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Breaker and wire size calculator (120/240 V, AWG)

The breaker and the AWG copper wire for an appliance or a circuit at 120, 208 or 240 V, by the US National Electrical Code.

Try the circuit in the labBuild it on the simulator's bench and measure it.

How it is worked out

The same two rules as everywhere that follows the NEC: a motor load or a continuous load (on for three hours or more: a water heater, an EV charger, lights) is sized at 125 % of its running current (NEC 210.19, 210.20), and the wire must be rated for the breaker, because the breaker protects the wire.

Ampacity is from the 75 °C column of NEC Table 310.16 for THHN/THWN in conduit and from the 60 °C column for NM-B (334.80), and the small-conductor rule 240.4(D) caps 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A.

Example: a 4,500 W water heater at 240 V draws 18.8 A; 125 % is 23.4 A, so a 25 A breaker on 10 AWG (a 30 A breaker on 10 AWG is the usual choice). A 1,500 W space heater at 120 V draws 12.5 A, 15.6 A at 125 %: a 20 A circuit on 12 AWG.

Air conditioners and heat pumps carry a nameplate MCA (minimum circuit ampacity) and MOCP (maximum breaker): when you have them, they decide. Voltage drop is a separate check: long runs need a bigger size than the current alone asks for.

For 230 V and mm² wire (the Philippines and other 230 V countries that use the PEC or the NEC), use the Philippine calculator; the answers for common appliances are on the answers page.

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