Overview NEC Reference
Article 314 of the National Electrical Code covers the installation and useof boxes.The article includes table references that guide the electricianin the selection of the proper box size necessary to safely accommodate
electrical service requirements.The box capacity table shown below isreproduced in part from the NEC as a quick reference and guide.TheNEC should be consulted for complete details.
Article 314 Boxes and Fittings
314.16.Number of Conductors in Outlet,Device and JunctionBoxes,and Conduit Bodies.
Metallic Boxes and Covers Boxes shall be of sufficient size to provide free space for all conductorsenclosed in the box.The provisions of this section shall not apply to terminal housings suppliedwith motors or generators.(See Section 430.12.)Boxes and conduit bodies containing conductors,size No.4 or larger,shallalso comply with the provisions of Section 314.28.
(b) Other Boxes.
Boxes 100 cubic inches or less,other than those described in Table
314.16(A),conduit bodies having provisions for more than two conduitentries and non-metallic boxes shall be durably and legibly marked by themanufacturer with their cubic inch capacity.The maximum number ofconductors permitted shall be computed using the volume per conductorlisted in Table 314.16(A),with the deductions provided for in Section
314.16(B),and these volume deductions shall be based on the largestconductor entering the box.Boxes described in Table 314.16(A) that have alarger cubic inch capacity than is designated in the table shall be permittedto have their cubic inch capacity marked as required by this section and themaximum number of conductors permitted shall be computed using the
volume per conductor listed in Table 314.16(B).
(a) Standard Boxes.
The maximum number of conductors permitted in standard boxes are listedin Table 314.16(A) reproduced in part on page A6.See Article 314.28(A)through (D) where boxes or conduit bodies are used as junction or pull boxes.(1) Table 314.16(A) shall apply where no fittings or devices,such as fixturestuds,
cable clamps,hickeys,
switches,or receptacles,are contained in
the box and where no grounding conductors are part of the wiring within
the box.Where one or more of these types of fittings,such as fixture
studs,
cable clamps,or hickeys are contained in the box,the number of
conductors shown in the table must be reduced by one for each type of
fitting; an additional deduction of one conductor must be made for each
strap containing one or more devices; and a further deduction of one
conductor shall be made for one or more grounding conductors entering
the box.Where a second set of equipment grounding conductors,as
permitted by Section 250.146(D),is present in the box,then an additional
deduction of one conductor must be made.A conductor running through
the box is counted as one conductor,and each conductor originating
outside of the box and terminating inside the box is counted as one
conductor.Conductors,no part of which leaves the box,shall not be
counted.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,etc.,that are marked with
their volume in cubic inches,or are made from boxes the dimensions of
which are listed in Table 314.16(A).(2) For combinations of conductor sizes shown in Table 314.16(A),themaximum number of conductors permitted is computed using the volume
per conductor listed in Table 314.16(B) with the deductions provided for
in Section 314.16(B) and these volume deductions shall be based on the
largest conductor entering the box.The maximum number and size of
conductors listed in Table 314.16(A) must not be exceeded.
(c) Conduit Bodies.
Conduit bodies enclosing 6AWG conductors or smaller other than short-
radius conduit bodies as described in 314.5,shall have a cross-sectionalarea not less than twice the cross-sectional area of the largest conduit towhich they can be attached.The maximum number of conductors permittedshall be the maximum number permitted by Table I,Chapter 9,for theconduit to which it is attached.Conduit bodies that are durably and legibly marked by a manufacturer withtheir volume shall be permitted to contain splices,taps or devices.Themaximum number of conductors shall be calculated in accordance with314.16(B) and are supported in a rigid and secure manner.
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