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Thursday, January 31, 2002
Question:
When wiring a 30-gallon water heater with a 240 volt, 4500 watt element, NEC 422-11(e) says the rating of the overcurrent protection shall not exceed that marked on the appliance and 422-13 says the branch circuit for the water heater shall have a rating not less than 125 percent of the nameplate rating. Does that mean you would use 10 AWG conductors and protect the circuit with a 20-ampere breaker?
A
Answer:
The 1999 NEC 422-11(e) requires that where the maximum overcurrent protection is marked on the appliance, the overcurrent protective device must not exceed the rating marked on the appliance. If the appliance is not marked, then the overcurrent device shall be sized as follows:
If the appliance does not exceed 13.3 amperes -- 20 amperes or less.
If the appliances exceeds 13.3 amperes – 150% or less of the appliance rating. Where 150% does not result in a standard size overcurrent device rating as listed in Section 240-6, then the next higher standard size is permitted.
The calculation for this water heater is 4500 watts ÷ 240 volts = 18.75 amperes. Since this exceeds 13.3 amperes, the overcurrent protection shall not exceed 18.75 x 1.50 = 28.125 amperes. This is not a standard size and the next higher standard size is 30 amperes.
NEC 422-13 requires a branch circuit rating of not less than 125 percent of the nameplate rating of the water heater. The nameplate rating of the water heater is 4500 watts @ 240 volts = 18.75 amperes x 1.25 = 23.4 amperes. This circuit will require
10 AWG copper conductors with overcurrent protection of 25 amperes. This will satisfy 422-11(e)(3) since it does not exceed 150 percent of the appliance rated current.