Box Fill Calculator — NEC 314.16
Enter your conductors, device yokes, internal clamps, and equipment grounds — get an instant cubic-inch total and pass/fail result based on NEC 314.16(B). Free, no account needed.
How the Calculation Works — NEC 314.16(B)
Box fill is a volume accounting exercise. Every item inside the box occupies space and is assigned a standardized allowance. Sum the allowances, convert to cubic inches, and compare to the box's marked capacity. If you exceed the capacity, you need a larger box.
The allowances come from NEC Table 314.16(B), which gives cubic inches per conductor allowance based on wire gauge:
| Wire size (AWG) | Per conductor (in³) | Per device yoke (2× — in³) |
|---|---|---|
| 14 AWG | 2.00 | 4.00 |
| 12 AWG | 2.25 | 4.50 |
| 10 AWG | 2.50 | 5.00 |
| 8 AWG | 3.00 | 6.00 |
| 6 AWG | 5.00 | 10.00 |
Counting rules
- Conductors — each insulated conductor that enters or leaves the box counts as 1 allowance. Pigtails that start and end inside the box without leaving do NOT count.
- Device yokes — each switch, receptacle, GFCI, or dimmer on a yoke counts as 2 allowances based on the largest conductor connected to it.
- Equipment grounds — all grounding conductors combined count as 1 allowance total, based on the largest ground wire gauge in the box.
- Internal clamps — all internal cable clamps combined count as 1 allowance total, based on the largest conductor in the box. External connectors do not count.
- Support fittings — fixture studs and hickeys each count as 1 allowance. Regular mounting screws do not count.
Counting each ground wire separately (they're all one allowance). Forgetting the device yoke is double the conductor allowance, not single. Counting pigtails. Using external connector fill instead of internal clamp fill.
Worked Examples
Example 1 — Single-gang switch box, 14 AWG
Incoming 14/2 power + outgoing 14/2 switch leg, 1 switch, internal clamps, bare grounds.
Example 2 — 2-gang switch box, 12 AWG
Two switches on 12 AWG circuits — one incoming cable, two switch legs out, internal clamps.
FAQ
Do pigtails count in box fill?
No. Short conductors that start and end inside the box without passing through the walls are not counted per NEC 314.16(B)(1).
How many grounds count as one allowance?
All equipment grounding conductors together count as one allowance regardless of how many wires there are — use the largest ground wire gauge for the allowance value.
External cable connectors — do they count?
No. Only internal clamps built into the box body count. Connectors installed from outside through knockouts do not count in box fill.
What about the 3-way switch traveler cable?
Every conductor that enters the box counts, including the white neutral that passes through a 3-way switch box per NEC 404.2(C). Count each physical conductor — black, red, and white — as separate allowances even though the white is spliced through and not connected to the switch.
NEC 314.16 box fill appears on most state licensing exams. The counting rules — conductors, device yokes (×2), combined grounds, combined clamps — are a common calculation question on PSI and Prometric tests. Use this calculator to check your hand-worked answers before exam day. The box fill guide walks through the full NEC logic with three worked examples.