gangable device boxes are available for use with
partitions specifically designed to separate the
wiring compartments.
Additional low voltage wiring methods are
similarly addressed in NEC® Articles 770 [770]
and 800 [800].
Voltage Between Adjacent Devices
NEC® Section (B) [380-8(b)] requires
that a snap switch shall not be grouped or
ganged in an enclosure with other snap
switches, receptacles or similar devices, unless
they are arranged so that the voltage between
adjacent devices does not exceed 300 volts. Use
of square or multi-gang type, or certain gangable
device boxes with provision for partitioning the
boxes to provide a barrier between such
devices, is an accepted method to comply with
this requirement.
Select the Outlet Box for the
Intended Location of Installation
General
The building construction style, materials
used, and environment are largely determined by
local electrical and building codes, depending on
the intended use for the finished structure.
Selection of components for the electrical
distribution system, including outlet boxes and
covers, is dependent upon building design,
system compatibility, and environmental
considerations. It is important to recognize that
several different environmental considerations
may affect the selection process for outlet boxes
and covers depending on the intended use of the
structure. Special consideration is needed when
selecting outlet boxes for use in fire resistance-
rated construction, and other special areas as
specifically addressed in applicable codes.
Other important selection criteria are the
structure and composition of the wall, ceiling,
floor, partition, or furniture on or in which the box
is to be mounted. Among the issues to consider:
Is the structural framework accessible for
attaching the box? What is the composition of
the structural framework—wood studs or steel
studs or concrete/masonry?
Electrical outlet boxes are available in
designs intended for installation on finished
surfaces, or flush mounted in walls, ceilings or
floors (See Figure 1). General-purpose outlet
and device boxes are designed for use indoors
in dry locations. When installed as intended with
weatherproof covers or cover plates, general-
purpose outlet boxes can be flush mounted in
the exterior walls of a structure. Special purpose
boxes such as weatherproof boxes and boxes
intended for use in swimming pool areas,
partitions, furniture and equipment, as well as
those for use in surface raceway systems are
also available, and require special consideration
during the selection process.
Boxes for Installation in or on Walls
or Ceilings
Surface Mounting Outlet Boxes
(Indoors and Outdoors)
When selecting an outlet box for surface
mounting, primary consideration is given to
providing maximum protection from physical
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damage both to the outlet box and its contents
during normal use (see section of Product
Installation). Since surface-mounted outlet boxes
are typically attached to surface mounted
raceways, surface mounted boxes typically must
have conduit entries. Gangable device boxes are
not typically recommended for surface mounting.
General-purpose outlet boxes are provided
with holes or other means for mounting the box
to a surface of the building structure.
Outlet and device boxes that are intended
for use in wet locations are often referred to as
“weatherproof” boxes and are provided with the
means to completely close all unused openings.
Metallic outlet boxes for these applications
typically have threaded conduit entries for
threaded Rigid Metal Conduit or Intermediate
Metal Conduit, while nonmetallic outlet boxes
typically have integrally molded sockets for Rigid
Nonmetallic Conduit. Manufacturer's instructions
may vary as to specific installation methods
necessary to ensure the proper finished
installation. These same boxes are also
acceptable for use as part of a pedestal-
mounted enclosure where a structural mounting
surface is not available. See Figure 1.
Flush Mounting Outlet and Device
Boxes in Hollow Walls or Ceilings
Many outlet and device boxes are designed
with specific features that assist in properly
mounting the box. When flush mounted in hollow
walls or ceilings, the face of the completed
enclosure—the cover plate, switch, receptacle,
or other device—will be essentially flush with the
finished surface. See Figure 1. The types of
boxes available for flush mounting include
gangable device boxes, square and octagonal
boxes with raised covers, and multiple gang
boxes.
When selecting outlet or switch boxes for
use in hollow walls, consideration should be
given to the depth of the wall cavity. The total
depth of the box including installed clamp screws
that will protrude out the back of the box must be
considered. Alternative box designs or
combinations of certain boxes and covers are
available to address this situation.
Where structural framework in a hollow wall
or ceiling is accessible for attaching the outlet
box (NEW WORK), many options are available.
These include pre-punched nail holes, integrally
molded slots with either nails or screws pre-
installed, brackets for mounting on vertical or
horizontal framing members, and bar hangers
that span and position the box between framing
members. Consideration should be given to
type of framing members (steel or wood) and
thickness of wall coverings, when selecting a
mounting bracket, or box/bracket assembly.
Boxes with adjustable brackets may be specified
when the finish wall surface thickness is
unknown. See Figure 2 for typical mounting
design options. When a bracket is supplied
separately from a listed box assembly, it shall be
listed for outlet box support. See Figure 3 for
typical separately supplied box supports.
Where structural framework in a hollow wall
or ceiling is not accessible for attaching the
outlet box (OLD WORK), outlet boxes are
available with special clips designed to secure
the box face flush to the structure's finish
surface covering. Outlet boxes provided with
these OLD WORK CLIPS are generally device
boxes intended to house a wiring device such as
a switch or receptacle, or to support a small
fixture such as a luminaire or other product.
Alternatively, separate box supports are
available which are intended to serve this
purpose. These separate supports shall be
listed for outlet box support.
Flush Mounting Boxes in Concrete
and Masonry Walls and Ceilings
A CONCRETE BOX or mud box, often called a
CONCRETE RING, is selected when the installation
calls for flush mounting the box by embedding it
in poured concrete during the casting process.
Where the application calls for flush
mounting of an outlet box in a hollow structure
with a masonry finish surface covering such as
tile or plaster, boxes typically designated
MASONRY BOXES are available. These specialized
box designs typically have a recessed flange
bracket. The flange bracket of these masonry or
tile boxes is intended to mount to the face of a
subsurface. When the tile or plaster is added,
the face of the box is flush with the finished
surface.
Masonry boxes are available in both single
and multiple gang styles for installation in a
concrete-block masonry wall having open cells.
A specialty “THROUGH-WALL BOX” is intended to
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provide two-way access from either end on both
sides of the wall. See Figure 1.
General Purpose Boxes Installed
Above Suspended Ceilings
General purpose outlet boxes, including
device boxes, may be installed in accessible
areas above suspended ceilings in accordance
with the requirements in NEC® Article 314 [370].
Boxes for Installation in Floors
General
All listed floor boxes are marked “Floor box”
on the product or its smallest unit shipping
carton, to indicate the intended use. All floor
boxes are designed for installation at or below
the floor surface. The first consideration in
selecting a floor box is the type of floor into
which it is to be installed. When fitted with the
appropriate cover and accessories, the interior of
the floor box and its contents are provided with a
degree of protection from physical damage and
the entrance of liquids (scrub water) or dirt.
Because of the high cost of expanding
branch circuits installed in floors and increasing
available capacity in floor boxes, special
consideration should be given during the design
phase to plan for future capacity needs and
access options. Often it is desirable to locate
additional floor boxes in a floor plan to provide
for future access. Many floor box designs have
optional “abandonment” kits that complete an
unused enclosure intended for planned or
unplanned future use. It is not necessary to
install devices and device access covers in
these boxes at the time of installation. Future
capacity considerations might also include
selecting a floor box that has provisions for
adding data and communications wiring circuits
as well as power.
Concrete Floors
Boxes specifically designed for installation in
concrete floors, and also marked “Concrete-
Tight” on the smallest unit shipping container,
may be encased in concrete slabs on grade, or
in concrete covered decking on the upper floors
of buildings. These boxes are positioned and
assembled to the raceway system prior to the
pouring of the concrete. The thickness of the
floor is another important consideration in
product selection.
Each floor box for concrete floors is provided
with a cover to protect against the entrance of
concrete aggregate during the pour. This cover
is often not intended as a permanent cover for
the enclosure. Some such covers may serve a
permanent function, to complete an enclosure
where the floor box is used as a junction box or
left empty for future use.
