Selection Guide
Article 314 – Boxes and fittings
6 STEEL CITY® METALLIC BOXES & COVERS
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
minimum volume for conduit bodies shall be as
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
(1) Standard boxes. The volumes of standard
boxes that are not marked with their volume
(2) Other boxes.
3
(100 in.
3
) or less,
and non-metallic boxes shall be durably and
legibly marked by the manufacturer with their
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.
(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
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
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.
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Overview
NEC® Reference
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 8) is reproduced
in part from the NEC® as a quick reference and guide. The NEC® should be
consulted for complete details.