Box fill

Check an outlet, device or junction box against NEC 314.16: the Table 314.16(A) or marked volume, every conductor, clamp, fitting, device yoke, grounding conductor and terminal block allowance, each line cited — NEC 2020 or 2023.

Check a box

Code basis

Standard metal box (Table 314.16(A))

…or volume marked on the box (in³)

Ring or domed cover, marked (in³)

Nonmetallic boxes, and boxes not in the table, carry their volume marked by the manufacturer. A marked volume, when given, is used instead of the table.

Counted conductors

14 AWG

12 AWG

10 AWG

8 AWG

Count once for each conductor that comes in and ends or is spliced in the box, once for each that passes straight through, twice for a loop at least twice the 300.14 free length, and not at all for a pigtail that never leaves the box. Grounding conductors go below.

18, 16 and 6 AWG

18 AWG

16 AWG

6 AWG

Device yokes (straps)

Largest conductor on a device

Grounding conductors entering

Largest grounding conductor

Internal cable clamps in the box

Support fittings, wide devices, terminal blocks

Support fitting types (stud, hickey)

Extra gangs for a device wider than 2 in.

Terminal blocks (NEC 2023)

Largest conductor on a block

How it works

The box's volume — from Table 314.16(A) for a standard metal box, or the volume marked on it — plus any marked plaster ring, extension ring or domed cover, against the sum of the allowances in 314.16(B): each counted conductor at its Table 314.16(B)(1) volume; one allowance for internal clamps; one per type of support fitting; two per device yoke; one for up to four grounding conductors and a quarter for each more; and, under NEC 2023, one per terminal block.

What it will not tell you

Conductors 4 AWG and larger — those boxes are sized by 314.28, not 314.16.

A box divided by a barrier — calculate each space on its own, with that space's volume.

Conduit bodies (314.16(C)).

The conductor count is yours: once for each conductor terminating or spliced, once passing through, twice for a long loop, zero for a pigtail. The page says the rule; it does not count a box it cannot see.

NEC 2020's listed clamp assemblies with cable terminations — their volume comes off the box, and that is the manufacturer's number.

No report, PDF or saved project — the answer is the whole of it.

Designing the whole house?

The paid module designs the circuits that land in these boxes — the load calculation, every feeder and branch circuit, panel schedules, the riser and the DXF — for the whole house.

See the plans