TradesHow this works
An overstuffed box is a real hazard. It damages insulation, traps heat, and makes the connections hard to fold in safely. The code turns that into a simple volume budget. Everything in the box counts as some number of conductor volumes, and the box has to be at least that big. This tool runs the count and picks the smallest common box that meets it.
The smallest legal box is not always the one to buy. A box with a little extra room is much easier to work in, so the tool shows the next size up too.
Common questions
How is box fill counted?
Each conductor counts as one unit of volume for its size. Every device like a switch or receptacle counts as two. All the grounding wires together count as one, and internal cable clamps count as one. Add the units, multiply by the volume for your largest conductor, and that is the space you need.
Why do devices count as two?
A device fills more of the box than a conductor because of the yoke and the body behind the plate. The code accounts for that by counting each strap as two conductor volumes at the largest conductor size.
Which conductor size do I use for the math?
The largest one in the box. If you have 12 and 14 gauge together, everything is figured at 12 gauge. This tool uses the size you pick as that largest conductor.
Sources: NEC 314.16 box fill (volume allowances and unit counts) (2023). Engine version 1.0.0. Risk tier 2.