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Thread: Short Circuit Calculations Help

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    Short Circuit Calculations Help

    Here is the information I received from the utility company

    "Service characteristics for the 600amp service is three (3) phase, 120/208 volt, four (4) wire.
    The maximum symmetrical short circuit duty is 27,000 amperes at the secondary terminals of
    the 75kva transformer bank."

    Now I need help finding the fault at the Main Distribution Panel (MDP).

    The distance from the transformer to the MDP is about 100 ft and the conduit that is running is 2 Sets of ( 4 # 350 kcmil + 1 # 2/0 kcmil G in 4" Conduit PVC)

    Using the Short-Circuit Current Calculations - Eaton Bussmann, refer Table 4 for "C" Values for Conductors

    To calculate the f factor, I used the following formula for Three Phase

    F = (1.732 X L X Isc)/(C * n * V)

    What would the value of C and n be in this example since we have the following information
    Isc = 27kA
    L = 100 ft
    V = 208V

    I tried using the Bussmann series FC2 Calculator but it seem's like I didn't have enough information to plug in
    Last edited by Jraef; 01-10-19 at 05:53 PM. Reason: Fixed spelling error in title

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    Quote Originally Posted by BatmanisWatching1987 View Post
    What would the value of C and n be in this example since we have the following information
    Isc = 27kA
    L = 100 ft
    V = 208V
    C and n relate to the Conductor Size (See table 4) and the number of conductors per phase (2, per the information you have given.)

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    Quote Originally Posted by david luchini View Post
    C and n relate to the Conductor Size (See table 4) and the number of conductors per phase (2, per the information you have given.)
    The confusing part is
    n = Number of Conductors per phase (Adjust C value for Parallel runs)

    How do I adjust C values based on the Parallel runs?

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    Quote Originally Posted by BatmanisWatching1987 View Post
    The confusing part is
    n = Number of Conductors per phase (Adjusts C value for Parallel runs)

    How do I adjust C values based on the Parallel runs?
    Multiply C times 2. (Your confusion is because you misread what it said.)

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    Thank you for your help.

    Also, if the Z% isn't given, what value should we typically assume?

    Is that when we us Table 1?

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    Quote Originally Posted by BatmanisWatching1987 View Post
    Thank you for your help.

    Also, if the Z% isn't given, what value should we typically assume?

    Is that when we us Table 1?
    You shouldn't need %Z. You are given available short circuit current at the transformer secondary

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    Quote Originally Posted by david luchini View Post
    You shouldn't need %Z. You are given available short circuit current at the transformer secondary
    I was asking since I was wondering how the came up with the value for Short Circuits Amps of 23132 from Table 1 for a 75 KVA (120/208V/3P).

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    Quote Originally Posted by BatmanisWatching1987 View Post
    I was asking since I was wondering how the came up with the value for Short Circuits Amps of 23132 from Table 1 for a 75 KVA (120/208V/3P).
    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

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    The 27kA is on LV side of transformer(s). You said "75KVA bank" which tells me you have 3 x 75KVA transformers which would make sense because you can't get 600A out of a single 75KVA transformer. To get that number the utility considered an infinite bus on the HV side.

    The easiest way to do what you need is to use the MVA Method. Convert the 27KA to MVA which is 9.73MVA, convert 100ft of cable to MVA which combined parallel run is approximately 21.6MVA in (PVC). Finally, the combined value is around 6.7 MVA, which equates to 18.6KA fault current at your MDP.

    I ran the same scenario in ETAP and came up with a very similar number.

    Don't forget that parallel MVA values combine like series impedance and series MVA values combine like parallel impedance.
    "Just because you're paranoid, doesn't mean they're not out to get you"

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    Quote Originally Posted by BatmanisWatching1987 View Post
    ...
    I tried using the Bussmann series FC2 Calculator but it seem's like I didn't have enough information to plug in
    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.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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