What Size Wire for an Electric Water Heater?

An electric water heater typically uses a 30A / 240V circuit, which calls for #10 copper (or #8 aluminum) conductors and a #10 copper equipment ground per NEC Table 250.122. This appliance is a continuous load, so the circuit is already sized at 125% of its draw (NEC 422.13). A standard 40-50 gallon, 4500W residential water heater draws 18.75A, and NEC 422.13 treats a fixed storage water heater as a continuous load, so it is sized at 125% (23.4A) onto a 30A / 240V circuit. Higher-wattage or commercial units need a larger circuit.

A 30A double-pole breaker. The #10 copper conductors are matched to that 30A rating. Always confirm against the appliance nameplate, which lists the exact minimum circuit and maximum breaker.

Yes. For the 30A circuit, aluminum needs #8 (versus #10 copper) at the 75C column. Aluminum is common on larger circuits and services to save cost; apply antioxidant to the terminations, torque to the listed spec, and remember aluminum drops slightly more voltage on long runs.