Electrical Box Fill Chart

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
How to count: each hot and neutral is one conductor; all grounds together count as one; each device (switch/receptacle) counts as two. Multiply the count by the volume and stay under the box rating.

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.

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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.