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Short Circuit formula using MI cable

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    Short Circuit formula using MI cable

    Does anyone have a formula to figure short circuit AIC ratings using MI cable as a feeder? I typically use the formula shown in the Bussman catalog which is the same as Mike's. The manufacturer sent me the AC resistance of the cable but I don't have a formula to plug the information into. Thanks for any input

    Marie Babbitt

    #2
    There is no formula to calculate an AIC rating. Are you trying to calculate availble fault current at a certian point? Do you have the fault current from the source already? Have you looked at the point to point method in the bussman guide?

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      #3
      Originally posted by marieb View Post
      Does anyone have a formula to figure short circuit AIC ratings using MI cable as a feeder? I typically use the formula shown in the Bussman catalog which is the same as Mike's. The manufacturer sent me the AC resistance of the cable but I don't have a formula to plug the information into. Thanks for any input

      Marie Babbitt
      Seems like some mixing of terms here, as to what data and for what purpose would be used. In short circuit calculations you're using the same formula, but an MI cable relevant data would be somewhat different from normally available cable's resistance/impedance data.

      However, the impact of the cable type is normally very small and only would represent meaningful difference when used in mass-length, like same type cable for transmission, distribution and branch feeders at significant length combinations. Using non-metallic jacketed cable or 3-1/c in PVC conduit data would get you close. I somewhat doubt that the data would be available from the MI cable manufacturer, but who knows, but if it is then you can plug that data into your program or longhand calcs.
      Last edited by weressl; 08-04-11, 09:42 AM.

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        #4
        The inductance portion of MI cable impedance is probalby less than the inductance of three single conductors in conduit or a 3/c cable because of the tighter conductor spacing.

        To be conservative, I would either neglect the cable impedance or only use the 60Hz resistance value in the short circuit calculations.
        Bob Wilson

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