TradeCalcs

Box Fill Calculator — NEC 314.16 Cubic Inches

Who this is for: For electricians roughing-in residential boxes — especially remodels where a deeper box is cheaper than a failed inspection — and inspectors double-checking a crowded multi-gang.

Enter the box volume and everything that lands in it — wires, clamps, devices, grounds — and see the 314.16 cubic-inch tally with pass/fail.

Quick answer: Box fill = every item's cubic-inch charge from NEC 314.16(B) summed against the box's marked volume. 14 AWG charges 2.00 in³ per conductor, clamps one charge per box, devices two conductors' worth each, grounds one charge total. A 14 in³ box holding three 14 AWG wires, a clamp, a device and grounds totals exactly 14.0 in³ — full.

in³

Marked on the box

2.00 in³ each

any # of clamps = 1 charge

all grounds = 1 charge

Verdict
Passes
14 of 14 in³
Used volume
14 in³
314.16 allowances
Spare space
0 in³
≈ 0 more 14 wire(s)
Itemized charges
3 × 14 AWG conductors6 in³
1 clamp allowance (14 AWG)2.00 in³
1 device(s)4 in³
ground wires (single allowance, 14 AWG)2.00 in³
Total14 in³ vs 14 in³
Allowances per NEC 314.16(B): conductors by size, one clamp allowance per box, devices/support fittings at 2× the largest conductor, grounds as a single allowance.
Core facts
Volume per conductor14 AWG 2.00 · 12 AWG 2.25 · 10 AWG 2.50 in³
Clamps1 allowance per box (largest conductor)
Devices & fittings2 × largest conductor, each
Ground wires1 allowance total
Example14 in³ box: 3×14 AWG + clamp + device + grounds = 14.0 in³ ✓
StatusFigures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code.

What counts in a box fill calculation?

NEC 314.16 assigns every item in the box a cubic-inch charge: each conductor by size (14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³), one clamp allowance per box regardless of clamp count (charged at the largest conductor present), each support fitting or device at twice the largest conductor's volume, and all equipment grounding conductors together as a single allowance. Add the charges up and they must fit the box's marked cubic-inch volume. Example: a 14 in³ box with three 14 AWG conductors, one clamp, one device and grounds totals 14.0 in³ — exactly full, so pass — and a fourth conductor tips it over.

Common uses

  • Checking whether a ceiling box can take one more cable before drywall
  • Sizing a multi-gang box for dimmers with pigtails everywhere
  • Explaining to a builder why the box went from 1-gang metal to a 22.5 in³ cube
  • Verifying a junction box buried behind insulation meets 314.16 and 314.29 access rules

Frequently Asked Questions

How much volume does each wire size take?
Per NEC 314.16(B): 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³, 8 AWG = 3.00 in³, 6 AWG = 5.00 in³ per conductor. Pigtails that originate and end inside the box count as zero — only conductors that enter or leave are charged.
Do clamps count once or per clamp?
One allowance, charged at the largest conductor in the box, no matter how many internal cable clamps it has. Two clamps, three clamps — still one charge.
What about the device (switch/receptacle)?
Each device or support fitting counts as two conductors of the largest size connected to it. A switch on 12 AWG charges 4.50 in³.
How are ground wires counted?
All equipment grounding conductors in the box together get a single allowance of one conductor at the largest size. Four grounds on 12 AWG still count 2.25 in³ total.
Does box fill apply to junction boxes too?
Yes — any box where splices or devices live. What's exempt is conduit bodies and wireways sized by their own rules. And remember 314.29: junction boxes must stay accessible, not buried behind finish.

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