Box Fill & Pull Boxes

Outlet/device/junction box fill per NEC 314.16 (2020), and pull/junction box sizing for 4 AWG and larger per 314.28.

1 · What's in the box

Insulated conductors (count each one entering & spliced/terminated, or passing through)

An unbroken conductor looped 12 in. or longer (twice the 300.14 length) counts as 2 — enter it twice. Pigtails, jumpers and fixture wires that don't leave the box are not counted, and up to four fixture wires smaller than 14 AWG plus an EGC from a domed canopy are exempt (314.16(B)(1) Exc.).

Devices / yokes (2 × largest conductor connected to the device)

Other allowances

EGCs entering the box Largest EGC size Support fittings — number of TYPES (studs, hickeys) Internal cable clamps

2 · Box choice

Mud/plaster ring, raised cover or domed cover — marked in³ Extension ring (adds its marked volume)

314.16(A): rings, extensions and covers count only when marked with their volume (or made from boxes listed in Table 314.16(A)). Use the marked number — values above are typical.

Metal boxes — Table 314.16(A)

Nonmetallic boxes — typical marked volumes

Nonmetallic boxes are rated by the volume molded/marked on the box (314.16(A)(2)). The sizes listed are common examples (18.0, 20.3, 22.5 in³ single-gang; 32/34 in³ two-gang; 20 in³ round ceiling). Always read the box.

3 · Reverse: max conductors of one size

BoxConductor size Or custom volume (in³)

Count only — subtract 2 for each device, 1 for clamps, 1 per fitting type, and EGC allowances (Table 314.16(A) assumes none of these).

4 · Pull & junction boxes (314.28, 4 AWG and larger)

Straight pull — 314.28(A)(1)

Largest raceway trade size (in.)

Angle or U pull — 314.28(A)(2)

Enter raceways entering ONE wall, comma-separated. Use a second row if there are two rows on that wall.

Wall A, row 1Wall A, row 2 Wall B, row 1Wall B, row 2

Distance between raceway entries enclosing the same conductor

Raceway 1 (in.)Raceway 2 (in.)

314.28: the dimensions are the minimum from the raceway entry wall to the opposite wall. Measure raceway-to-raceway spacing between the nearest edges of the entries. Boxes with conductors 4 AWG and larger also need 314.28(A)(3) smaller dimensions only where permitted and listed, accessible covers (314.28(C)), and racking/support in boxes over 6 ft (314.28(B)).

Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC