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Thread: Short circuit current questions

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    Short circuit current questions

    two quick (I hope) questions:

    Assume a fairly high SCCA (100KA) at the meter of a 600A, 120/208V, 3Ph (100KA) .


    Can a main fused disconnect with current limiting fuses significantly reduce the required (SCCR) rating of load side circuit breakers?


    An existing 600 fused disconnect - sight unseen, would it be safe an prudent to assume the existing fuses are non-current limiting?

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    Quote Originally Posted by olc View Post
    two quick (I hope) questions:

    Assume a fairly high SCCA (100KA) at the meter of a 600A, 120/208V, 3Ph (100KA) .


    Can a main fused disconnect with current limiting fuses significantly reduce the required (SCCR) rating of load side circuit breakers?


    An existing 600 fused disconnect - sight unseen, would it be safe an prudent to assume the existing fuses are non-current limiting?
    It doesnt really matter if they are current limiting or not. What matters is if there is a tested series rated combination with the downstream circuit breakers. Look up the series ratings for the manufacturer and type of circuit breakers you have and see if there is a tested combination that gets you where you need to be.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Agree with electrofelon but note that SquareD does show a 22k breaker series rating with a 600 amp CL fuse but emphasizing as he says, the devil is in the detail so you need to check your specific application.
    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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    See below link (for Siemens). First page fuse->breaker.
    https://www.google.com/url?sa=t&sour...qRkHJg1zaEnQjE

    Sent from my ONEPLUS A6013 using Tapatalk

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    In the example, the fused disconnect is a main service disconnect that would feed a 600A MPD (Square D I Line for example) with five feeder circuit breakers.
    400, 100, 200, 200, 250
    So all five would have to have a series rating with the same 600A fuse? Possible?
    Last edited by olc; 04-22-19 at 08:06 AM.

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    Quote Originally Posted by olc View Post
    In the example, the fused disconnect is a main service disconnect that would feed a 600A MPD (Square D I Line for example) with five feeder circuit breakers.
    400, 100, 200, 200, 250
    So all five would have to have a series rating with the same 600A fuse? Possible?
    Yes probable and likely. Also dont forgot to look at your branch breakers to make sure they are sufficiently rated.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Yes probable and likely. Also dont forgot to look at your branch breakers to make sure they are sufficiently rated.
    My understanding those branch breakers might be series rated with the breaker in the distribution panel, but are not "double series rated" and could not be used in series with another breaker that is already being used in a series rating. Not that they couldn't test this combination but likely they haven't.

    That said, unless your branch circuit panel is in immediate vicinity as the distribution panel, there is a good chance that the resistance of the feeder reduces the available fault current at the branch circuit panel to a level that is acceptable for your branch breakers.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    My understanding those branch breakers might be series rated with the breaker in the distribution panel, but are not "double series rated" and could not be used in series with another breaker that is already being used in a series rating. Not that they couldn't test this combination but likely they haven't.

    That said, unless your branch circuit panel is in immediate vicinity as the distribution panel, there is a good chance that the resistance of the feeder reduces the available fault current at the branch circuit panel to a level that is acceptable for your branch breakers.
    Yeah that's why it is good to look at the entire system as a whole. One might focus on the main and the distribution ocpd's and get everything hunky dory but not consider the branch breakers. For example one job I recently had, it worked out well to use a MLO with 6 mains @65k and those series rated with regular 10k branches. It might have been tempting to use a main and use series rated distribution breakers, but then I would have been screwed on the branches as you typically won't find any series ratings from an over 250 amp frame @65k down to regular 10k branches. You really want to avoid having to use 22k "H' branch breakers for cost reasons, unless as you noted there is enough conductor to get down to 10k without needing a series rating.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Yeah that's why it is good to look at the entire system as a whole. One might focus on the main and the distribution ocpd's and get everything hunky dory but not consider the branch breakers. For example one job I recently had, it worked out well to use a MLO with 6 mains @65k and those series rated with regular 10k branches. It might have been tempting to use a main and use series rated distribution breakers, but then I would have been screwed on the branches as you typically won't find any series ratings from an over 250 amp frame @65k down to regular 10k branches. You really want to avoid having to use 22k "H' branch breakers for cost reasons, unless as you noted there is enough conductor to get down to 10k without needing a series rating.
    And it doesn't take a lot of conductor length to make a big difference, especially for 200 amp or less conductor sizes. Placing that branch circuit panel for the mechanical room right next to the distribution panel with just a offset nipple between is likely not going to be enough conductor length to get below that 10k level unless you want to somehow put enough extra conductor in the cabinet(s) involved to reduce the available current through that length. Running the feeder to the ceiling across the room and back down to a panel there, might be enough though, depends on what starting level and what you have for conductors and even raceway type and motor contributions.
    I live for today, I'm just a day behind.

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    Thanks for the replies.
    It would be somewhat important to know if the SCCR at the branch panels depends on what we are starting with at the main feeder breaker.

    On the other hand the SCCA at the main breaker may have to be significantly lower to make a difference at the branch. I did some rough quick comparison calcs and, for example, starting with 100K at the service, the SCCA at a branch was greater than 10KA. And using a lower SCCA at the main circuit breaker still resulted in >10KA. I think for my example, the only difference would be the SCCR of the main distribution panel (65KA vs 100KA or what ever).

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