Through-Floor Penetrations
Poke-through floor fittings are specially
designed floor outlet assemblies that when
installed as intended, fully penetrate the concrete
floor in which it is installed. The electrical outlet
box component of the assembly is suspended in
the open area below the concrete floor after
installation. This open area is also the area
above the ceiling of the floor below. Additionally,
if the space below the concrete floor is used for
the transfer of environmental air, the outlet box
component may have to meet the requirements
for use in such locations. See Clause ,
Plenum Boxes.
Hollow Floors
Floor boxes intended for installation in
hollow floors are designed for support by either
the sub-floor or a structural member.
Raised Floors
Floor boxes intended for installation in raised
floors, such as computer room floors, are
designed to be supported by insertion into
access holes that are cut into raised floor panels.
Floor Box Covers
Floor box covers are required to complete
the enclosure. Care should be taken to select a
floor box cover that is designed to mount as
intended to the floor box. The desired finished
floor covering will help to determine the selection
of the appropriate cover assembly. The cover
assembly includes the cover itself and any
necessary accessories such as carpet flanges,
tile rings, fasteners, etc. The selection of the
cover will depend on such factors as the
required service to be provided (power, data,
communication, etc.), finished floor covering
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(carpet, tile, wood, etc.), and aesthetic
preferences (colors, finishes, etc.).
Boxes for Other Locations
Pendant Box Assemblies
For the purpose of these guidelines, a
pendant assembly typically consists of a junction
box on one end, from which a flexible cord or
raceway is used to suspend a device box. See
Figure 1. Similar assemblies are also common
on cord reels. The assembly shall be approved
for the specific application. The device box shall
provide the mechanical protection required of a
box installed in areas that may be subject to
physical damage. Cast metal or plastic molded
weatherproof boxes (such as FS and FD), and
conduit bodies are most often recommended to
house the devices in a pendant box assembly.
Particular product features may be considered in
order to comply with the rules for pendant
mounting in NEC® Section (H) [370-
23(h)].
Pedestal Box Assemblies
Boxes intended for support by approved
non-flexible raceways must have threaded
conduit entries. The assembly shall be
approved for the specific application. The device
box shall provide the mechanical protection
required of a box installed in areas that may be
subject to physical damage. Cast metal or
plastic molded weatherproof boxes (such as FS
and FD), and conduit bodies are most often
recommended to house the devices in a
pedestal box assembly.
Plenum Boxes
Special-purpose box and cover enclosure
assemblies are available, specifically designed
for installation either flush or suspended in areas
classified as air-handling spaces. All unused
openings in such boxes must be sealed and the
cover interface with the box must be gasketed to
form a reliable seal when completely installed.
For the purposes of these guidelines, air-
handling spaces are ducts or plenums used for
environmental air; other spaces used for
environmental air such as above hung ceilings or
below raised floors. This does not include
plenums used for the removal of dust, loose
stock, or vapor, or for ventilation of commercial
type cooking equipment. Refer to NEC® Section
(A) through (D) [300-22 (a) through (d)].
Plenums and air-handling spaces are within
the jurisdiction of Mechanical Code officials.
However, many jurisdictions still consider small
discrete products such as nonmetallic outlet
boxes, as minor combustible components with
respect to this application.
Swimming Pool Boxes
A swimming pool box is a specially designed
Junction Box for the connection of lighting
circuits for luminaires (fixtures) installed in or
near a swimming pool. NEC Article 680 [680]
includes requirements for swimming pools,
fountains, and similar installations.
A swimming pool junction box is typically
required to have threaded entries or hubs for
rigid steel conduit or sockets for rigid PVC
conduit. Threaded entries may also be used to
assemble fittings designed to accept flexible
cords. Conduit entries are only permitted to be in
the bottom of the junction box. Because of the
corrosive nature of the environment around a
swimming pool or similar location, swimming
pool junction boxes must be constructed of
metallic materials that are inherently resistant to
corrosion, such as copper or brass, or of a
suitable nonmetallic material. Swimming pool
junction boxes are provided for mounting either
flush with the deck surrounding a swimming pool
or for installation pedestal style supported above
the deck and water's surface. All components
necessary for the assembly of a swimming pool
junction box are provided with an assembly
including the box, its cover, grounding bus,
gaskets, sealing materials, etc., and any
hardware necessary to complete the enclosure.
Boxes for Use in Wet and Damp
Locations and Adverse Environments
An enclosure intended for use in wet, damp,
or adverse environments consists of a box, a
cover, and possibly other materials or
accessories, and may involve special installation
methods.
General-purpose flush mounted outlet boxes
and device boxes are suitable for use in wet or
damp locations, when installed with an
appropriate cover.
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Boxes for surface mounting, such as FS and
FD boxes and conduit bodies having threaded
conduit entries, and such nonmetallic boxes with
sockets suitable for use with Rigid Nonmetallic
Conduit, are suitable for use in wet or damp
locations when installed with an appropriate
cover.
The cover of a junction box intended for use
in wet locations must be marked “Wet
Locations.”
The suitability of covers used with boxes
intended for use in wet and damp locations, is
dependent upon several conditions such as:
a) Attended or Unattended Outdoor Wet
Locations (15 or 20 A, 125 or 250 V
receptacle): A cover plate marked “Wet
Locations” or a hood marked “Wet Location
Only When Hood Closed” or equivalent must
be used with a device box for mounting a 15
or 20 ampere, 125 or 250 V receptacle
outdoors in a wet location. A cover plate or
hood with this marking is intended for
installation in accordance with NEC® Section
(B)(1) [410-57(b)(1)]. Effective 2002,
this requirement applies to all such cover
plates or hoods installed outdoors.
b) Unattended Indoor (Wet Locations (all
receptacles) or Unattended Outdoor Wet
Locations (other than a 15 or 20 A, 125 or
250 V receptacle): A cover plate or hood
used with a device box for mounting other
than a 15 or 20 ampere, 125 or 250 V
receptacles intended for unattended use in
wet locations with a power-supply cord
attached, must be marked as described in a)
above. A cover plate or hood with this
marking is intended for installation in
accordance with NEC® Section
(B)(2)(a) [410-57(b)(2)].
c) Attended Indoor Wet Locations (all
receptacles) or Attended Outdoor Wet
Locations (other than 15 or 20 A, 125 or 250
V receptacle: A cover plate used with a
device box for mounting receptacles
intended for use in wet locations only while
attended, with the power-supply cord
attached, must be marked “Wet Locations
Only When Cover Closed.” A cover plate or
hood marked as described in a) is also
suitable for this application. Cover plates or
hoods with these markings are intended for
installation in accordance with NEC Section
(B)(2)(b) [410-57(b)(2)].
d) A cover plate or outlet box hood used
with a device box for use in damp locations
must be marked “Damp Locations.” Either
of these, when used with a device box for
use in wet locations can also be used in
damp locations.
e) An enclosure intended for use in
adverse environments will be marked with a
Type rating in accordance with ANSI/NEMA
250 or ANSI/UL 50, which reflects the
environmental conditions for which the box
or enclosure is suitable.
Boxes for Direct Burial in Earth
When listed for the purpose and approved
by the local jurisdiction as meeting the
requirements for accessibility in accordance with
NEC® Section [370-29], an outlet box
such as a junction or pull box, or conduit body is
permitted for direct burial.
Boxes in or on Partitions
A box for use in a partition, such as a wall in
a modular office, is either a listed component of
a manufactured assembly, or a general-purpose
box that is surface mounted.
Boxes in or on Furniture
A box for use in furniture is either a listed
component of a manufactured assembly, or a
general-purpose box that is surface mounted.
Boxes in Surface Raceway Systems
A box for use in surface raceway systems
typically is a component of the listed system.
Where components of separate systems are to
be used in an installation, consideration shall be
given to compatibility of the components.
Outlet and Device Boxes for Use in
Fire Resistance-Rated Construction
General
The fire-resistance rating for boxes and
poke-through floor fittings must have a rating in
hours, that is at least that of the wall, ceiling or
floor assembly into which it is to be installed.
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Metallic Boxes
Listed metallic single and double gang outlet
and device boxes with metallic or nonmetallic
cover plates are suitable for flush mounting in
ceilings, and in bearing and non-bearing wood
stud and steel stud walls with fire-resistance
ratings not exceeding 2 hours. Metallic boxes do
not require any markings other than the listing
mark.
