Box Fill Chart (NEC Table 314.16(A))
The most conductors of one size each standard metal box can hold when nothing else is in it, from the box volumes in NEC Table 314.16(A) and the per-conductor allowances in Table 314.16(B).
| Box | Volume (in³) | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|
| 4" round/octagonal, 1-1/4" deep | 12.5 | 6 | 5 | 5 | 4 | 2 |
| 4" round/octagonal, 1-1/2" deep | 15.5 | 7 | 6 | 6 | 5 | 3 |
| 4" round/octagonal, 2-1/8" deep | 21.5 | 10 | 9 | 8 | 7 | 4 |
| 4" square, 1-1/4" deep | 18.0 | 9 | 8 | 7 | 6 | 3 |
| 4" square, 1-1/2" deep | 21.0 | 10 | 9 | 8 | 7 | 4 |
| 4-11/16" square, 1-1/2" deep | 29.5 | 14 | 13 | 11 | 9 | 5 |
| 4" square, 2-1/8" deep | 30.3 | 15 | 13 | 12 | 10 | 6 |
| 4-11/16" square, 2-1/8" deep | 42.0 | 21 | 18 | 16 | 14 | 8 |
| 3x2x1-1/2" device box | 7.5 | 3 | 3 | 3 | 2 | 1 |
| 3x2x2" device box | 10.0 | 5 | 4 | 4 | 3 | 2 |
| 3x2x2-1/4" device box | 10.5 | 5 | 4 | 4 | 3 | 2 |
| 3x2x2-1/2" device box | 12.5 | 6 | 5 | 5 | 4 | 2 |
| 3x2x2-3/4" device box | 14.0 | 7 | 6 | 5 | 4 | 2 |
| 3x2x3-1/2" device box | 18.0 | 9 | 8 | 7 | 6 | 3 |
Counts assume an empty box: subtract allowances for every device, clamp, fitting and the grounds before relying on a number. Volumes are the table minimums; a box stamped with a larger volume may use the stamped figure.
Volume allowance per conductor
Table 314.16(B) assigns each conductor a volume by size. The chart divides the box volume by these figures and rounds down.
| Trade size | Allowance |
|---|---|
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
What else uses up box volume
- Each device yoke (switch, receptacle): two allowances, sized to the largest conductor connected to it.
- All equipment grounds together: one allowance, sized to the largest ground.
- Internal cable clamps, if present: one allowance.
- Each fixture stud or hickey: one allowance.
- Pigtails that never leave the box do not count.
To add devices, grounds and clamps to a real box, use the box fill calculator.
In-depth guide: How to calculate box fill under NEC 314.16
Worked example
A 4" square box, 1-1/2" deep, is 21.0 cu in.
For 12 AWG (2.25 cu in each) it holds 21.0 ÷ 2.25 = 9.33, so 9 conductors with nothing else in the box.
Add one receptacle (2 × 2.25 = 4.5) and the grounds (2.25): 21.0 − 6.75 = 14.25 cu in left, room for 6 conductors of 12 AWG.
Frequently asked questions
How many 12 AWG wires in a 4x4 box?
A 4" square box 1-1/2" deep (21.0 cu in) holds nine 12 AWG conductors when empty; the 2-1/8" deep version (30.3 cu in) holds thirteen.
How many 14 AWG wires in a single-gang box?
A 3×2×2-1/2" device box (12.5 cu in) holds six; 3×2×3-1/2" (18.0 cu in) holds nine. Subtract two for the switch or receptacle and one for the grounds.
Do I count the ground wires separately?
All equipment grounds together count as one conductor, sized to the largest ground in the box.
Can I use the volume stamped on the box?
Yes. Boxes marked with their volume, and plastic boxes, use the marked figure; mud rings and extension rings marked with a volume add to it.
Sources
- NFPA 70, National Electrical Code (free read-only access)
- EC&M, Code Quiz: box volume calculations and EGCs (2020 NEC change)
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