Box Fill Calculator
Check an electrical enclosure against its rated fill capacity from conductor count, gauge, devices and grounds — IEC/CENELEC 60670 framing for the UK.
Disclaimer
Estimate only — verify with a licensed engineer or your local building code before construction.
How many wires can go in an electrical box?
An electrical junction or device box has a legal limit on how many conductors, devices and fittings can fit inside it, based on the box's rated interior volume. Overfilling a box is a recognised fire hazard -- it traps heat around wire nuts and terminations -- and makes future repairs or additions far harder to carry out safely. This calculator applies the NEC (National Electrical Code) Article 314.16(B) volume-allowance method: enter the conductor count and gauge, any clamps, fittings and devices, and the grounding conductors, and it checks the total required volume against the box's rated capacity.
How to use the box fill calculator
- Enter the number of current-carrying conductors entering the box and the largest gauge (AWG) among them.
- Add any internal cable clamps, support fittings and devices (switches or receptacles) -- each counts toward the total fill.
- Enter the grounding conductors and the box's rated interior volume; the calculator totals the required volume and checks it against that capacity.
The box fill formula
Required volume = (conductors x allowance) + (1 x allowance, if clamps present) + (1 x allowance, if fittings present) + (2 x allowance x device count) + (grounding allowance x allowance). "Allowance" is the NEC Table 314.16(B) cubic-inch figure for the largest conductor gauge present -- 2.25 in³ for 12 AWG, for example -- and up to four grounding conductors together count as a single allowance.
Worked example: 3 conductors and 1 device, all 12 AWG, plus 1 ground, no clamps or fittings. Required volume = (3 x 2.25) + (2 x 2.25) + (1 x 2.25) = 6.75 + 4.5 + 2.25 = 13.5 in³. An 18 in³ box has 4.5 in³ to spare, so it passes.
Common box-fill mistakes
As a typical UK example, having a qualified electrician check an enclosure and swap in a larger one typically costs around £90-£300 for the callout.
- This calculator sizes conductor, clamp, fitting and device allowances by the largest conductor gauge present -- mix in one larger wire and it drives the whole calculation, not just its own allowance.
- A device (switch or receptacle) always counts as two conductor-equivalent allowances, not one -- a box that looks fine for the wires alone can still fail once devices are added.
