Calculate the required volume for an electrical box based on NEC standards for conductors, devices, and clamps.
Last updated: March 2026
Switches/Outlets
First 4 = 1 count; additional = 1/4 each
All clamps = 1 count
Disclaimer: This calculator provides general estimates based on NEC guidelines. Actual box fill requirements may vary depending on conductor types, installation conditions, local code interpretations, and amperage. Always consult the latest electrical code or a licensed electrician for final determinations.
Box fill refers to the maximum number of conductors, devices, and clamps allowed in an electrical box. This is regulated by the National Electrical Code (NEC) Article 314.16 to prevent overheating, fire hazards, and difficulty installing devices safely.
The volume of each component is calculated based on the size of the largest wire in the box. Each wire gauge has a specific cubic inch allowance. Devices count as two conductor volumes, all ground wires together count as one, and all internal clamps together count as one. Overfilling a box is a code violation and creates safety risks—conductors can be damaged, insulation can be compromised, and heat dissipation is reduced.
Calculate box size for: 4 wires (14 AWG), 1 switch, 1 ground wire, no clamps:
Use an 18 cu in or larger box (standard 3×2×2.5" box = 12.5 cu in is too small)
Under NEC rules, all ground wires in a box count as a single conductor based on the largest ground wire present. You don't count each one individually.
A device is any strap-mounted component like a single-pole switch, duplex receptacle, GFCI, or dimmer. Each strap counts as two conductors based on the largest wire connected to it.
No, wire nuts, pigtails, and cable connectors that remain outside the box do not count towards the fill volume. Only items inside the box count.
Calculate each wire size separately. For devices and grounds, use the volume allowance for the largest wire connected to them. Add all volumes together.
No. Using a box smaller than the calculated minimum is a code violation and creates safety hazards. Always meet or exceed the minimum required volume.
Standard single-gang boxes: 18 cu in (3×2×3.5"), 22.5 cu in (4" square × 1.5" deep). Double-gang: 25-30 cu in. Check box markings for exact volume.
Conduit bodies and fittings may affect fill calculations; consult NEC guidance and local code—some accessories have specific allowances or exceptions.
NEC allowances are based on conductor gauge regardless of solid or stranded construction. When in doubt, use the published cubic inch values for the largest conductor present.
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