How do I calculate box fill?

Q: How many wires can I put in a single-gang box?

It depends on the box’s cubic-inch volume and wire size. A common 18 cu in single-gang box allows eight 14 AWG conductors before any device or clamp deductions — but a switch or receptacle (a two-conductor allowance) plus the ground reduce that to about five incoming circuit conductors. Always calculate from the box’s marked volume, not a fixed wire count.

Q: Does a receptacle count toward box fill?

Yes. Each device yoke or strap (receptacle or switch) counts as two conductor volume allowances, based on the largest conductor connected to it, per NEC 314.16(B)(4). A device that spans two gangs still counts as two. This is on top of the conductor, clamp, and grounding allowances, so a crowded device box fills up faster than the wire count alone suggests.

Question

How do I calculate box fill?

Accepted Answer

Add up volume allowances per NEC 314.16(B): one for each conductor passing through or terminating, two for each device yoke (based on the largest conductor on it), one total for all cable clamps, and one total for all equipment grounding conductors. Multiply the count by the per-size volume (for example, 2.25 cu in for 14 AWG, 2.00 for 12 AWG) and compare to the box’s marked volume.

How do I calculate box fill?

Answers