Nonmetallic Boxes
Listed nonmetallic outlet and device boxes,
marked to indicate their classification and fire-
resistance rating in hours, are suitable for use
with metallic or nonmetallic cover plates, in fire-
resistance rated bearing and non-bearing wood
stud and steel stud walls, and ceilings.
Poke-Through Floor Fittings
Listed poke-through floor fittings marked to
indicate classification and fire resistance rating in
hours, are suitable for use in fire-resistance
rated floor construction.
Special Use Boxes
Both general-purpose boxes and specially
designed boxes, such as boxes without backs,
or combination high-low voltage boxes, are
available for mounting of Class 2 wiring such as
communication and other low voltage cables and
devices (See ).
Covers and Cover Plates for Dry
Locations
Covers and cover plates installed on a box
complete the enclosure. See Figure 1 for typical
covers and cover plates. Both metal and
nonmetallic covers and cover plates are
available. The appropriate cover or cover plate
should be selected for the intended application.
Covers and cover plates may be blank, have
knockouts or threaded entries, or have openings
for wiring devices or pendant cords. Holes
provided for mounting covers or cover plates to
outlet boxes and device boxes are standardized
in accordance with ANSI/NEMA OS 1 and
ANSI/NEMA OS 2, with respect to minimum
dimensions. Holes and other openings in cover
plates that are designed for use with various
types of devices are standardized in accordance
with NEMA WD 6 and NEMA OS 1 and OS 2.
The durability of a cover is an important
consideration when selecting a cover for a box to
be installed in areas that are subject to physical
damage. Outlet and device box covers are
intended to provide the same degree of
protection as the boxes to which they are
attached. They are available for both surface
mounting and flush mounting. A flush device
cover plate is intended only for flush mounting
applications.
A FLAT COVER for a flush or surface mounted
outlet box may be blank, have a knockout in the
middle, or be designed for attachment of a wiring
device directly to the flat cover. This assembly
completes the enclosure.
A RAISED SURFACE COVER for a surface
mounted outlet box (typically a square box) is
designed for attachment of a wiring device
directly to the RAISED SURFACE COVER and
provides additional internal volume to the
enclosure. This assembly completes the
enclosure.
A RAISED COVER (also called a plaster ring or
mud ring) for a flush mounted outlet box is
designed for attachment of a wiring device
directly to the RAISED COVER and provides
additional internal volume to the enclosure.
Since in this installation, the box is mounted
flush with the structural framing member,
consideration must be given to the thickness of
the finished wall surface in selecting the
appropriate height of the raised cover. The
raised cover alone does not complete the
enclosure. A cover plate is required to finish the
enclosure (See Figure 1).
A COVER PLATE (face plate or wall plate) for
flush-mounted wiring devices is a cover intended
to close a flush-device box and cover the edges
of the wall opening in which the box is located.
The cover is either a blank plate fastened to the
box or a plate fastened directly to a wiring
device. This type of cover completes the
enclosure.
Covers and cover plates for floor boxes and
those specifically intended to provide degrees of
protection in damp and wet locations and
adverse environments are addressed separately
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in this section under “Boxes for Installation in
Floors” (See ).
Extension Rings and Box Extenders
Extension rings are commonly used to
extend the walls of an outlet box or a flush
device box in order to increase the box depth,
volume or both. Since they function as an
extension of the walls of the outlet box they are
required to provide the same degree of
protection as the box to which they are attached.
A box extender is a device installed in or on
a box that is mounted in a finished structure.
The box extender rests on the edge of the
existing box or extends into the box, but is not
mechanically secured to the box. The flange of
the box extender, if provided, rests on the
finished surface. The combination of the box,
the box extender, and the appropriate cover
plate forms a complete enclosure.
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Figure 1-1
Typical Box Mounting Positions
Flush Mount – New Work Flush Mount – Old Work Flush Mount – New Work – Ceiling
Surface Mount – Walls Surface Mount –
Exposed Ceiling Stud
Surface Mount – Exposed Surface Mount – Protected Pendant Mount
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Figure 1-1 (continued)
Typical Box Mounting Positions
Flush Mount Pedestal Mount Pedestal Mount
Square Box with Raised Cover
Through–Wall Flush Mount
Masonry Box Old Work – Ceiling
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Figure 1-2
Typical Box Mounting Styles
Nonmetallic ceiling box
with bar bracket Nonmetallic wall mount
2 gang box with nails
Metallic wall mount
box with nails
Nonmetallic wall mount
box with nails
Nonmetallic wall mount
box with bracket
Nonmetallic ceiling box
with bracket
Metallic ceiling box
with bracket
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Figure 1-3
Separately Supplied Box Supports
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Figure 1-3 (continued)
Separately Supplied Box Supports
BOX TO T-BAR
OLD WORK BOX SUPPORT
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Section 2
Product Installation
Introduction
Having selected the right box for the
application, there are both general and specific
guidelines for installation of an electrical outlet
box related to each of the general selection
criteria, restated as follows:
� Type of wiring system
� Contents and function after installation
� Location and type of construction
Storage and Preparation for
Installation
When received on the jobsite, prior to
installation, boxes should be stored in a clean
dry location that is not subject to physical abuse.
Immediately prior to installation, inspect
boxes to confirm that appropriate product
selection has been made in accordance with
Section 1. To avoid unforeseen problems during
installation, boxes should be inspected to identify
damaged or missing features or parts, including,
where applicable:
Knockouts
Closure plugs
Cable clamps
Cover mounting means
Box mounting means
Sides of gangable boxes
Threads in threaded holes or entries
Staking and leveling means for concrete
floor boxes
Coordinating Trades (After Boxes are
Mounted)
The quality of a box installation can be
seriously compromised when an effort is not
made to coordinate the installation of the
electrical outlet boxes, and enclosed devices
and wiring, with other construction trades. In
particular, installation problems have been
evidenced where there was insufficient
coordination with the trades that apply finished
surfaces and poured concrete. Every effort
should be made to coordinate with these and
other trades who, through lack of careful
consideration, may compromise the quality of
the installation.
Fire Resistance-Rated
Construction
The installation of an outlet box in a fire
resistance rated wall or floor-ceiling assembly
requires particular attention to detail. After the
installation has been completed, the penetration
that is made in the wall, ceiling, or floor to
accommodate the box must not compromise the
designed fire resistance rating of the assembly.
General installation rules apply to metallic
outlet boxes in these special installations.
Particular installation instructions must be
followed for nonmetallic outlet boxes that have
been classified for this use. Both general and
particular instructions are detailed in published
directories such as the UL Fire Resistance
Directory.
The following summarizes the general rules
for installation of outlet boxes as described in the
UL Fire Resistance Directory.
Metallic Boxes
Listed single and double gang metallic outlet
and switch boxes with metallic and nonmetallic
cover plates may be used in bearing and
nonbearing wood stud and steel stud walls with
ratings not exceeding 2 (hours). These walls
shall have gypsum wallboard facings similar to
those shown in Design Nos. U301, U411 and
U425. The metallic outlet or switch boxes shall
be securely fastened to the studs and the
opening in the wallboard facing shall be cut so
that the clearance between the box and the
wallboard does not exceed 1/8 inch. The
surface area of individual metallic outlet and
switch boxes shall not exceed 16 sq. in. The
aggregate surface area of the boxes shall not
exceed 100 sq. in. per 100 sq. ft. of wall
surface.
Metallic outlet boxes located on opposite sides
of walls or partition, shall be separated by a
minimum horizontal distance of 24 in. This
minimum separation distance between metallic
boxes may be reduced when “Wall Opening
Protective Materials” (CLIV) are installed
according to the requirements of their
Classification.
Page 24 NEMA OS 3-2002
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Metallic boxes shall not be installed on opposite
side of walls or partitions of staggered stud
construction unless “Wall Opening Protection
Materials” are installed with the metallic boxes
in accordance with Classification requirements
for the protective materials.
