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Thread: Location used for calculating utility available fault current

  1. #11
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    Quote Originally Posted by Natfuelbilll View Post
    For a system supplying many motors - yes.
    Even without many motors, the secondary fault current can be higher than primary fault current and usually is.
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  2. #12
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    Quote Originally Posted by mivey View Post
    For equipment sizing you want to allow for system configuration changes including kVA upsizing, as well as lower impedance due to transformer replacement, primary wiring upgrades/feed changes, substation upgrades, transmission/generation changes, etc.

    For arc flash you want the current system state.
    Ok, that makes sense.


    So for equipment KAIC selection, you'd want to be prepared for any possible fault current to be available at your transformer primary, such that no matter what the utility does in transmission/distribution upgrades, your system remains safe. The "infinite bus" calculation, which calculates the upper limit on secondary fault current from transformer KVA & impedance, assuming available primary fault current is infinite.

  3. #13
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    Quote Originally Posted by Carultch View Post
    Ok, that makes sense.


    So for equipment KAIC selection, you'd want to be prepared for any possible fault current to be available at your transformer primary, such that no matter what the utility does in transmission/distribution upgrades, your system remains safe. The "infinite bus" calculation, which calculates the upper limit on secondary fault current from transformer KVA & impedance, assuming available primary fault current is infinite.
    Yes. Infinite source admittance may be extreme in some cases but not too far off in others. It is conservative but not necessarily so if other variances are ignored (which they are many times).

    You also need to allow for impedance variance ranges (use low end of range for fault current, high end for flicker analysis).

    You also should allow for reasonable transformer size increase if it is reasonable to expect growth (like commercial build_out, building additions, beyond temporaries, etc.).

    Sometimes you see infinite bus (conservative) and ignoring service drop (conservative) and ignoring variance (liberal) with the prayer that all will be ok. I also see completely ignoring the fault calc altogether and just installing whatever is handy (moreso with small commercial and residential): sloppy work and sloppy inspections.
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  4. #14
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    Quote Originally Posted by Natfuelbilll View Post
    The source is characterized by available fault current amps and impedance.
    So no voltage?
    no location? ie xfmr primary terminals
    what was the form of the impedance? x/r or R + jX ???

    what were the actual values or the sheet they sent you
    service info, voltages, kva, etc
    can you scan it? Or post a pic



  5. #15
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    Quote Originally Posted by Ingenieur View Post
    So no voltage?
    no location? ie xfmr primary terminals
    what was the form of the impedance? x/r or R + jX ???

    what were the actual values or the sheet they sent you
    service info, voltages, kva, etc
    can you scan it? Or post a pic
    It would be revealing to see what the utility provided.
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  6. #16
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    I routinely see utilities give, what I consider to be, unreasonably high figures for fault current. Last time I did a new medium sized 480 service, the utility gave me a figure of 39K, and when I calculated it from the transformer assuming infinite buss, it was a third of that. Some people say, "you have to use what the utility gives you" but now I would pretty much always go with the actual transformer data if it is available or I can get it. I can see providing for a larger transformer due to customer expansion, if that is of significant likelihood, but as far as the theory that utility may replace it with a a larger unit "they have on hand" if it fails, or with one with an impedance drastically lower (which I have never seen in 20 years), those are too far in the "what if" field for me.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  7. #17
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    Quote Originally Posted by electrofelon View Post
    I routinely see utilities give, what I consider to be, unreasonably high figures for fault current. Last time I did a new medium sized 480 service, the utility gave me a figure of 39K, and when I calculated it from the transformer assuming infinite buss, it was a third of that. Some people say, "you have to use what the utility gives you" but now I would pretty much always go with the actual transformer data if it is available or I can get it. I can see providing for a larger transformer due to customer expansion, if that is of significant likelihood, but as far as the theory that utility may replace it with a a larger unit "they have on hand" if it fails, or with one with an impedance drastically lower (which I have never seen in 20 years), those are too far in the "what if" field for me.
    The strike odds are in your favor if that is how you want to do it. Not sure about the financial odds but maybe those too.
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  8. #18
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    If this is Con Ed in NY, then it was probably 200kA with no other supporting information. We take that number to be at the service transformer paralleled bus. If it is a large facility with their own vaulted transformers, the number would be at the buss'd service end box.
    Ron

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