Wiring hot water heaters requires paying attention to the National Electrical Code, which specifies wire gauge sizes and circuit breaker sizes based on the current draw of the hot water heater.
Most electric hot water heaters operate on 220 to 250 volts AC. This voltage in homes requires a double circuit breaker rated for the amperage draw of the hot water heaters.
Residential water heaters use varying amounts of power depending on the size of their heating elements. The power amount is given in watts for the heater, and the wire gauge size is dependent on the amps drawn by the heater. Divide watts by voltage to find amps.
Common Current and Wire Gauge Sizes
Wire a hot water heater with wire that's sized to accommodate the heater's current requirement. For example, a 20-amp hot water heater needs 12-gauge wire, a 25-amp hot water heater requires 10-gauge wire, and a 30- to 40-amp hot water heater requires 8-gauge wire. Some large heaters draw up to 50 amps, requiring 6-gauge wire.