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Thread: hot water heater calculation

  1. #1

    Question hot water heater calculation

    My question is if you have a single phase 240 volt hot water heater with dual 4500 watt elements.And when it is on how many amps per leg would you have.This is what i came up with. 18.75 per leg at 240 volts = 9000 watts is this correct and if so how come it takes 9000 watts to operate a 4500 watt heating element. is it do to the heating of the element. because the name plate says its a 4.5kw heater. And if this is correct on my calculation in the code book it says use the name plate value,help please......:confused:

  2. #2
    Join Date
    May 2004
    Location
    MA
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    1,472
    18.75 X 240 = 4500 Watts.

    don't mutiply the load current by 2, on a two wire circuit the current is in series. Draw a diagram of a single load connected to the source.

    single phase kVA is = VA/1000.
    Charlie

  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
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    17,159
    I think you need to re-read the nameplate more carefully. Does it say something along the lines of “4.5kW with dual elements,” or “dual 4.5 kW elements”?

    If it is a 4.5kW heater, and if it has dual elements, then each element uses 2,250 watts. That's 18.75 amps.

    If it has dual 4.5kW elements, it is a 9000 watt heater. That's 37.5 amps.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  4. #4
    Join Date
    Feb 2003
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    Minnesota
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    4,987
    The common electric water heater with dual elements only operates one element at a time.

    Water stratifies when being heated. . .hot on top, cold water on bottom. When the upper element senses that the water (that it is heating) has come to temperature, the upper element control switches off and simultaneously energizes the lower element.
    Another Al in Minnesota

  5. #5
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Quote Originally Posted by al hildenbrand
    The common electric water heater with dual elements only operates one element at a time.

    Water stratifies when being heated. . .hot on top, cold water on bottom. When the upper element senses that the water (that it is heating) has come to temperature, the upper element control switches off and simultaneously energizes the lower element.
    The clue is something to the effect of "upper element - 4500W; lower element - 4500W; total - 4500W."

    Under normal usage, the lower element does most of the heating, by the way, which is why it's the one most often needing replacement; that and mineral deposits.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #6
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    26,450
    Quote Originally Posted by Joe livewire
    My question is if you have a single phase 240 volt hot water heater with dual 4500 watt elements.And when it is on how many amps per leg would you have.This is what i came up with. 18.75 per leg at 240 volts = 9000 watts is this correct and if so how come it takes 9000 watts to operate a 4500 watt heating element. is it do to the heating of the element. because the name plate says its a 4.5kw heater. And if this is correct on my calculation in the code book it says use the name plate value,help please......:confused:
    It's like Al says-- there are 2 4500 elements in the heater and only one element can come on at a time. The top of the tank has a single pole double throw thermostat that lets the top of the tank heat first when that is satisfied the thermostat switches to the bottom element. That element has a single pole T-stat. If the water heater need to heat the bottom it will. If in the meantime the top gets cold then the top T-stat will switch to the top element even if the bottom element is calling for heat.

    Simple concept but I probably made it difficult to understand.

  7. #7
    I know this doesn't apply, but I got a buddy that corrects me when I say it so here goes.

    Is there such thing as a HOT water heater??? If the water was already hot why would you heat it??


    Sorry I just had to, it is all in fun.

  8. #8

    Smile thanks for all the input.

    I just want to thank everybody for their input it really helped me to understand the consept.:smile:

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