Listed metallic outlet boxes with metallic or
nonmetallic cover plates may be used in floor-
ceiling and roof-ceiling assemblies with ratings
not exceeding 2 hours. These assemblies shall
have gypsum wallboard membranes. The
metallic outlet boxes shall be securely fastened
to the joists and the opening in the wallboard
facing shall be cut so that the clearance
between the box and the gypsum wallboard
does not exceed 1/8 in. The surface area of
individual boxes shall not exceed 16 sq. in. The
aggregate surface area of the boxes shall not
exceed 100 sq. in. per 100 sq. ft of ceiling
surface.
(Copyright–2001 Underwriters Laboratories Inc.;
reprinted from 2001 Fire Resistance Directory (BXUZ)
with permission from Underwriters Laboratories Inc.)
When applying the requirement for surface
area of individual outlet and switch boxes (16 sq.
in.), many jurisdictions permit installation of
standard 4-11/16 inch square trade size boxes,
which are installed in wall penetrations that do
not exceed 16 square inches. These boxes are
typically covered by a raised cover (see Figure 2-
1).
Nonmetallic Boxes
Listed nonmetallic outlet and device boxes
that are identified with a specific fire resistance
rating in hours are to be installed according to
specific instructions shown in the UL Fire
Resistance Directory or provided by the
manufacturer. Often installation instructions may
vary slightly requiring particular attention to the
details.
The following summarizes the general
direction provided in the UL Fire Resistance
Directory relative to nonmetallic boxes.
The category of “Outlet Boxes and Fittings
Classified for Fire Resistance” (CEYY) includes
Classifications for nonmetallic outlet and switch
boxes for use in wall or partition assemblies.
The information provided for each Classification
includes the model numbers for the Classified
products, a description of the rated assemblies,
the spacing limitations for the boxes and the
installation details. Nonmetallic outlet boxes
shall not be installed on opposite sides of walls
or partitions of staggered stud construction
unless Classified for use in such constructions.
Nonmetallic outlet boxes evaluated for
installation in floor-ceiling or roof-ceiling
assemblies are included in Outlet Boxes and
Fittings Classified for Fire Resistance (CEYY).
(Copyright–2001 Underwriters Laboratories Inc.;
reprinted from 2001 Fire Resistance Directory (CEYY)
with permission from Underwriters Laboratories Inc.)
When installed in accordance with the
requirements for “Wall Opening Protective
Materials” (CLIV), additional installation options
may be available for specified nonmetallic outlet
and device boxes.
Mounting and Securing of
Electrical Boxes
General
The National Electrical Code® requires that
electrical boxes be securely mounted in place so
that they remain secure during the construction
process (raceway attachment, wire pulling, dry
wall installation, etc.). Mounting hardware
(screws, nails, etc.) provided with the box is not
to be substituted. When the box is not supplied
with mounting hardware, the hardware shall be
acceptable for the installation by the local
authority having jurisdiction (AHJ). When
installing nonmetallic boxes, only holes provided
for mounting shall be used. Brackets intended to
support electrical boxes that are supplied
separately from the box should be listed for the
specific purpose.
Surface Mounting on Walls or Ceilings
Surface mounted boxes should be mounted
in a manner that prevents the box from turning.
This can be accomplished by relying on the
attached raceway for additional support. The
back of surface mounted boxes should be
secured against a surface that provides
adequate support for the box. Placement of
surface mounted boxes should take the
exposure to possible physical damage into
consideration. For instance, placing the box in
the U-shaped channel portion of an I-beam
would provide more protection than mounting it
to the exposed surface of the flange (See Figure
1, Pole Mount-Protected).
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The hardware used to mount a box shall be
suitable for the surface to which it is mounted.
For surface mounting on drywall or other hollow
wall or ceiling surfaces, where a nail or screw is
not able to securely attach to a structural framing
member, suitable wall anchors and appropriate
fasteners must be used. Suitable concrete
anchors or special masonry nails may be used
with concrete or masonry block construction.
The heads of nails or screws inside a metallic
box shall be driven to where they do not present
a risk of damaging conductors during installation
or in normal use. Listed nonmetallic boxes have
a molded pocket or barrier around the screw or
nail head. Where it is necessary to install a
ground screw, consideration should be given
before installation to providing clearance for the
end of the screw.
Boxes Supported by Raceways
Under certain circumstances, NEC® Section
[370-23] permits boxes to be supported
by raceways. Where boxes are located on the
surface of a structure and supported in an
approved manner by a raceway, the box is not
required to be secured to the surface of the
structure.
Flush Mounting in Walls and Ceilings
Flush mounting means that an enclosed
wiring device, when mounted as intended, will be
essentially flush with the finished wall or ceiling
surface. Therefore, a flush device box should be
mounted with an offset from the stud or joist that
will accommodate the thickness of the wallboard
that will be installed later. Most flush device
boxes have convenience markings that can be
used to allow for proper alignment with the edge
of the stud.
New Work Installations
Flush device boxes for direct mounting to a
wood stud are provided with mounting brackets,
nails, or clearance holes for nails or screws,
which permit the box to be properly positioned
and secured to the stud. Boxes for mounting to
a steel stud have brackets that attach to the stud
by various means. If the box being installed has
a bracket that attaches to the face of the stud,
screws or nails with low profile heads should be
used to minimize the amount of bulge that the
finished wall will have at the box location. NEC®
(B) [370-23(b)] specifies the conditions
for structural mounting of a box.
NEC® [370-20] requires that a box
installed in a wall or ceiling made of non-
combustible material such as concrete, tile or
gypsum may be recessed a maximum of 6 mm
(1/4 in.) from the wall surface. When the box is
installed in a wall or ceiling that is made of
combustible material or has a combustible
surface material, the face of the box must be
flush or project slightly from the finished surface
of the wall or ceiling. The box should be installed
so that the wiring device cover plate will be in
contact with the wall surface.
Square boxes being used for flush mounting
should be mounted close to flush with the stud or
joist, with consideration given to the head of the
screw and the thickness of the cover (see Figure
1, Flush Mount – Square Box with Raised
Cover). This will allow the properly selected
raised cover to fit flush with the installed surface.
Boxes with brackets on the side can be
secured using nails or screws installed through
the holes provided in the bracket. The screw or
nail should be suitable for the material (wood or
metal stud) to which the box is being secured.
Single gang boxes without brackets can be
mounted using nails or screws that pass through
both walls of the box. The nails or screws should
be long enough to adequately engage the wood.
Only those holes provided in boxes for through-
nailing should be used as they have been
examined for proper clearance of devices and
conductors.
Toe-nailing of square boxes without brackets
is not recommended where it can be avoided.
When using this practice however, care should
be taken to keep the head of the nail or screw
from interfering with the conductors, and to
prevent splitting of wood studs or joists. Toe-
nailing of single gang flush device boxes is not
recommended.
Mounting screws that pass through the
interior of metallic boxes must have all exposed
threads covered to prevent damage to the
insulation on the conductors. Listed nonmetallic
boxes are required to have design elements that
protect conductors from exposed screw threads.
Page 26 NEMA OS 3-2002
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Screws for conduit or cable clamps, that will
protrude out the back of the box must be allowed
sufficient space for tightening in the wall cavity
behind where the box is to be mounted. The
amount of available space should be verified
during installation. If it is found that the screws
will hit the opposite side wallboard then
adjustments should be made. Moving the box
slightly forward will often times solve this
problem. If the box cannot be moved far enough
forward then a different box design or box-cover
combination should be selected.
Old Work Installations
When structural framing is not accessible
(old work), flush device boxes may be installed
by attaching the box directly to the finished wall
surface, such as gypsum wallboard, by means of
clips or brackets. NEC® (C) [370-23(c)]
requires the box to be rigidly mounted in the
finished surface and secured by methods
intended for the application. Particular care
must be taken when mounting the box so that a
finished wall surface (such as paneling) will
permit mounting of a wiring device in accordance
with NEC (B) [380-10(b)] and (A)
[410-56(f)(1)].
