Electrical boxes have a volume limit so wires are not crammed in. Each conductor counts for a set volume; add them up and the total must fit the box. This chart gives the volume per conductor.
| Wire size | Volume per conductor |
|---|---|
| 14 AWG | 2.00 cu in |
| 12 AWG | 2.25 cu in |
| 10 AWG | 2.50 cu in |
| 8 AWG | 3.00 cu in |
FAQ
What happens if an electrical box is overfilled? Overfilling a box leaves too little room for wires to bend safely behind devices, increasing heat buildup and the risk of damaged insulation or a poor connection. Box fill calculations exist specifically to keep enough free volume for safe wire management.
See the Conduit Fill Chart and How to Wire a Receptacle.
More than just the wires
The conductors in the chart are only part of the count. A switch or receptacle counts as two conductors of the largest wire connected to it, internal cable clamps count as one conductor as a group, and all the equipment grounding conductors together count as one. Fixture studs and similar fittings add allowances too.
Checking the box
Add up the allowances and compare the total with the volume marked on the box. If the box is too small, use a deeper box or a larger one rather than forcing the wires in, since crowded boxes overheat and are hard to work on. Wire size matters because larger wires take more space. Always confirm requirements with the electrical code that applies to your area.