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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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6 Responses to “Wiring An Electric Water Heater”

  1. Ed Williams on January 27th, 2007 11:18 pm

    I have a new electric furnace. It has a 15kv heater strip with it. On the box of the heater it says its 15kv. When I look at the heater it has to separate heaters in it and two circuits . So is this 15 kv total or each. What size 220 volt wire would I use.
    Thanks Ed

  2. Administrator on January 28th, 2007 7:23 pm

    Hi Ed,

    Thank you for your electrical question.

    I’m afraid that I am the wrong person to ask questions about your furnace. You should consult with a heating expert. I’m sure they could tell you how this works.

    If I had to guess, I would assume that your furnace is 15kW total. However, I’m not certain. I have never worked on an electric furnace before.

    I would assume that the 2 heating elements are for 2 stage heating. This is typically done to start one element and the blower moter at the same time and after a short period, the second element would kick in. This decreases the surge wattage during start up.

    To size the wire that feeds your furnace, you need to do a simple ohm’s law calculation. 15kW (15,000 watts) divided by 220 volts = 69 amps approximately. You will need a 2-pole, 70 amp circuit breaker and # 4 copper or # 3 aluminum cable minimum.

    If the furnace is new, it should have instructions. The instructions will tell you the recommended overload protection (circuit breaker or fuses) and wire size you need for this installation.

  3. don on September 20th, 2010 5:28 pm

    how to wire a 208/240 single phase water heater

  4. Alan Moore on February 16th, 2011 7:10 am

    Are there also requirements on the wall side for the placement of the electrical outlet box?. I’m installing a larger/taller electric tank and have to extent the water supply lines higher inside the wall. I will also move the existing electric outlet box & grounding wire upward too – but are there minumum clearance requirements to the water supply pipes or separation requirements (e.g. wood blocking inside the wall – or similar?) (Arizona)


  5. Glenn Noone on May 11th, 2011 8:30 am

    is it okay to feed two water heaters with the same wire and breaker that fed one.

  6. Glenn Noone on May 11th, 2011 9:36 am

    dim planning on adding a second water heater and would like to not have to run a new line.

    i plan to take the 220 line and put it against the wall in a timmer than out of the timer into two water heaters