How must a branch circuit feeding a domestic hot water heater of 120 gallons or less be sized?

• A. 100% of its nameplate rating
• B. 110% of its nameplate rating
• C. 125% of its nameplate rating
• D. 150% of its nameplate rating

Answer: (C)

Electric water heaters can run for extended periods while heating water, so the circuits feeding them are treated as loads that may stay on continuously. To provide enough current headroom for sustained operation and any inrush when the element first heats, the branch circuit must be sized at 125% of the heater’s nameplate rating. This extra 25% helps prevent overheating of conductors or nuisance tripping of the breaker during regular use.

In practice, you take the heater’s nameplate wattage and voltage to find the full-load current, multiply by 1.25, and then choose the next standard breaker size and conductor accordingly. For example, a 4500 W heater at 240 V draws about 18.75 A; multiplying by 1.25 gives about 23.4 A, so you would typically use a 25 A circuit with the appropriate conductors.