Box fill
Conduit fill asks how much a raceway will hold; box fill asks whether there is room to make the joints. The rule is a volume budget — every conductor, every clamp, every device and one allowance for all the equipment grounds together consumes cubic inches, and the box has to have enough of them.
| Item | Count | Volume |
|---|---|---|
| Ungrounded conductors | 3 | 6.75 in³ |
| Grounded conductors | 2 | 4.50 in³ |
| Equipment grounds (one allowance) | 1 | 2.25 in³ |
| Devices (two allowances each) | 1 | 4.50 in³ |
| Clamps | 1 | 2.25 in³ |
| Support fittings | 0 | 0.00 in³ |
| Luminaire wires | 0 | 0.00 in³ |
| Total required | — | 20.25 in³ |
| Box volume | — | 30.30 in³ |
| Spare | — | 10.05 in³ |
| Smallest box that passes | — | 4 × 4 × 1½ in square |
§314.16(B)(5) is the rule people miss: all the equipment grounding conductors together take one allowance, not one each. A device takes two allowances, because it displaces conductors on both sides.
(ungrounded + grounded) × A · One allowance each.
1 × A · All of them together, §314.16(B)(5).
devices × 2 × A · §314.16(B)(4).
Σ ≤ box volume · Table 314.16(A).
A clear path from inputs to results.
Conduit fill asks how much a raceway will hold; box fill asks whether there is room to make the joints. The rule is a volume budget — every conductor, every clamp, every device and one allowance for all the equipment grounds together consumes cubic inches.
How to use
- Choose the box and the conductor size.
- Count the ungrounded, grounded and grounding conductors.
- Add devices, clamps, fittings and any luminaire wires.
- Read the required volume against the box volume.
The formula
Reference: NFPA 70 §314.16, Table 314.16(A) for box volumes and Table 314.16(B) for the conductor allowance.
Worked example
Three hots, two neutrals, one ground, one device in a 4×4×2⅛ box
- 4 × 4 × 2⅛ in square box, 30.3 in³.
- 12 AWG conductors at 2.25 in³ each.
- 3 ungrounded, 2 grounded, 1 ground, 1 device, 1 clamp.
- 1
Five conductors take 5 × 2.25 = 11.25 in³.
- 2
All the equipment grounds together take one allowance, 2.25 in³.
- 3
The device takes two allowances, 4.5 in³, and the clamp one, 2.25 in³.
- 4
The total is 20.25 in³ against 30.3 in³ available.
- 5
The box passes with 10.05 in³ to spare, and would hold 13 conductors of this size.
20.25 in³ required · 30.3 in³ available · passes.
Interpreting the result
The rule people miss is that all the equipment grounding conductors share one allowance. A box with four grounds is not charged four times; a box with four devices is, because each device displaces conductors on both sides. That is why a two-gang box fills up faster than the conductor count suggests.
Assumptions & limitations
- Metal boxes with the Table 314.16(A) volumes.
- All conductors are the same size; mixed sizes are calculated at the largest allowance.
- One allowance for all equipment grounding conductors.
- Boxes with a marked volume may use the marked figure.
Frequently asked questions
Why do all the grounds share one allowance?
Because they are not current-carrying and they terminate together. §314.16(B)(5) gives them a single allowance based on the largest one.
What about a box with 14 and 12 AWG?
Every conductor is calculated at the larger allowance, so mixing sizes in a small box runs out of volume quickly.
What if the box is non-metallic?
Use the volume marked on the box; the Table 314.16(A) figures apply to metal boxes only.
Does a receptacle count as two conductors?
It counts as two allowances, not two conductors — the device occupies space on both sides of the box, so §314.16(B)(4) charges it double.
Do cable clamps count?
Yes, one allowance each, unless the clamp is listed as not requiring one — which is marked on the box.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.