When a new wall surface is being applied to
an existing wall where there is a box already
properly installed, an accessory such as a box
extender may be necessary to maintain the
proper mounting position of the device, with
respect to the new wall surface.
Pendant Mounting
A pendant assembly typically consists of two
boxes. The first is a junction box that may be
surface mounted or flush mounted. Mounting of
this junction box shall be in accordance with the
guidelines for the applicable mounting. This
junction box may be placed in either an
accessible or inaccessible location. The second
box typically houses a switch or device or
supports a fixture such as a luminaire. Particular
installation methods, such as strain relief or anti-
rotational locking, may be required to comply
with pendant mounting rules in NEC Section
(H) [370-23(h)].
The location for access to pendant-hung
device boxes should not pose a hazard to
personnel when used. Pendant-mounted device
boxes should not be hung in high traffic areas.
Guidelines for attachment of wiring systems
to pendant boxes are found in Clause .
Pedestal Mounting
Wherever possible, pedestal mounted boxes
should be located where they are provided with
maximum protection from physical damage.
Where this is not possible, consideration should
be given to providing additional support for the
assembly. NEC Section (E) and (F) [370-
23(e) and (f)] covers support of pedestal boxes.
A pedestal box assembly should not be located
in a high traffic area. Pedestal boxes are not
typically permitted to be supported by a single
raceway.
Guidelines for the attachment of wiring
systems to pedestal mounted boxes are found in
Clause .
Masonry Wall Mounting
Single and multiple gang outlet boxes are
able to be installed in concrete-block masonry
walls having open cells. Sufficient support for
these installations is provided by the tightness of
fit in the concrete block and the attachment of a
non-flexible raceway. A close fitting opening
should be cut in the concrete block and any area
around the box should be mortared.
Floor Box Mounting
Because of the many unique floor box
design features, care should be taken to closely
adhere to the manufacturer’s installation
instructions. Check to be sure all components
necessary to complete the floor box installation
in accordance with the instructions provided by
the manufacturer are on hand.
Hollow Floors
Floor boxes in hollow floors, in new or old
construction, are typically mounted in a hole that
is cut through a sub-floor. The hole should be cut
so that all flanges provided with the floor box are
in secure contact and able to be secured as
instructed to the sub-floor surface. In old
construction, the security of the sub-floor should
be inspected first. The depth of the sub-floor
surface in relation to the finished floor surface
should be predetermined to ensure proper cover
mounting.
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Concrete Floors
Be certain to stake or otherwise secure the
box and attached raceway in place in
accordance with manufacturer’s instructions
(See “Floor Boxes in Concrete Floors” in Section
, Connection to Wiring Systems.) In fire
resistance rated floor/ceiling assemblies, the box
should be positioned and mounted to ensure a
minimum of 25 mm (1 in.) of concrete will flow
and set beneath the floor box.
Level the box using the means described in
the manufacturer’s instructions.
Secure in place the cover or other means
provided to ensure that concrete does not enter
the box during the pour.
A metal floor box installed in poured
concrete floors is typically placed at a distance
above the bottom of the form (or grade on the
ground floor) that will allow access to the box
opening after the pour without requiring removal
of a significant amount of concrete above the
box. As a helpful guide, the maximum
adjustment permitted by the adjusting ring
portion of the box will help determine the
maximum depth for locating the top of the box
before the concrete pour.
Nonmetallic floor boxes are typically
designed to protrude out of the poured concrete
and need to be located at a height that will allow
this.
Poke-Through Floor Fitting Mounting
Local building codes contain explicit limits on
the type, size, number, and placement of
through-floor penetrations in concrete floors and
decks. Since many poke-through floor fittings
are designed for installation in fire resistant rated
floor/ceiling assemblies, the installation
instructions required with these products must
be strictly adhered to.
Poke-through floor fittings are mounted into
holes that are core drilled through pre-cast
concrete floors and decks of variable depth. It is
very important that the appropriate type and size
of equipment, and core drilling technique is
used. Be certain the poke-through is the correct
size for the depth of the floor before installing.
Other Boxes
Swimming Pool Boxes
When installing a swimming pool junction
box, installation according to manufacturer's
instruction is important. The bottom of the box
containing the conduit entries must always be
positioned facing downward. General guidelines
for assembly to threaded entries or sockets
provided in Section are to be followed.
Where flexible cord is to be installed using a
specific fitting assembled to a threaded entry in a
box, adequate strain relief for the cord should be
provided. Often, the manufacturer will
recommend the use of a potting compound to
help seal the cord entry. Be certain all gaskets
are in place.
Where the swimming pool junction box is
installed pedestal style above the pool's deck
and water's surface, adequate support should be
provided for the box. See guidelines in Section
for support of pedestal mounted outlet
boxes.
Boxes for Use in Wet and Damp
Locations and Adverse Environments
The installation guidelines for flush mounting
or surface mounting apply.
Boxes for Direct Burial in Earth
Boxes for direct burial in earth are not within
the scope of these guidelines.
Boxes in or on Partitions
The installation guidelines for flush mounting
or surface mounting apply. Special
consideration as described in boxes for fire
resistance-rated construction should be given to
partitions with fire-resistance ratings.
Boxes in or on Furniture or
Equipment
For field installation, guidelines for flush and
surface mounting apply. Boxes that are
provided as part of furniture or equipment
assemblies should be installed according to the
recommendations of the furniture or equipment
manufacturer.
Page 28 NEMA OS 3-2002
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Boxes in Surface Raceway Systems
Boxes in surface raceway systems are not
within the scope of these guidelines. Only boxes
that are specifically designed for use in these
systems are to be installed.
Special Use Boxes
Special use boxes such as those that are
without backs, for mounting of Class 2 wiring,
must be installed in accordance with the
manufacturers instructions included with the box.
Connection to Wiring Systems
General
Proper installation of outlet boxes will
provide for secure mechanical attachment of the
conduit, tubing, or cable at the point of entry. In
addition, proper installation of metallic outlet
boxes connected to metallic conduit, tubing, or
metal-jacketed cables will provide a reliable
connection to ground.
See NEMA FB , Selection and
Installation Guidelines for Fittings for Non-flexible
Conduit and Tubing, and NEMA FB ,
Selection and Installation Guidelines for Fittings
for Flexible Conduit, Tubing and Cable for
guidance in the connection of fittings to conduit
and cable entries in
Where a fitting or box manufacturer marks a
particular tightening torque for installing the
securement means provided for connection of a
wiring system, this torque should be measured
at the time of installation and verified when the
installation has been completed.
Connection to Conduit Entries
Threaded Entries for Threaded
Conduit or Threaded Fittings
Where a conduit entry has threads tapped all
the way through (no integral bushing), care must be
taken to ensure that at least 3½ threads of the
conduit or threaded fitting are fully engaged with the
threads of the conduit entry when wrench tightened.
When a conduit entry has an integral bushing, care
must be taken to ensure that at least 5 threads of
1 These documents are available free of charge for
downloading at .
the conduit and three threads of a fitting are fully
engaged with the threads of the conduit entry when
wrench tightened (see Figure 2-2).
After installation is completed all unused
conduit entries must be plugged with properly
sized closure plugs.
Unthreaded Entries for Unthreaded
Conduit or Tubing
Knockouts
Knockouts used for connection of conduit
and tubing are to be completely removed. When
removing the inner segment of a combination
knockout, care should be taken so as not to
break the connection between the box and the
remaining segment.
For metallic boxes, listed reducing washers,
where needed, may be used on both sides of a
knockout to allow installation of smaller sized
conduit or tubing. If any knockout is
inadvertently removed and not used, it is to be
plugged with a properly sized closure plug.
Boxes such as junction and pull boxes
supplied without conduit entries are intended to
have such entries made in the field in
accordance with manufacturer’s instructions.
For nonmetallic boxes, if a knockout is
inadvertently removed and not used, the
knockout is to be effectively closed with an
approved closure plug. If one is not available the
box is to be discarded and replaced.
