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Determine the approximate short circuit current available

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    Determine the approximate short circuit current available

    Using the given information, determine the approximate short circuit current available at the transformer secondary: 13,800-277Y/480V, 2500 kVA, 5% Impedance

    60,000 A
    54,500 A
    42,000 A
    37,500 A
    25,000 A


    first should be 480Y/277 not 277y/480.

    basically this would be the calculation. I= (2500*1000)/(480*1.732)= 3007.08
    z= (100/5)= 20
    short circuit = 3007.08*20= 60189

    so i would just go with the closest answer correct? and also they should fix the error on 277Y.

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    Agreed

    I work it out to the same answer. Seems you did it right and I would go for the closest answer.



    Quote Originally Posted by rasilva View Post
    Using the given information, determine the approximate short circuit current available at the transformer secondary: 13,800-277Y/480V, 2500 kVA, 5% Impedance

    60,000 A
    54,500 A
    42,000 A
    37,500 A
    25,000 A


    first should be 480Y/277 not 277y/480.

    basically this would be the calculation. I= (2500*1000)/(480*1.732)= 3007.08
    z= (100/5)= 20
    short circuit = 3007.08*20= 60189

    so i would just go with the closest answer correct? and also they should fix the error on 277Y.

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  5. madMAX's Avatar
    madMAX is offline Seasoned Member
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    The questions clearly states "determine the approximate short circuit current". Go with the closest answer.

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    Quote Originally Posted by madMAX View Post
    The questions clearly states "determine the approximate short circuit current". Go with the closest answer.

    yes sir but its wrong wording that what confused 277y/480 doesn't exist, 480y/277 exist

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    fault current

    Quote Originally Posted by rasilva View Post
    Using the given information, determine the approximate short circuit current available at the transformer secondary: 13,800-277Y/480V, 2500 kVA, 5% Impedance

    60,000 A
    54,500 A
    42,000 A
    37,500 A
    25,000 A


    first should be 480Y/277 not 277y/480.

    basically this would be the calculation. I= (2500*1000)/(480*1.732)= 3007.08
    z= (100/5)= 20
    short circuit = 3007.08*20= 60189

    so i would just go with the closest answer correct? and also they should fix the error on 277Y.
    2500 x 100
    1.732 x .480 x5

    250,000/4.1568= 60,142

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    Explain the math

    Could someone explain the math a little more in depth? Never mind I was confused in Z, but looking over it again I understand.
    Last edited by Ronwilson1801; December 29, 2018 at 09:27 AM.

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  13. carl21 is offline Junior Member Pro Subscriber
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    A simple question hopefully

    480 *1.732= 831.36

    why do you multiply 480 by 1.732?

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  15. Kalbi_Rob's Avatar
    Kalbi_Rob is offline Experienced Member Pro Subscriber
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    Quote Originally Posted by carl21 View Post
    480 *1.732= 831.36

    why do you multiply 480 by 1.732?
    Original equation is for single phase transformer:
    IFLA=(KVA*1000)/EL-L

    To calculate for 3 phase voltage, the square root of 3 (1.732) must be multiplied by the voltage:
    IFLA=(KVA*1000)/(EL-L*1.732)

    EL-L for both of theses transformers is 480V, one is single phase and the other is 3 phase.

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