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Thread: Simple Arc Flash Question(s)

  1. #1
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    Simple Arc Flash Question(s)

    Hi all,

    Looked thru the archives and google and just can't find an answer to a simple question(s)-

    If therre is a change to the load side does that affect the calcs and if so--How big of a change is needed?

    Example--If we were to require and arc flash analysis to be included as part of the contract documents--then there is a future (couple years later) load change of 20 amps to an existing 50 amp load, for a total of 70 amps--will this require a recalculation --or what would trigger a recalc for a load side addition?

    Or--am I over thinking this?

    Thanks all

    RichB
    Last edited by RichB; 05-14-18 at 10:18 AM. Reason: Spelling/addition
    RichB N7NEC

  2. #2
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    I think if load change is such that bigger size conductors are required and fault level increased as a result, arc flash study necessary.

  3. #3
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    Are you changing a 50A load to a 70A load? If so, most likely you are changing a breaker/fuse and changing conductor size. Those changes will make the incident energy calculation on that circuit invalid, so at the very least, that circuit needs to be re-analyzed.

    If you are adding a new 20A circuit in a panel that has a 50A load, then you have a new circuit that needs to be analyzed.

  4. #4
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    Quote Originally Posted by wbdvt View Post
    If you are adding a new 20A circuit in a panel that has a 50A load, then you have a new circuit that needs to be analyzed.
    It won't change the IE available in the panel so why would a new arc flash study be needed?
    Bob

  5. #5
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    Quote Originally Posted by petersonra View Post
    It won't change the IE available in the panel so why would a new arc flash study be needed?
    I agree. On something that small, it isn't going to change the Incident Energy of the system. If you were adding a large 3 phase motor to a system it could contribute to the available fault current, but that's not what's being described here.
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  6. #6
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    Thanks for all the replies guys!!

    Yes--I was looking at the IE available to the system--

    You all just confirmed what I thoought---

    Jref--Yes we would be adding a new 20 amp load--

    I just pulled those numbers as representative of what we have--

    This would be for Highway systems--lighting/cameras/data station type stuff.

    We typically have less than 100 amps on 200 amp service and usually a large addition is around 20 amps max.

    So to summarize and ensure I am reading wjhat you folks wrote--A small load increase such as described would not increase the system IE, triggering a new analysis- and if it doesn't change the IE available at that panel then it also won't trrigger a new study.

    Last question--is there a rule of thumb for an increase--in load not necessarily type of load (Jref's large 3 phase motor, and I believe I read somewhere that a 50 HP and above is what is called out) that would/could trigger the need for a new study

    Thanks again !!
    RichB N7NEC

  7. #7
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    Quote Originally Posted by RichB View Post
    Thanks for all the replies guys!!

    Yes--I was looking at the IE available to the system--

    You all just confirmed what I thoought---

    Jref--Yes we would be adding a new 20 amp load--

    I just pulled those numbers as representative of what we have--

    This would be for Highway systems--lighting/cameras/data station type stuff.

    We typically have less than 100 amps on 200 amp service and usually a large addition is around 20 amps max.

    So to summarize and ensure I am reading wjhat you folks wrote--A small load increase such as described would not increase the system IE, triggering a new analysis- and if it doesn't change the IE available at that panel then it also won't trrigger a new study.

    Last question--is there a rule of thumb for an increase--in load not necessarily type of load (Jref's large 3 phase motor, and I believe I read somewhere that a 50 HP and above is what is called out) that would/could trigger the need for a new study

    Thanks again !!
    I would caution against a "rule of thumb" in this type of analysis, but do agree with Jraef. Both the size and the nature of the load you're altering would need to be considered whenever you alter the system. Don't paint yourself into a corner by setting up a rule of thumb based on specific horsepower or load rating, just think about the scenario like you're doing now.

    A large motor load or other specialty loads such as large UPS unit could have an effect. Adding 20A of general load would not really impact the label on the upstream panel in the system you described.

  8. #8
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    Quote Originally Posted by skeshesh View Post
    I would caution against a "rule of thumb" in this type of analysis, but do agree with Jraef. Both the size and the nature of the load you're altering would need to be considered whenever you alter the system. Don't paint yourself into a corner by setting up a rule of thumb based on specific horsepower or load rating, just think about the scenario like you're doing now.

    A large motor load or other specialty loads such as large UPS unit could have an effect. Adding 20A of general load would not really impact the label on the upstream panel in the system you described.
    how would a UPS change anything? It is not like a motor where it generates energy that can be back fed into the system.

    The label at the panel says how much SCCR and IE there is at the panel. Unless you change something feeding the panel, neither of those can change.
    Bob

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