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Thread: Formula for kW -> amps

  1. #1
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    Sep 2008
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    Formula for kW -> amps

    This is probably easy for you folks, just not me:

    Could someone provide the formula to calculate the amperage rating for a given load when the power source is known?

    I hope I've asked that right. My exact need is the amp rating for a 45 kW load on 208VAC 3-phase power.

    Thank you

  2. #2
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    The multiplier for 3-phase 208 volt is 360. (208x1.73). 45000/360=125

  3. #3
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    If it's a continuous load add 1.25% to what ruwired said.

  4. #4
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    Exclamation Power Formula

    I googled it since my mind is gone..

    KW = KV X Amps X PF X 1.73 so assuming 0.8 power factor:

    45KW = 0.208KV X Amps X 0.8 X 1.73
    Amps = 45 / .208 X .8 x 1.73
    Amps = 45 / .2879
    Amps = 156.3A

    ? someone pls check this
    Am I in shape?? I get plenty of exercise pushing my luck!!

  5. #5
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    Quote Originally Posted by tonytonon

    ? someone pls check this
    See Ricks post.

    Roger
    Moderator

  6. #6
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    Quote Originally Posted by tonytonon
    I googled it since my mind is gone..

    KW = KV X Amps X PF X 1.73 so assuming 0.8 power factor:

    45KW = 0.208KV X Amps X 0.8 X 1.73
    Amps = 45 / .208 X .8 x 1.73
    Amps = 45 / .2879
    Amps = 156.3A

    ? someone pls check this
    This is close enough if it is a motor or other inductive load, also a safe bet if it is a mixed load. If it is a purely resistive load, i.e. heating elements or incandescent lights, then the power factor is unnecessary. But few loads are purely resistive.

  7. #7
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    Quote Originally Posted by Jraef
    This is close enough if it is a motor or other inductive load, also a safe bet if it is a mixed load.
    I agree, just don't convert HP straight to KW.
    Just because you can, doesn't mean you should.

  8. #8
    Join Date
    Sep 2008
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    Thank you all.

    The load is an FM broadcast tube-type transmitter. The mftr is calling for a 250A FUSED disconnect, which seemed high.

    I guess that leads to this question then: Does the use of fuses require an increase in the disconnect's amp rating?

  9. #9
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    Quote Originally Posted by jim dungar
    I agree, just don't convert HP straight to KW.
    I don't follow you.
    In what circumstances would the conversion be the wrong thing to do?

  10. #10
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    Quote Originally Posted by Besoeker
    I don't follow you.
    In what circumstances would the conversion be the wrong thing to do?
    When many people try to determine the input KW of a motor which is rated in output HP, by simply converting HP to KW, they forget to account for the efficiency of the motor.
    Just because you can, doesn't mean you should.

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