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Thread: Short Circuit Calculations Help

  1. #11
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    Quote Originally Posted by david luchini View Post
    They divide the transformer rated secondary current by the percent impedance...But since the percent impedance can have a 10% tolerance, the use the low side of 0.9%Z, which would give the highest possible short circuit value.

    208.19A/0.9%=23132A
    How do you figure out the percent impedance is 10%Z and use the low side of 9%Z

  2. #12
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    Quote Originally Posted by BatmanisWatching1987 View Post
    How do you figure out the percent impedance is 10%Z and use the low side of 9%Z
    When the utility gave you the available short-circuit value of 27kA at the transformer terminals, it means the %Z being considered was FLA/Isc = 624A/27,000A = 2.31%!

  3. #13
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    Quote Originally Posted by BatmanisWatching1987 View Post
    How do you figure out the percent impedance is 10%Z and use the low side of 9%Z
    I'm not sure what you are asking. The table that you referenced lists 1%Z for the 75kva transformer.

    1%Z +10% gives a range of 0.9%Z to 1.1%Z.

  4. #14
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    The short circuit value wouldn't utilize a tolerance from the utility. They know what the transformer impedance is, or is going to be. The tolerance is only used by manufacturing to IEEE standards.

    A utility transformer with an impedance of 2.31% is very realistic.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  5. #15
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    Is the transformer primary voltage relevant?

    Thanks

  6. #16
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    IMO, once the utility gives you the available current at their transformer terminals, the transformer, impedance, etc. become irrelevant. All you need is their number.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  7. #17
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    Quote Originally Posted by augie47 View Post
    IMO, once the utility gives you the available current at their transformer terminals, the transformer, impedance, etc. become irrelevant. All you need is their number.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  8. #18
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    Quote Originally Posted by don_resqcapt19 View Post
    You have all the information you need to use the Bussmann app for this calculation. You just start with "add a conductor" to my system.
    Using your information in the app, and assuming steel conduit, I get 17,191 amps at the end of your 100' run.
    I'm not sure what I am doing incorrectly?


    I selected "No, Assume Infinite Current on Primary"

    What was the Z% value you inputted into the system

    Or did I fill out the conductor information incorrectly.

    Click image for larger version. 

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    Also if I wanted to add my MDP at the end of the line, why does it ask for distance again when inputting a Bus Run to the system?

  9. #19
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    Quote Originally Posted by BatmanisWatching1987 View Post
    I'm not sure what I am doing incorrectly?


    I selected "No, Assume Infinite Current on Primary"

    What was the Z% value you inputted into the system

    Or did I fill out the conductor information incorrectly.


    Also if I wanted to add my MDP at the end of the line, why does it ask for distance again when inputting a Bus Run to the system?
    Forget the part about a transformer. Start your calculation with "Add conductor run to system."

    Click image for larger version. 

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    Last edited by david luchini; 01-11-19 at 02:48 PM.

  10. #20
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    Quote Originally Posted by don_resqcapt19 View Post
    You have all the information you need to use the Bussmann app for this calculation. You just start with "add a conductor" to my system.
    Using your information in the app, and assuming steel conduit, I get 17,191 amps at the end of your 100' run.
    same
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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