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Water Heater Overcurrent Protection

November 27, 2005

Q: What is the minimum and maximum breaker size for an electric water heater? What is the minimum wire size for a 4,500 watt, 240 volt single-phase water heater?

A: Let’s calculate the minimum ampacity of the branch circuit conductors first. 422.13 states “Storage-Type Water Heaters. A branch circuit supplying a fixed storage-type water heater that has a capacity of 120 gallons (450 L) or less shall have a rating not less than 125 percent of the nameplate rating of the water heater.” (4,500 divided by 240 x 1.25 = 23.4 amps) This means you must use 10 AWG copper conductors.

The minimum size overcurrent protection for this water heater is 25 amperes. This is the next standard size circuit breaker, according to 240.6. Even though the NEC says this is a standard size, no one stocks this size.

If the water heater nameplate does not list a maximum size overcurrent device, 422.11(E) states “If the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes, not to exceed 150 percent of the appliance rated current. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.” (4,500 divided by 240 x 1.5 = 28 amperes) The next standard size circuit breaker is 30 amperes, according to 240.6.


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