Unthreaded Sockets (Hubs)
Set screws provided in unthreaded sockets
(hubs) of metallic boxes to attach Rigid Metal
Conduit, Intermediate Metal Conduit (IMC) or
Electrical Metallic Tubing (EMT), shall be
adequately tightened to ensure a secure
mechanical and electrical connection. Unless
otherwise specified in the manufacturer’s
instructions, the reference maximum tightening
torque shall be as follows:
For screws tightened with a screwdriver:
No. 10 and larger, 35 pound-inches ( N•m)
For wrench-tightened bolthead screws:
160 pound-inches ( N•m)
Over-tightening should be avoided as this
can cause damage to the raceway.
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Sockets (integrally molded hubs) for
attachment of Rigid Nonmetallic Conduit (RNC)
or Electrical Nonmetallic Tubing (ENT) in
nonmetallic boxes are typically designed for one
size of conduit or tubing. Where a smaller trade
size conduit or tubing is to be installed, an
appropriate sized reducer bushing must be
cemented into the socket before inserting the
conduit or tubing. The recommended method
for cementing the reducer in place is to apply the
appropriate solvent cement both inside of the
hub and on the surface of the reducer. After
completely inserting the reducer, to where it is
seated against the end stop of the socket, the
reducer is then rotated 1/4 turn. Similarly, the
solvent cement is applied to both the surface of
the conduit or tubing and inside the socket. The
conduit or tubing is then completely inserted into
the socket so that it is seated against the
socket’s end stop and rotated 1/4 turn.
PVC conduit should be cut to length using a
hacksaw, fine-toothed saw, rotating pipe cutter,
or a shear type cutting tool. All cuts should be
clean and square with the barrel of the conduit,
all burrs or ridges from the conduit removed with
a knife, file, or abrasive paper, and joint surfaces
wiped clean of dirt, moisture, or other
contaminants. Use PVC cleaner or primer if
necessary. Dry fit the joint to assure conduit and
the socket can be joined. Apply the cement
quickly and evenly to the inside of the socket and
to the end of the conduit. Immediately insert the
conduit fully into the socket and apply a one-
quarter twist on the conduit as insertion is made
to evenly distribute the cement. Hold the joint
firmly together for 10 to 20 seconds without
movement. Cold weather applications may
require a longer holding time. A small bead of
cement should appear around the lip of the
socket if adequate cement has been applied;
wipe off this excess bead after the joint has set.
Handle the newly assembled joint carefully until
the cement has gone through an adequate set
time. The recommended set times for cemented
joints are related to the temperature of the joint
(See Table 2-1).
Table 2-1
Recommended Set Times for Cemented Joints
Application temperature during
cementing
Minimum set time
15 to 40ºC (60 to 100ºF) 30 minutes
5 to 15ºC (40 to 60ºF) 1 hour
-5 to 5ºC (20 to 40ºF) 2 hours
-20 to -5ºC (0 to 20ºF) 4 hours
Floor Boxes in Concrete Floors
Because of the permanence of the
installation, the following special precautions are
necessary before the concrete is poured.
Double-check all conduit entries. Be certain
conduit connections are secured. Perform an
electrical continuity test on metallic raceway and
box systems. Be certain all unused conduit
entries are securely closed with appropriate
closure plugs.
Connection to Cable Entries
General
Cable may enter a box through a knockout
when assembled to an appropriate cable fitting.
In this case, installation of a cable fitting in a
knockout is the same as described for conduit
entries, above.
Metallic Boxes
General
When assembling cable, small diameter
tubing, or flexible metal conduit through a pryout
in a metallic outlet box, remove the pryout
completely, making sure that no sharp edges
remain. Insert the conduit, tubing, or cable
through the pryout and the securement clamp
provided with the box. NEC® (300-14)
provides the required minimum length of
conductors in a box. Depending on the type of
wiring system used, the method of installation
will vary.
Nonmetallic-Sheathed Cable
Once the appropriate length cable is inserted
into the box, the cable jacket is secured with a
minimum of 6 mm (1/4 in.) of the cable jacket
extended beyond the securement clamp. Only
after the appropriate length of cable jacket has
been removed should the securement clamp be
tightened in place. Particular care should be
taken to position the cable’s jacket in the clamp
so that it will not be abraded by exposed screw
threads when the securement clamp is
tightened. The securement screw, when
provided, should be tightened so that the cable is
secure. Overtightening of the screw is to be
avoided as damage to the outer jacket of the
cable can occur, either at the time of installation
or, over time as a result of cold flow of the jacket
material under excessive pressure of the clamp.
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Metal Jacketed Cables
Once the appropriate length of cable jacket
has been removed, and the end of the metal
jacket squared, the cable is inserted into the box.
The conductors are extended beyond the
securement clamp and the squared end of the
metal jacket is seated against the end stop
provided in typical clamp designs. The bushing
required for Armored Cable must be held
securely in place. The clamp is then tightened
so that it seats on the metal jacket of the cable.
Be certain that the clamp is properly seated on
cable jackets having irregular surfaces. Seating
can be ensured by hand-tightening the clamp
securement screw then slightly rotating the cable
until the clamp seats itself. The securement
screw can then be tightened in place.
Flexible Metal Conduit or Tubing
Once the end of the flexible metal conduit or
tubing is squared it is inserted into the box. The
squared end of the conduit or tubing is seated
against the securement clamp’s smooth rounded
end stop. Be certain that the clamp is properly
seated as described for metal jacketed cables.
Screw Torque
Unless otherwise specified in the
manufacturer’s instructions, No. 8 securement
screws are tightened to a torque of 20 pound-
inches ( N•m), and No. 10 securement
screws are tightened to a torque of 35 pound-
inches ( N•m). Over-tightening should be
avoided as this can cause damage to the cable,
conduit or tubing.
Nonmetallic Boxes
Nonmetallic-Sheathed Cable
If a cable entry contains a knockout that
must be removed for the cable to be inserted, it
is to be removed cleanly and completely. Then
the appropriate length of cable is inserted into
the box, and the cable jacket is secured with a
minimum of 6 mm (1/4 in.) of the cable jacket
extending beyond the securement clamp. Care
should be taken when stripping the outer jacket
of the cable to avoid unnecessary strain on the
securement clamp. When using a screw-type
securement clamp, the screw should be
tightened in place only after the appropriate
length of cable jacket has been removed,
applying the same torque indicated above for
clamps in metal boxes.
Electrical connection – A box intended for
connection to metal raceway or metal enclosed
cable must provide a continuous electrical
grounding connection. A metallic outlet box
connected to a nonmetallic conduit, tubing, or
cable must have provision for connection of a
separate grounding conductor. A nonmetallic
outlet box may be connected to a metal
enclosed cable or metallic raceway, as long as a
bonding jumper is installed between the
raceways or cables. See Section .
Connection to High and Low Voltage
Wiring Circuits
Where both a high voltage wiring circuit
(Electric light, power, and Class 1 circuits) and a
low voltage wiring circuit (Communications and
fiber optic circuits, and Class 2 or 3 circuits) is to
be installed in the same outlet or device box, the
high and low voltage wiring must enter the box
through separate conduit or cable entries and be
separated in discrete wiring compartments.
Proper installation of partitions provided with
multi-gang boxes, or proper assembly of
modular, gangable boxes according to
manufacturer’s instructions assures the required
separation of these wiring circuits. All such
installations must conform to the requirements
detailed in NEC® Articles 725 [725], 770 [770]
and 800 [800].
Completing the Installation
Introduction
At this point in the process, the right box for
the application has been selected, and the box
has been mounted as intended and the raceway
or cable has been secured to the box. All that
remains is to complete the enclosure so that it is
equipped to perform its intended function. As
the saying goes, “the devil is in the details.” The
variety of specific uses that an electrical box can
serve at this point is staggering. However, in
large part, the most important guidelines for
completing the installation of a box can be
covered in a few general categories. Specific
guidelines for particular types of boxes and
installations are covered separately.
Make sure that construction debris such as
sawdust, plaster dust, and Kraft paper is removed
from the box before installing a wiring device.
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Grounding and Bonding
Metal Boxes
The NEC® requires that metal boxes be
grounded independently of any devices installed.
This may be accomplished through the use of a
continuous grounded raceway or cable armor, or
through the use of separate grounding
conductors. If grounding conductors are used,
the following methods may be employed to
ground the box. If multiple grounding conductors
are present they must be spliced together before
attaching to the box.
1) Attach the conductor to the box using a 10-32
ground screw. This is the preferred method.
Whenever there is paint or corrosion on the
box surface, under a screw head, the area
should be scraped or sanded shiny before the
ground wire is wrapped around the screw.
Other holes, including tapped holes containing
other screws, are not intended for grounding.
2) Use a listed ground clip on the side of the box.
Such clips are only suitable for use on flat
surfaces. Ground clips are listed for the
thicknesses of boxes for which they have been
evaluated. If the clips have been evaluated for
a range of thicknesses, the package of clips
will be marked accordingly
Devices Installed in Surface-Mounted
Metal Boxes (. Device (Utility) Boxes; FS
and FD Boxes; Square, Round, or Octagon
Boxes)
Outlet devices such as receptacles that are
depended upon to carry the fault current from
equipment (appliances, power tools, etc.) can be
bonded by either of the following two methods:
1) Run a bonding conductor or pigtail from the
green hex shaped ground screw on the device
to the box or to spliced grounding conductors
attached to the box.
With a rectangular box (single- or two-gang),
remove non-conductive screw retainers
located between device yoke(s) and box, and
screw the device(s) down tight to the box,
establishing metal-to-metal contact between
the device yoke(s) and the grounded box.
Then screw the cover plate to the device.
Other approaches that were once common no
longer comply with the NEC®. These include
mounting a device to a metal cover over a 4 X
4-inch square box or an octagon box, where
the bonding connection depends on the metal
screws holding the cover to the box. This is
now permissible only if the box and cover
combination are listed as providing satisfactory
ground continuity between the box and
receptacle.
2) Other devices such as switches are required to
be effectively grounded and to provide means
to ground metal faceplates. This can be
accomplished by either of the following
methods:
The equipment grounding conductor or
equipment bonding jumper is connected to the
equipment grounding terminal on the switch,
when provided.
The device is mounted, using metal screws, to
a nonmetallic box with integral means for
grounding via the mounting.
Devices Installed in Flush-Mounted
Metal Boxes
Outlet devices such as receptacles that are
depended upon to carry the fault current from
attached equipment (appliances, power tools,
etc.) can be bonded by either of the following two
methods.
1) Run a bonding conductor or pigtail from the
green hexagonally-shaped ground screw on
the device to the box or to spliced grounding
conductors attached to the box.
Use a receptacle that includes a yoke or
automatic ground clip specifically listed for the
purpose of establishing the ground connection
to the box in conjunction with the metal
mounting screws.
2) Other devices such as switches are required to
be effectively grounded and provide means to
ground a metal faceplate. This can be
accomplished as follows:
An equipment grounding conductor or
equipment bonding jumper is connected to the
equipment grounding terminal on the switch,
when provided.
The device is mounted using metal screws to a
metal box or mud ring, or to a nonmetallic box
with integral means for grounding via the
mounting.
Page 32 NEMA OS 3-2002
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Common Grounding/Bonding Errors
Beware and avoid these three common
means of attempting to ground/bond devices and
metal boxes. The NEC® does not permit these!
1) Connecting the grounding wire to a device and
ignoring box grounding, or relying on the wiring
devices mounting to bond the box.
2) Connecting the wire to a screw that is
simultaneously performing another function,
such as securing a cable clamp.
3) Wrapping the wire under or around something,
such as a cable sheath, that will be squeezed
by metallic hardware in contact with the box,
such as a cable clamp or part of a connector.
Installation of Wiring Devices
General
Device boxes and box/cover combinations
intended for the installation of wiring devices
such as switches, receptacles, dimmers, GFCIs
and other similar products have mounting holes
that are spaced to accept the screws provided in
the mounting straps of wiring devices. The
spacing of these mounting holes is standard for
all wiring devices. The wiring device is secured
to the box using the screws provided with the
device. The screws should be tightened to
securely mount the device but care should be
taken not to overtighten the mounting screws, in
order to prevent deforming the wiring device
mounting strap or yoke, or stripping the threads
in the mounting holes of the box.
Wiring devices may also be mounted on box
extenders, extension rings, and raised covers if
they have the appropriate mounting holes for
accepting the wiring device mounting screws.
Longer screws than those supplied with the
device may be needed when mounting box
extenders. Typically, box extenders are
provided with the necessary longer screws.
Where longer screws are not provided, use only
the type of screw provided with the wiring device.
An appropriate cover or cover plate must be
used to complete all wiring device installations.
Voltage Between Adjacent Devices
Where the voltage between a snap switch
and adjacent snap switches, a receptacle or
similar device will exceed 300 volts, the snap
switch is not permitted to be grouped or ganged
in an enclosure. Only when the devices are
arranged to eliminate this condition, or where a
properly installed barrier is provided, are the
devices permitted to be grouped or ganged in
the same enclosure. See in the Selection
section.
Boxes Intended to Support Fixtures/
Luminaires, Ceiling-Suspended Fans, and
Other Products
Due to the importance for safety and the
uniqueness of the design of boxes intended to
support fixtures/luminaires, ceiling suspended
fans, and other products, specific markings
described in Section must be verified prior
to installation.
Boxes Intended to Support
Fixtures/Luminaires
In old work installations, the installer must be
aware that a box intended to support a
fixture/luminaire is not supported by a structural
member. Rather, the box is supported by the
finished surface. Surfaces such as drywall may
not provide adequate strength to support the box
and the fixture. Strong consideration should be
given before supporting a luminaire weighing
more than 7 kg (15 lbs.) without additional
supporting means.
Boxes Intended to Support Ceiling-
Suspended Fans
Ceiling fan boxes are uniquely designed to
support the dynamic loads generated by ceiling
fans in normal use. Always use a listed ceiling
fan box to support ceiling fans. All listed fan
boxes have a permanent label, stamping, or
marking stating “acceptable for fan support.”
Make sure the box has this mark.
Another factor to consider is the total weight
of the fan and the weight rating of the box. The
total weight is the fan weight plus any add-ons
such as light fixtures. If a box does not state a
weight rating, then it can support a total weight
up to 16 kg (35 pounds). Specially listed fan
boxes are allowed to support a total fan weight
up to 33 kg (70 pounds). If the box is listed for
fan support over 16 kg (35 pounds), then its
weight rating must be marked on the box.
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NEMA OS 3-2002 Page 33
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Where the box will provide support for the
weight of the fan, the instructions provided with
the fan box for mounting the box to the structure
shall always be followed. The installer must
carefully consider both the installation
instructions provided by the fan box
manufacturer and those provided by the
manufacturer of the fan when mounting the fan
to the box. The fan manufacturer may provide
alternative mounting instructions and hardware
for mounting the fan to the box. If the preferred
alternative mounting method is in conflict with
the instructions provided with the fan box, the
fan box manufacturer should be consulted for
recommendations. The screws provided with
the fan box for securing the fan to the box shall
be used in all cases. Where the weight of the
fan is to be supported by the structure,
independently of the box, the instructions
provided by the fan manufacturer shall always be
followed.
Boxes for Fire Resistance-Rated
Construction
For specific installation requirements for
listed boxes, refer to directories such as the UL
Fire Resistance Directory. Boxes classified for
this application that are included in the UL Fire
Resistance Directory typically comply with model
building codes. See Section .
Floor Boxes
Because of the many unique floor box design
features, care should be taken to closely adhere
to the manufacturer’s installation instructions
when completing the installation.
The main areas of consideration at this
phase in the installation of a floor box assembly
are: final leveling adjustments and attachment of
the floor box cover assembly to the finished floor
surface.
Final Leveling Adjustments—Most floor
boxes intended for installation in concrete
provide options for final leveling adjustments
after the concrete is poured and set. For floor
boxes of nonmetallic material, one of these
options is to trim the edge of the box opening so
that it is level to the poured concrete surface.
