Why box fill matters more than it looks
A box that is overfilled has three problems. The conductors are compressed against each other and can abrade when the device is pushed back. Heat from terminations has less room to dissipate. And connections get strained when the device is folded in, which is a leading cause of loose neutrals and intermittent faults that are hard to find later.
The calculation is not about the physical emptiness of the box — a box can look half empty and still be overfilled by the code, because the allowance is based on wire volume plus allowances for hardware that also takes up space.
The allowances, in full
| Item | Allowances counted | Article |
|---|---|---|
| Each current-carrying conductor | 1 | 314.16(B)(1) |
| All equipment grounding conductors, together | 1 total | 314.16(B)(5) |
| Each device yoke (switch, receptacle) | 2 per yoke | 314.16(B)(4) |
| Cable clamps inside the box | 1 | 314.16(B)(2) |
| Fixture studs, hickeys | 1 | 314.16(B)(3) |
| Pigtails and wire nuts staying in the box | 0 | 314.16(B)(1) exception |
The volume per allowance is set by the largest conductor present: 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, 3.00 for 8 AWG and 5.00 for 6 AWG.
Two worked examples
1. A single-gang box with one 12 AWG cable
One cable brings a hot, a neutral and a ground, feeding one receptacle. Current-carrying conductors: 2 (hot and neutral). Grounds: 1 allowance. Device: 2 allowances. Total 5 allowances at 2.25 cubic inches each, or 11.25 cubic inches. It is inside a 20.5 cubic inch plastic box by a wide margin. This is the standard case and it is why single-gang boxes are almost never a fill problem.
2. A junction box where two cables splice
Two 12 AWG cables enter a metal 4 inch square box, and each conductor is spliced through. That gives 4 current-carrying conductors (2 hots, 2 neutrals), 1 ground allowance, 1 clamp allowance, and no devices. Total 6 allowances at 2.25 cubic inches, or 13.5 cubic inches. A 2-1/8 inch deep 4 inch square box has 30.3 cubic inches, so it passes easily. Add a receptacle and two more cables and the picture changes quickly — that is where installers get caught.
Where people actually fail
- Adding a receptacle to an existing junction box. Each device adds two allowances. A box that was fine as a splice point may fail once a device goes in.
- Counting grounds individually. Ten grounding conductors count as one allowance, not ten. Installers sometimes over-count and buy an unnecessarily large box.
- Using the wrong conductor size. The allowance is based on the largest conductor in the box. One 10 AWG conductor among 12 AWG conductors raises every allowance to 2.50 cubic inches.
- Using an unmarked box. A box with no marked volume cannot be used for this calculation at all. Metal boxes are marked on the box or on a label inside the cover.
- Forgetting the clamp. Metal boxes with internal cable clamps cost one allowance. Plastic boxes with integral clamps do not.
How this calculator is verified
Volume allowances reproduce NEC 314.16(B). Box volume presets use the standard volumes marked on commonly available boxes; always confirm against the marking on your actual box, because manufacturers vary and older stock may differ.
- NFPA 70, National Electrical Code — Article 314.16, including the conductor volume table and the allowances for devices, clamps and grounds.
- NEMA — outlet box and enclosure dimensional standards.
- OSHA 29 CFR 1910 Subpart S — installation requirements referencing the code.
Volume allowance table and worked examples last verified: 19 September 2026.