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    Nathonian's Avatar
    Nathonian Posts: 1, Reputation: 1
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    #1

    Apr 6, 2007, 12:59 PM
    Step down transformers
    How many amps, at 400 volts, can I get from a 300 kva [ 480v/ 400v] transformer?
    Morrowrj's Avatar
    Morrowrj Posts: 345, Reputation: 22
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    #2

    Apr 8, 2007, 06:12 AM
    Quote Originally Posted by Nathonian
    How many amps, at 400 volts, can I get from a 300 kva [ 480v/ 400v] transformer?
    This answer can vary depending on the type of load. Assuming a non-inductive load,(no large motors) then your answer would be 300kva/400volts output=750 amps. This is in an ideal situation. In reality you have a less then perfect energy transfer, the "standard" for industrial applications is usually to caclulate for a 60% efficiency or Power Factor(PF). In this case 300kva*.6PF =180000/400volts yields you approximatley 450 Amps.

    Hope This Helps,
    Morrowrj
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #3

    Apr 8, 2007, 08:12 AM
    The expression Volt Amps of AC power, will include any power factor consideration on the circuit. A transformer rated in KVA will not need to be derated for any inductive or capacitive load.

    Assuming this is a 3 phase transformer, due to the size and voltages involved, the amps available on the 400 volt secondary side is calculated:

    300,000 VA / 400 volts * 1.73= 1297.5 or 1300 amps
    Morrowrj's Avatar
    Morrowrj Posts: 345, Reputation: 22
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    #4

    Apr 8, 2007, 04:30 PM
    Hmm, I normally work single phase... However it still seems like your calculation is off? Is it possible that the equation is KVa divided by the quantity of volts times 1.73 ?

    300,000VA / (400 V * 1.73)

    or

    300,000Va / 692 = 433 Amps.

    Please see here :Rex Power Magnetics - Home Page

    Or here : How To Size Phase Transformers

    Please let me know if I am missing something.



    Morrowrj
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #5

    Apr 10, 2007, 01:26 AM
    Sorry I missed this, thought I would get subscribed just by answering.

    You are right ,I can't believe I said that. The brackets mean to calc phase to phase voltage of 692, then divide.

    Morrow is correct, the amps is 433.
    MRCHAD's Avatar
    MRCHAD Posts: 3, Reputation: 1
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    #6

    Jun 1, 2007, 05:52 PM
    Hello There
    MRCHAD's Avatar
    MRCHAD Posts: 3, Reputation: 1
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    #7

    Jun 1, 2007, 05:54 PM
    Sorry New, What Would Be The Right Size Transformer.

    Power Coming In Would Be Single Phase 480

    Output Is 120v , About 4.2a

    I Figure About 1kva Would Be Plenty
    Morrowrj's Avatar
    Morrowrj Posts: 345, Reputation: 22
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    #8

    Jun 3, 2007, 05:55 AM
    Yes, I agree 1kva should be good. Did you check the links I referenced above?



    Morrowrj
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