Conduit BendingBy Blissom · Updated 2026-10-23
Short answer: add up an allowance for every conductor entering the box (#14 = 2.00 in³, #12 = 2.25 in³, #10 = 2.50 in³), two allowances per device yoke, one for internal clamps, one for each type of support fitting, one per terminal-block assembly (2023), and one for up to four grounds (plus a quarter for each ground over four). The total must not exceed the box volume. The calculator covers #18 to #6 conductors, yokes, clamps, grounds, pass-throughs, long loops, support fittings and terminal blocks, in the listed metal boxes or any box with a marked volume.
What counts, and how much (314.16(B))
| Item | Counts as |
|---|---|
| Each conductor that enters and ends or is spliced | 1 allowance of its size |
| Conductor passing through unbroken | 1 allowance |
| Unbroken loop or coil at least twice the free length 300.14 requires | 2 allowances |
| Pigtails and jumpers that do not leave the box | 0 |
| Internal cable clamps (any number) | 1 allowance, largest conductor |
| Support fittings (luminaire studs, hickeys) | 1 allowance for each type present, largest conductor |
| Each device yoke or strap | 2 allowances, largest conductor on that device (per gang if wider than 2") |
| Each terminal-block assembly (2023 NEC, 314.16(B)(6)) | 1 allowance, largest conductor on that block |
| Equipment grounding conductors, up to 4 | 1 allowance, largest ground |
| Each ground beyond 4 (2020 NEC and later) | 1/4 allowance |
Volume per conductor (Table 314.16(B)(1) in NEC 2023, 314.16(B) in 2020)
| Size | in³ each |
|---|---|
| #6 AWG | 5.00 |
| #8 AWG | 3.00 |
| #10 AWG | 2.50 |
| #12 AWG | 2.25 |
| #14 AWG | 2.00 |
| #16 AWG | 1.75 |
| #18 AWG | 1.50 |
Metal box volumes (Table 314.16(A))
| Box | Volume in³ | Max #14 | Max #12 |
|---|---|---|---|
| 4 × 1-1/4 round / octagon | 12.5 | 6 | 5 |
| 4 × 1-1/2 round / octagon | 15.5 | 7 | 6 |
| 4 × 2-1/8 round / octagon | 21.5 | 10 | 9 |
| 4 × 1-1/4 square | 18.0 | 9 | 8 |
| 4 × 1-1/2 square | 21.0 | 10 | 9 |
| 4 × 2-1/8 square | 30.3 | 15 | 13 |
| 4-11/16 × 1-1/4 square | 25.5 | 12 | 11 |
| 4-11/16 × 1-1/2 square | 29.5 | 14 | 13 |
| 4-11/16 × 2-1/8 square | 42.0 | 21 | 18 |
| 3 × 2 × 1-1/2 device | 7.5 | 3 | 3 |
| 3 × 2 × 2 device | 10.0 | 5 | 4 |
| 3 × 2 × 2-1/4 device | 10.5 | 5 | 4 |
| 3 × 2 × 2-1/2 device | 12.5 | 6 | 5 |
| 3 × 2 × 2-3/4 device | 14.0 | 7 | 6 |
| 3 × 2 × 3-1/2 device | 18.0 | 9 | 8 |
| 4 × 2-1/8 × 1-1/2 device | 10.3 | 5 | 4 |
| 4 × 2-1/8 × 1-7/8 device | 13.0 | 6 | 5 |
| 4 × 2-1/8 × 2-1/8 device | 14.5 | 7 | 6 |
"Max" is the number of allowances the box holds, before counting devices, clamps and grounds as allowances. Plastic and nonmetallic boxes use the volume marked on the box.
Worked example: receptacle in a 4" square box
A 4" square × 1-1/2" box (21.0 in³) with four #12 conductors, one receptacle, internal clamps and two grounds:
- 4 × #12 = 4 × 2.25 = 9.00 in³
- Receptacle yoke = 2 × 2.25 = 4.50 in³
- Clamps = 2.25 in³
- Grounds (two) = 2.25 in³
- Total 18.00 in³ ≤ 21.0 in³: the box is big enough.
The grounding rule since 2020
Before the 2020 NEC, equipment grounding conductors generally shared one allowance, with an extra allowance for an additional set of isolated-ground conductors (250.146(D)). Since 2020, up to four grounding conductors entering the box share one allowance and each one beyond four adds a quarter allowance. Boxes with many grounds that passed under older editions can now fail. Check which edition your jurisdiction enforces, and see the conduit fill calculator for the raceway side.
Sources: NFPA 70 (NEC) 2020 and 2023: 314.16, Table 314.16(A), Table 314.16(B)(1) (314.16(B) in 2020), 314.16(B)(1)–(6) and 300.14; Box manufacturer markings for nonmetallic and listed boxes. Method.
Questions electricians ask
How many 12 AWG wires can go in a 4x4 box?
A 4" square × 1-1/2" deep box (21 in³) holds nine #12 conductor allowances (9 × 2.25 = 20.25 in³). Remember that a device counts as two allowances, clamps as one and the grounds as one.
How do devices count in box fill?
Each yoke or strap holding one or more devices (switch, receptacle) counts as two conductors of the largest size connected to it (314.16(B)(4)). A device wider than 2" counts two allowances for each gang it needs.
Do ground wires count in box fill?
Since the 2020 NEC, up to four equipment grounding conductors entering the box count as one allowance of the largest ground, and each one beyond four adds a quarter allowance. Earlier editions counted them as one allowance, plus one for an additional isolated-ground set.
Do pigtails count in box fill?
No. A conductor that starts and ends inside the box, such as a pigtail or a jumper, is not counted (314.16(B)(1) exception). Fixture wires coming from a canopy also do not count in some cases.
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