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A-4
Overview
Boxes & Covers — Steel City
®
Metallic Boxes
Article 314 of the National Electrical Code
®
covers the installation and use of
boxes. The article includes table references that guide the electrician in the
selection of the proper box size necessary to safely accommodate electrical
service requirements. The box capacity table shown (page A-5) is reproduced
in part from the NEC
®
as a quick reference and guide. The NEC
®
should be
consulted for complete details.
NEC
®
Reference
Article 314 — Boxes and Fittings
314.16 Number of Conductors in Outlet, Device, and Junction
Boxes, and Conduit Bodies.
Boxes and conduit bodies shall be of sufficient size to provide free space
for all enclosed conductors. In no case shall the volume of the box, as
calculated in 314.16(A), be less than the fill calculation as calculated in
314.16(B). The minimum volume for conduit bodies shall be as calculated
in 314.16(C).
The provisions of this section shall not apply to terminal housings supplied
with motors or generators. Informational Note: For volume requirements
of motor or generator terminal housings, see Article 430.12. Boxes and
conduit bodies enclosing conductors #4 AWG or larger shall also comply
with the provisions of 314.28.
(A) Box Volume Calculations.
The volume of a wiring enclosure (box) shall be the total volume of
the assembled sections and, where used, the space provided by plaster
rings, domed covers, extension rings and so forth, that are marked with
their volume or are made from boxes the dimensions of which are listed
in Table 314.16(A).
(1) Standard Boxes. The volumes of standard boxes that are not marked
with their volume shall be as given in Table 314.16(A).
(2) Other Boxes. Boxes 1650 cm
3
(100 in.
3
) or less, other than those
described in Table 314.16(A), and nonmetallic boxes shall be durably
and legibly marked by the manufacturer with their volume. Boxes
described in Table 314.16(A) that have a volume larger than is
designated in the table shall be permitted to have their volume
marked as required by this section.
(B) Box Fill Calculations.
The volumes in paragraphs 314.16(B)(I) through (B)(5), as applicable, shall
be added together. No allowance shall be required for small fittings such
as locknuts and bushings.
(1) Conductor Fill. Each conductor that originates outside the box
and terminates or is spliced within the box shall be counted once,
and each conductor that passes through the box without splice or
termination shall be counted once. Each loop or coil of unbroken
conductor not less than twice the minimum length required for free
conductors in 300.14 shall be counted twice. The conductor fill shall be
calculated using Table 314.16(B). A conductor, no part of which leaves
the box, shall not be counted.
Exception: An equipment grounding conductor or conductors not over
four fixture wires smaller than #14 AWG, or both, shall be permitted to
be omitted from the calculations where they enter a box from a domed
luminaire or similar canopy and terminate within that box.
(2) Clamp Fill. Where one or more internal cable clamps, whether factory
or field supplied, are present in the box, a single volume allowance in
accordance with Table 314.16(B) shall be made based on the largest
conductor present in the box. No allowance shall be required for a cable
connector with its clamping mechanism outside the box.
(3) Support Fittings Fill. Where one or more luminaire studs or hickeys
are present in the box, a single volume allowance in accordance with
Table 314.16(B) shall be made for each type of fitting based on the
largest conductor present in the box.
(4) Device or Equipment Fill. For each yoke or strap containing one or
more devices or equipment, a double volume allowance in accordance
with Table 314.16(B) shall be made for each yoke or strap based on the
largest conductor connected to a device(s) or equipment supported by
that yoke or strap. A device or utilization equipment wider than a single
50mm (2 in.) device box as described in Table 314.16(A) shall have
double volume allowances provided for each gang required for mounting.
(5) Equipment Grounding Conductor Fill. Where one or more equipment
grounding conductors or equipment bonding jumpers enter a box,
a single volume allowance in accordance with Table 314.16(B) shall
be made based on the largest equipment grounding conductor or
equipment bonding jumper present in the box. Where an additional set
of equipment grounding conductors, as permitted by 250.146(D), is
present in the box, an additional volume allowance shall be made based
on the largest equipment grounding conductor in the additional set.
(C) Conduit Bodies.
(1) General. Conduit bodies enclosing #6 AWG conductors or smaller,
other than short-radius conduit bodies as described in 314.16(C)(2),
shall have a cross-sectional area not less than twice the cross-sectional
area ofthe largest conduit or tubing to which they can be attached.
The maximum number of conductors permitted shall be the maximum
number permitted by Table 1 of Chapter 9 for the conduit or tubing to
which it is attached.
(2) With Splices, Taps or Devices. Only those conduit bodies that are
durably and legibly marked by the manufacturer with their volume shall
be permitted to contain splices, taps or devices. The maximum number
of conductors shall be calculated in accordance with 314.16(B). Conduit
bodies shall be supported in a rigid and secure manner.
(3) Short-Radius Conduit Bodies. Conduit bodies such as capped elbows
and service-entrance elbows that enclose conductors #6 AWG or
smaller, and are only intended to enable the installation of the raceway
and the contained conductors, shall not contain splices, taps or devices
and shall be of sufficient size to provide free space for all conductors
enclosed in the conduit body.