Formula
Each conductor that enters and is spliced or terminates counts once, at its own size from Table 314.16(B). Conductors that pass through count once. A loop at least twice the free-conductor length counts as two, and you add that second count yourself. Pigtails that start and end in the box do not count. All equipment grounding conductors together count as one allowance of the largest one. Internal clamps count as one of the largest conductor. Each device yoke counts as two of the largest conductor connected to it. A fixture stud or hickey counts as one.
total = Σ (count × allowance) + ground + clamps + 2 × each yoke + stud
remaining = box volume − total
The volumes are NEC 2023 Table 314.16(A) for the common metal boxes, and 314.16(B) for the allowance per conductor. A 14 AWG allowance is 2.00 in³, 12 AWG is 2.25, 10 AWG is 2.50. The same numbers were in the 2020 edition. They were not re-read from the NFPA PDF. A plastic box is marked with its volume. Use Custom and type that mark. Being under the table is not “safe,” and it is not a code approval. Local amendments and the electrician’s count control. The wire size and voltage drop pages are the load side of the same job.
Worked examples
4×4×1½ square, four 14 AWG, one yoke on 14 AWG, grounds on 14, clamps on 14. 8.0 + 4.0 + 2.0 + 2.0 = 16.0 in³ in a 21.0 in³ box. Remaining 5.0. The count is under the table. It is still not a statement that the box is safe.
A 3×2×2 device box, 10.0 in³, with the same count. 16 is over 10. The page says the count is over the tabulated volume. It does not design a fix. A deeper box or a ring is a decision for the person doing the work.
Reference
Table 314.16(B) volume allowance per conductor, NEC 2023. Confirm the adopted edition.
| AWG | in³ |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
A 14 AWG allowance is 2.00 in³. Common box volumes are in the selector.
Questions
How full is a 4×1½ square with four 14 AWG conductors, a device, grounds, and clamps?
Table 314.16(A) gives that box 21.0 in³. Four 14 AWG conductors are 8.0. One yoke is two allowances, 4.0. All the equipment grounds together are one allowance, 2.0. Internal clamps are one allowance, 2.0. The total is 16.0 in³, with 5.0 in³ left. That means the count is under the tabulated volume. It does not mean the installation is safe or code-compliant. Check the adopted code and an electrician.
Do pigtails count?
A conductor that starts and ends inside the box does not count. A conductor that passes through counts once. A loop that is twice the minimum free-conductor length or longer counts as two. This page does not guess which of those you have. You enter the count.
Which code edition is this?
The allowances match NEC 2023 Tables 314.16(A) and 314.16(B), which did not change from 2020. They were not re-read from the NFPA PDF for this page. The authority having jurisdiction adopts the edition that counts.
Sources
- NFPA 70, 2023, 314.16(B) counting rules and Table 314.16(B) allowances: 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³.
- Table 314.16(A) metal-box volumes, including 4×1½ square at 21.0 in³ and 3×2×2 device at 10.0 in³. The 2020 tables match. Not re-read from the NFPA PDF.
- Verify with the code adopted locally and with an electrician. A total under the table is not a finding that the box is safe.