Wiring An Electric Water Heater
March 6, 2006
Q: I’m trying to wire a 50-gallon electric water heater that has 4,500W interlocked heating elements and a voltage rating of 240 single phase. Do I use 10 AWG copper branch circuit conductors or can I use 12 AWG copper conductors with an ampacity of 25 as shown in Table 310.16? Is a 25A circuit breaker the right size for this load?
A: The full-load current for this water heater is 18.75A (4,500 divided by 240). A storage-type water heater with this size tank must be supplied by a branch circuit with a rating of not less than 125 percent of the water heater’s full-load current. Therefore, the minimum rating of the overcurrent device is 25A. If there is no maximum size overcurrent device marked on the water heater nameplate, a 30A 2-pole circuit breaker may be used. This larger size circuit breaker is permitted by 422.11(E)(3).
Notice that the ampacities for 14, 12, and 10 AWG copper conductors shown in Table 310.16 are followed by an asterisk which references 240.4(D) at the bottom of the table, and this part (D) limits overcurrent protection to 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper conductors. The branch circuit conductors cannot be smaller than 10 AWG copper.
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