Box volume · NEC 314.16(B)

Box Fill Calculator

Count conductors by size, device yokes, grounds, and internal clamps to estimate the required outlet or device box volume in cubic inches using NEC 314.16(B) volume allowances — then compare it against a box you have on the truck.

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NEC 314.16(B)
Yokes · grounds · clamps
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Box volume

NEC 314.16(B)

Formula

Vol = Σ conductors + yokes + grounds + clamps

Field inputsBox volume

Conductors: by AWG
Yokes: × 2 each
Grounds: × 1 group
Compare: your box

Tells you the cubic inches required, then checks a box you carry.

Box contents

What lands in the box

Insulated conductors (each counts once at its own size)

Conductor group 1

Conductor size (AWG)

Quantity

Device yokes / straps (×2 each)

Grounding conductor groups (×1)

Support fittings (studs/hickeys)

Internal cable clamps (×1)

Compare against a known box volume

Box volume (cu in)

Assumptions: volume allowances from NEC 314.16(B). Device yokes count as two equivalents, all grounds together as one, one or more internal clamps as one, and each support fitting as one — all at the largest conductor's allowance. Each insulated conductor counts at its own size.

Required box volume

Box fill

Box fits

Required volume

18.00 cu in

Total conductor equivalents: 9
Largest conductor (#14): 2.00 cu in / unit
Conductors (5 eq): 10.00 cu in
Device yokes (2 eq): 4.00 cu in
Grounds (1 eq): 2.00 cu in
Internal clamps (1 eq): 2.00 cu in
Your box volume: 18.00 cu in

Calculation

Required = Σ(conductors at own size) + (yokes + grounds + clamps + supports) × 2.00 = 18.00 cu in

Source: NEC 314.16(B).

Safety & accuracy notice

This box fill calculator is a planning aid only. It is not engineering, a permit, or design approval, and it is not legal or code certification. It covers NEC 314.16(B) volume fill, not 314.16(A) standard-box tables or conductor pull-space rules.

Always verify against the current adopted NEC (National Electrical Code), the equipment listings and manufacturer instructions, and your local AHJ (Authority Having Jurisdiction — the inspector or department that enforces the code). The final box selection should be reviewed and approved by a licensed electrician or professional engineer.

Box fill FAQ

How is box fill calculated under the NEC?

NEC 314.16(B) assigns a volume allowance per conductor equivalent by AWG: 18 AWG = 1.50, 16 = 1.75, 14 = 2.00, 12 = 2.25, 10 = 2.50, 8 = 3.00, and 6 = 5.00 cubic inches. Each insulated conductor counts at its own size; each device yoke or strap counts as two; all grounding conductors together count as one; one or more internal clamps count as one. Yokes, grounds, and clamps use the volume allowance of the largest conductor in the box.

Does a cable passing through count?

A conductor that runs through the box without splice or termination counts once. A conductor that originates outside and terminates inside also counts once. This tool counts each conductor you enter as one equivalent at its size.