How it works
Required fill = Σ(conductor allowances) + clamp + support + 2×device yokes + EGC + terminal blocks — each allowance in cu in from Table 314.16(B) by AWG.
Every insulated conductor entering and terminating in the box counts as one allowance at its own AWG (NEC 314.16(B)(1)). All internal cable clamps together add one allowance at the largest conductor (B)(2); fixture studs and hickeys add one allowance **per type** present, not per piece, at the largest conductor (B)(3); each device yoke (receptacle or switch) adds **two** at the largest conductor connected to it (B)(4). Equipment grounding conductors get one combined allowance for up to four, plus a quarter allowance for each one beyond four (B)(5). New in NEC 2023, each terminal block adds one allowance (B)(6). Allowances per AWG are 18=1.50, 16=1.75, 14=2.00, 12=2.25, 10=2.50, 8=3.00, 6=5.00 cu in. Pigtails that start and end inside the box are not counted. Conductors 4 AWG and larger are not sized this way at all — they fall under NEC 314.28 pull/junction box rules (minimum length = 8× the largest raceway trade size). After sizing the box, confirm the conduit feeding it also meets the fill limits.
Code references
- Standard box volumes NEC 2023, 314.16(A) and Table 314.16(A)
- Volume allowance per conductor by AWG NEC 2023, Table 314.16(B)
- Conductor fill and pigtails NEC 2023, 314.16(B)(1)
- Clamp, support fitting and device fill NEC 2023, 314.16(B)(2), (B)(3), (B)(4)
- Graduated equipment grounding conductor fill NEC 2023, 314.16(B)(5)
- Terminal block fill (new in 2023) NEC 2023, 314.16(B)(6)
- Pull/junction boxes for 4 AWG and larger NEC 2023, 314.28
FAQ
What happens when I have two different wire sizes in the same box?
Each conductor uses its OWN volume allowance from Table 314.16(B) — a 10 AWG counts as 2.50 cu in while a 12 AWG counts as 2.25. The clamp, device-yoke and terminal-block allowances, however, all use the LARGEST conductor connected (NEC 314.16(B)(2), (B)(4), (B)(6)). Add a row per wire size above and the calculator applies each correctly. Note: where a box mixes sizes, the calculator conservatively uses the largest conductor in the box for these allowances; the code bases the yoke allowance on the largest conductor connected to that specific yoke, so if a big feed only passes through and a smaller wire actually lands on the device, the tool may size slightly high (never low).
Do pigtails count toward box fill?
No. A pigtail that both originates and terminates inside the box (for example a short jumper from the bundle to a device) is not counted — NEC 314.16(B)(1). Only conductors that enter the box from outside count. A looped conductor passing through counts as one, unless it is long enough to be cut into two 6-inch pieces, in which case it counts as two.
How many ground wires can I put in a box?
As many as fit, but for fill they are generous: up to four equipment grounding conductors share a single allowance, sized to the largest EGC. Each ground beyond four adds only a quarter allowance (NEC 314.16(B)(5), the 2020-and-later graduated rule). So six 12 AWG grounds count as 1 + 2×0.25 = 1.5 allowances = 3.375 cu in, not six full ones.
Can I use an extension ring to fix an overfilled box?
Yes. An extension ring (or a "box extender") adds its own marked volume to the assembly, which counts toward the total available (NEC 314.16(A)). If you are over by 3 cu in, a listed 4 or 6 cu-in extension ring brings you back into compliance — the calculator’s verdict suggests the minimum size needed.
When does NEC 314.28 apply instead of 314.16?
When any conductor is 4 AWG or larger. Those boxes are sized by dimensional pull/junction-box rules in NEC 314.28 (minimum straight-pull length of 8× the largest raceway trade size, plus angle-pull and conductor-spacing rules), not by the cubic-inch volume method. This calculator flags that case and stops, because the volume method would give a wrong answer.
Does a metal box change the result versus plastic?
Only through the clamp allowance. Metal boxes typically have internal cable clamps that must be counted (one allowance at the largest conductor), while plastic/NM boxes often clamp externally or use a built-in strain relief that is not counted. The calculator defaults the clamp toggle from the box material — verify by looking inside the box.
This calculator is provided for estimation purposes and covers the NEC 314.16 volume method for conductors 6 AWG and smaller only. Always verify results against the current NEC edition and local amendments with a licensed electrician before closing a box, and size boxes for 4 AWG and larger conductors under NEC 314.28.