Water heaters having a capacity of ___ gallons or less shall have a branch-circuit amp rating not less than 125% of the amp rating of the water heater?

Water heaters having a capacity of ___ gallons or less shall have a branch-circuit amp rating not less than 125% of the amp rating of the water heater?

• A. 60
• B. 75
• C. 90
• D. 120

Answer: (D)

Sizing the branch circuit for an electric water heater uses a safety margin because the heater can draw more current than its nominal running value, especially during heating cycles. For water heaters with a capacity up to 120 gallons, the circuit must be rated at least 125% of the heater’s full-load current. This 125% allowance accounts for startup inrush and the possibility that more than one element may be energized during operation, ensuring the wiring and overcurrent protection won’t be stressed. To apply it, you compute the heater’s current (wattage divided by voltage), multiply by 1.25, and then select conductors and a breaker that meet or exceed that amperage. The 120-gallon threshold is the cutoff used on the rule you’re studying; larger units use different sizing rules, so the 125% requirement applies specifically to those smaller (120 gallons or less) heaters. For example, a 4.5 kW heater at 240 V draws about 18.75 A. 125% of that is roughly 23.4 A, so you'd size the circuit for at least 25 A (typically a 30 A circuit in standard practice).

Sizing the branch circuit for an electric water heater uses a safety margin because the heater can draw more current than its nominal running value, especially during heating cycles. For water heaters with a capacity up to 120 gallons, the circuit must be rated at least 125% of the heater’s full-load current. This 125% allowance accounts for startup inrush and the possibility that more than one element may be energized during operation, ensuring the wiring and overcurrent protection won’t be stressed.

To apply it, you compute the heater’s current (wattage divided by voltage), multiply by 1.25, and then select conductors and a breaker that meet or exceed that amperage. The 120-gallon threshold is the cutoff used on the rule you’re studying; larger units use different sizing rules, so the 125% requirement applies specifically to those smaller (120 gallons or less) heaters.

For example, a 4.5 kW heater at 240 V draws about 18.75 A. 125% of that is roughly 23.4 A, so you'd size the circuit for at least 25 A (typically a 30 A circuit in standard practice).