The short answer
Add up the volume each item takes in cubic inches and compare it with the box's volume (NEC 314.16). Each conductor counts its size's allowance: 2 in³ for 14 AWG, 2.25 for 12 AWG, 2.5 for 10 AWG. A device yoke counts two of the largest conductor connected to it; all equipment grounding conductors together count one; internal clamps count one.
Step by step
- Count each insulated conductor that enters the box and is spliced or terminated there, or passes through it, once at its own size's allowance; an unbroken loop of 12 inches or more counts twice.
- Add one allowance for all equipment grounding conductors together, sized by the largest of them, plus a quarter allowance for each one beyond four.
- Add one allowance of the largest conductor in the box if it has internal cable clamps, and one for each type of support fitting, such as fixture studs or hickeys.
- Add two allowances of the largest conductor connected to each device yoke (a receptacle or switch).
- Compare the total with the box's marked volume or its Table 314.16(A) volume; the box fits if its volume is at least the total.
Worked example
A receptacle fed by two 12/2 cables with ground in a box with internal clamps: four 12 AWG conductors, two equipment grounding conductors and one device yoke need 18 in³. The smallest standard boxes that hold it are 3 x 2 x 3-1/2 device and 4 x 1-1/4 square (18 in³).
| Item | Count | Each | Volume |
|---|---|---|---|
| 12 AWG conductors | 4 | 2.25 in³ | 9 in³ |
| Internal cable clamps | 1 | 2.25 in³ | 2.25 in³ |
| Device yokes (2 each) | 2 | 2.25 in³ | 4.5 in³ |
| Equipment grounding conductors | 1 | 2.25 in³ | 2.25 in³ |
Volume per conductor (Table 314.16(B)(1))
| Conductor | Volume |
|---|---|
| 14 AWG | 2 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.5 in³ |
| 8 AWG | 3 in³ |
| 6 AWG | 5 in³ |
Standard metal boxes (Table 314.16(A))
| Box | Volume | Max 12 AWG | Max 14 AWG |
|---|---|---|---|
| 4 x 1-1/4 round/octagonal | 12.5 in³ | 5 | 6 |
| 4 x 1-1/2 round/octagonal | 15.5 in³ | 6 | 7 |
| 4 x 2-1/8 round/octagonal | 21.5 in³ | 9 | 10 |
| 4 x 1-1/4 square | 18 in³ | 8 | 9 |
| 4 x 1-1/2 square | 21 in³ | 9 | 10 |
| 4 x 2-1/8 square | 30.3 in³ | 13 | 15 |
| 4-11/16 x 1-1/4 square | 25.5 in³ | 11 | 12 |
| 4-11/16 x 1-1/2 square | 29.5 in³ | 13 | 14 |
| 4-11/16 x 2-1/8 square | 42 in³ | 18 | 21 |
| 3 x 2 x 1-1/2 device | 7.5 in³ | 3 | 3 |
| 3 x 2 x 2 device | 10 in³ | 4 | 5 |
| 3 x 2 x 2-1/4 device | 10.5 in³ | 4 | 5 |
| 3 x 2 x 2-1/2 device | 12.5 in³ | 5 | 6 |
| 3 x 2 x 2-3/4 device | 14 in³ | 6 | 7 |
| 3 x 2 x 3-1/2 device | 18 in³ | 8 | 9 |
| 4 x 2-1/8 x 1-1/2 device | 10.3 in³ | 4 | 5 |
| 4 x 2-1/8 x 1-7/8 device | 13 in³ | 5 | 6 |
| 4 x 2-1/8 x 2-1/8 device | 14.5 in³ | 6 | 7 |
Common mistakes
- Counting each equipment grounding conductor separately: together they count one allowance (plus a quarter for each beyond four).
- Forgetting the device yoke: a receptacle or switch takes two allowances of the largest conductor connected to it.
- Counting pigtails and wire connectors: a conductor that starts and ends inside the box, and small fittings such as wire connectors, are not counted.
- Leaving out ring and cover volume, or counting it unmarked: plaster rings, extension rings and domed covers add to the box's volume only when they are marked with their volume or made from a box in Table 314.16(A).
Code numbers and editions
Section numbers here are NEC 2023's, which most states enforce. Conductors larger than 6 AWG are sized for pull and junction boxes under 314.28, not by volume.
Code adoption cycles change. Always confirm the edition and amendments in force with your Authority Having Jurisdiction (AHJ) before permitting work.