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Thread: 40 gallon Electric water heater wire and breaker sizing question

  1. #1
    DIY Junior Member dcorazal's Avatar
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    Default 40 gallon Electric water heater wire and breaker sizing question

    I bought a house that the copper was stolen including the water heater. I plan on installing a cheap 40 gal Rheem 6year.

    I have to run wiring because that is also gone. I am confused about the wiring and breaker.
    One feature of the heater is that it has Dual 4500 watt heating elements for faster recovery. It seems that most of the water heater have dual elements.
    If I do the math correctly then
    4500watts x 2 = 9000 watts
    9000watts/240volts = 37.5 amps
    Therefore I would need a 50 amp 220 breaker and run 6 gage wires.

    Does this make sense for a 40 gallon electric water heater? It seems to high for this size heater

    There apears to be a 30amp breaker and 10-guage wire in the area of where I think the old heater was? Do the new heaters require that much more power??

    Any advice would be appreciated

  2. #2
    Nuclear Engineer nukeman's Avatar
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    The elements won't run at the same time, so use 4500W. I bet the specs on that WH are for a 30A breaker, 10ga wire. This is common for just about any "full size" electric WH. My WH uses 2 5400W elements and calls for a 30A circuit.

  3. #3
    Jack of all trades DonL's Avatar
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    Welcome to Terry's Forum dcorazal,

    The Thermostats are wired so that both elements do not draw current at the same time as nukeman said.


    Good Luck on your project.


    DonL
    Theory only works perfect in a vacuum.

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  4. #4
    DIY Junior Member dcorazal's Avatar
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    I was wondering about the elements on at the same time. Glad to hear that. I was looking at the price of 100 feet of 6 guage and that is almost more then the cost of the heater.

    Thank you Experts!

  5. #5
    DIY Senior Member JerryR's Avatar
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    As stated, both elements do not run at the same time. The upper element comes on first and when the water at the top of the tank gets to temp the upper element turns off and redirects to the lower element to heat the water at the bottom of the tank.

    The heater should state on it what the current draw is. The rule of thumb is that the circuit capacity (wiring) and protection (breaker) should exceed the max curent draw by 25%. Another way to look at it is that the max current draw should be 80% of the breaker and wiring capacity. So a 16 amp heater should be wired and protected by a 20amp circuit or a 20 amp breaker should only feed a max of 16 amp load.

    Bottom line, For a 4500w heater w/240v it will draw 4500/240 or 18.75 amps so #10 wire and either 25 or 30 amp breaker, depending on code.
    Last edited by JerryR; 07-31-2011 at 12:36 PM.
    JR

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