NEC Box Fill Calculator (314.16) — Free
Calculate device and junction box fill per NEC 314.16 by counting conductors, devices, clamps, and grounding conductors to confirm the box is large enough.
- Count conductors Each current-carrying conductor entering and not terminating counts as one volume allowance at the largest conductor size.
- Add device and clamp allowances Each device yoke counts as two conductors; all internal clamps together count as one.
- Add the grounding allowance All equipment grounding conductors together count as one conductor.
- Compare to box volume Multiply total counts by the per-conductor volume from Table 314.16(B) and compare to the box’s cubic-inch capacity.
How do I calculate box fill?
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 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.
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.