Trimming must be neat and even to ensure
proper seating of the floor box cover.
Many floor box assemblies also include
leveling rings that are mounted to an installed
floor box and provide the means for mounting
the floor box cover level with the floors finished
surface covering.
Attachment of the Floor Box Cover
Assembly—Since all floor boxes are installed
beneath the finished floor surface, the floor box
cover and sometimes, additional accessories are
depended upon to complete the enclosure and
interface with a particular floor covering (,
carpet, wood, tile, etc.). In normal use, the floor
box cover assembly is expected to prevent the
entry of foreign material into the wiring
compartments of a floor box and enclosed wiring
devices. A Listed floor box cover is designed to
prevent the entry of dust and dirt. Listed floor box
covers intended for use on carpet or tile covered
floors are also designed to prevent the entry of
scrubwater such as they might be exposed to in
the normal maintenance of these floors. Close
attention to the proper installation of these
covers and specific accessories such as carpet
flanges, tile rings, etc., is important to achieve
the designed protection. At the interface of the
cover or accessory and the floor or floor
covering, carefully follow the manufacturer’s
instructions for trimming the tile or carpet.
Manufacturer’s instructions may also call for use
of sealant or other materials as a provision for
prevention of the entry of scrubwater into the
floor box. Spaces between tiles should be
grouted level where they enter under a floor box
cover to prevent a channel for the entry of
scrubwater.
Any concrete or debris that may enter the
box during the construction process should be
removed. The box should be covered to prevent
entrance of foreign materials before and after
the installation is completed.
If the floor box is not intended to be activated
(completed by assembly of wiring devices) at the
time that the electrical conductors are pulled,
sufficient wire to complete the intended
installation in the future shall be provided. The
wiring should be looped inside the box, and the
ends of the conductors shall be protected.
Poke-Through Floor Fittings
For attachment of the cover assembly to a
poke-through floor fitting, refer the section above
“Attachment of the Floor Box Cover Assembly.”
Page 34 NEMA OS 3-2002
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Flush Device Boxes: Coordination
with Finished Wall Surface Installation
In new work installations, finished wall
surfaces and wall coverings are installed after
mounting of electrical boxes. Often, the wiring
system has been installed and de-energized
conductors are present in the box. It is not
uncommon for wiring devices such as switches
and receptacles to have already been installed.
Whenever possible, installation of wiring devices
should be scheduled after the finished wall
surface is installed and wall coverings (paint or
wallpaper) are complete. Scheduling the work in
this sequence will provide easier installation of
the finished wall surface and avoid unintended
damage to the wiring device. It will also give the
electrician full knowledge of the final installation
and the ability to make any necessary minor
adjustments at the time the installation is
completed.
The electrician should take the responsibility
to provide all necessary protection for electrical
conductors in the box to avoid damage or
contamination by paint or other materials used
during the installation of the finished wall
covering and surface treatment. This
preparation may include coiling the wires without
stripping the conductor insulation or cable jacket,
protecting exposed conductor ends, and stuffing
the box with non-absorbent paper.
When wiring devices will be installed prior to
installation of the finished wall surface and wall
coverings, they should be adequately masked
and protected by the electrician to prevent
contamination or unintentional damage. The wall
surface installer should be trained and made
responsible for avoiding damage to the box and
its contents during the installation of the wall
surface and coverings. The common practice of
using a hand router and the outside contour of
the box as a template guide for cutting openings
in the wall surface has resulted in many
problems. The electrician must inspect the box
and its contents after installation of the wall
surface to identify and replace any damaged
electrical components.
Installing Cover and/or Cover Plate
Damp or Wet Locations
After the device is installed, the properly
selected weatherproof cover should be installed
using either the mounting holes located on the
device strap or the holes located in the box itself.
Care should be taken to ensure that the cover
and gasket are properly placed on the box with
the gasket edges exposed on all sides of the
box. This will enable the cover to provide the
proper resistance to the entrance of water.
When a weatherproof cover is installed on a
flush mounted, dry location box, check to ensure
that the cover and gasket are seated against the
building material, and apply a sealing compound
if necessary. Self-closing lids and hoods should
be checked to ensure that they work properly.
Dry Locations
Covers for exposed work such as raised
surface covers, as described in the Selection
section in , require that the device be
attached to the cover and the cover then
attached to the box. The device should be
attached to the cover using at least two
mounting points. Listed covers contain these
provisions and include the necessary hardware.
Check to ensure that the faces of receptacles
protrude from the face of metallic covers and are
at least flush with the face of nonmetallic covers.
Check to ensure that the covers do not allow any
gaps between the boxes and covers.
Cover plates (face plates, wall plates) for
flush-mounted wiring devices should allow the
face of receptacles to protrude from the face of
metallic cover plates and be at least flush with
the face of nonmetallic cover plates. This also
applies to surface mounted single gang boxes.
For flush mounted boxes, check to see that the
cover plate completely covers the opening
between the box and the wallboard. Inform the
wallboard trades of any repairs necessary to
meet this requirement.
Verification of Installation
Verify secureness of all connections of the
wiring system to the box. Where a manufacturer
marks a specific tightening torque for a wiring
system securement means, the torque is to be
verified using an appropriate torque verification
tool. For wiring system connections that will not
be accessible after the assembly of finished
building surfaces, verification should take place
prior to final building surface assembly.
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NEMA OS 3-2002 Page 35
© National Electrical Manufacturers Association. It is illegal to resell or modify this publication.
Figure 2-1
Square Boxes in 4” X 4” Wall Opening
4” Square Box And Cover 4-11/16” Square Box And Cover
with 4” x 4” wall opening with 4” x 4” wall opening
Figure 2-2
Typical Threaded Conduit Entries
Integral Bushing
Thread Engagement
Page 36 NEMA OS 3-2002
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NEMA Publication OS 3-2002
Section 1
General
Connection to the Wiring System
The outlet box is an integral part of the wiring system, and as such, the box selected must be provided with a means to connect to the wiring system.
Mechanical connection—Every outlet box must have a means for entry of conduit, tubing, or cable. In addition, outlet boxes must provide for secure mechanical attachment of the conduit, tubing or cable at the point of entry. Single-gang nonmetallic out
Wiring Systems Employing Tubing or Conduit
Wiring Systems Employing Metal Jacketed or Nonmetallic-Sheathed Cable
Grounding
Select the Box Based on Contents and Function After Installation
Outlet and Device Boxes—Mounting of Wiring Devices
A device box has a provision for attaching and housing electrical switches, receptacles, or similar wiring devices in accordance with NEMA OS 1 and NEMA OS 2. Types of device boxes include single device (single gang), multiple device (multiple gang), an
to 23 (6 to 50) New Work Only
Boxes for High and Low Voltage Wiring Circuits (Class 1, Class 2, and Class 3 Circuits)
Low Voltage Wiring Circuits (Class 2 and Class 3 Circuits)
Although the distinction between high and low voltage is not clearly defined in the NEC, Class 2 and Class 3 wiring circuits and devices are not typically required to be installed in an electrical outlet box unless the installation will be subject to phy
Flush Mount – New Work Flush Mount – Old Work Flush Mount – New Work – Ceiling
Surface Mount – Exposed Surface Mount – Protected Pendant Mount
Square Box with Raised Cover
Masonry Box Old Work – Ceiling
Section 2
Metallic Boxes
Mounting and Securing of
Electrical Boxes
Surface Mounting on Walls or Ceilings
Connection to Wiring Systems
General
Proper installation of outlet boxes will provide for secure mechanical attachment of the conduit, tubing, or cable at the point of entry. In addition, proper installation of metallic outlet boxes connected to metallic conduit, tubing, or metal-jacketed
Connection to High and Low Voltage Wiring Circuits
Where both a high voltage wiring circuit (Electric light, power, and Class 1 circuits) and a low voltage wiring circuit (Communications and fiber optic circuits, and Class 2 or 3 circuits) is to be installed in the same outlet or device box, the high and
Completing the Installation
Floor Boxes
Poke-Through Floor Fittings
Integral Bushing
Thread Engagement
