Wire Ampacity & NEC Sizing Calculator (AWG)
Determine maximum electrical wire ampacity, recommended circuit breaker size, and conductor gauge (14 AWG to 4/0 AWG) for copper and aluminum wire per NEC 310.16 tables.
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Frequently Asked Questions
What wire gauge do I need for standard household circuit breakers?
Under the National Electrical Code (NEC): 15-Amp breaker requires 14 AWG copper wire; 20-Amp breaker requires 12 AWG copper wire; 30-Amp breaker requires 10 AWG copper wire; 40-Amp breaker requires 8 AWG copper wire; 50-Amp breaker requires 6 AWG copper wire.
What is the NEC 80% continuous load rule?
Article 210.20 of the NEC states that an electrical branch circuit breaker must not be loaded beyond 80% of its rated capacity for continuous loads (loads operating for 3 hours or longer, such as EV chargers, water heaters, and electric heating). Alternatively, size wire and breaker to 125% of continuous load.
Why does Romex NM-B cable have to use the 60°C column?
Per NEC 334.80, Nonmetallic-Sheathed Cable (Type NM-B Romex) conductors must be sized according to the 60°C ampacity column, even though individual conductors inside are insulated with 90°C THHN material. This prevents overheating inside insulated wall cavities.
Why is aluminum wire sized larger than copper wire?
Aluminum has higher electrical resistivity than copper. To safely carry the same amperage without overheating, aluminum wire must generally be one to two AWG sizes thicker than equivalent copper wire.