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Box Fill Calculator

Calculate the required electrical box volume from conductors, devices, clamps and grounds using NEC 314.16 volume allowances.

Largest conductor

Insulated conductors entering

Devices (switches/receptacles)

Internal cable clamps?

Equipment grounds present?

Box volume (stamped) (in³)

How to use it

Count each insulated conductor that enters and terminates or splices in the box (pigtails that stay inside are not counted).
Each device counts as 2 conductors of the largest size connected to it.
All clamps together count as 1; all grounds together count as 1.

How it works

Required volume = (conductors + 2 × devices + clamps + grounds) × the allowance for the largest conductor (NEC Table 314.16(B)).

FAQ

How many wires can go in a single gang box?

An 18 in³ plastic box holds 9 × 14 AWG or 8 × 12 AWG allowances in total.

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