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Thread: SCCR NEC409

  1. #1
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    SCCR NEC409

    I have a question about calculating the SCCR for a panel I am designing for a machine.
    In my MCC panel I have a circuit breaker that is feeding a separate hoist control panel, separately from my panel.
    That panel comes prebuilt as part of a hoist assembly and it has an SCCR rating of 5ka.
    Since technically I am only calculating my SCCR to the line side of the branch circuit OCD can I ignore the 5ka rating of the hoist control box
    while calculating my cabinets SCCR ?

    The separate hoist control panel contains the starters and O.L.'s for the hoist motor.
    I will try to attach a drawing of the 480 dist page..
    Attached Images Attached Images

  2. #2
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    Quote Originally Posted by milmat1 View Post
    I have a question about calculating the SCCR for a panel I am designing for a machine.
    Is this panel going to be listed/labeled as meeting 'standards'? If so follow their procedures.
    If not, I do not see how you can rate the panel SCCR as being any higher than its lowest component.
    Just because you can, doesn't mean you should.

  3. #3
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    Quote Originally Posted by jim dungar View Post
    Is this panel going to be listed/labeled as meeting 'standards'? If so follow their procedures.
    If not, I do not see how you can rate the panel SCCR as being any higher than its lowest component.

    That is sort of my question. Since the panel I'm feeding is a separate panel with its own SCCR rating. And will be non compliant because my panel will have an AFC (going to that panel) much higher than the 5ka rating of the sub. And the sub panel being supplied by a different manufacturer.

  4. #4
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    Quote Originally Posted by milmat1 View Post
    That is sort of my question. Since the panel I'm feeding is a separate panel with its own SCCR rating. And will be non compliant because my panel will have an AFC (going to that panel) much higher than the 5ka rating of the sub. And the sub panel being supplied by a different manufacturer.
    Your panel rating is independent of the sub-panel's rating, unless your contract says otherwise. It is up to the installer to follow NEC 110.10 - a common methods is to add enough impedance (e.g. long lengths of small conductors) to lower the available Short Circuit Amps.

    Has anyone calculated what fault current is likely to exist at both panels?
    Just because you can, doesn't mean you should.

  5. #5
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    "Panel" is being used a little flippantly for my taste here, I'm not sure which "panel" you are referring to and where the "MCC" is in all of this. Nor am I clear on your role here; are you the installing contractor, a panel builder, the end user?

    Assuming you are a panel builder, if the "panel" in your drawing, let's call it "7549", is the one YOU are building, the "hoist panel" is external and is essentially a load fed FROM this panel. So you do not need to account for the SCCR of that remote device in THIS panel's listing, that's their problem. You can list 7549 at a level commensurate with the SCCR of the devices inside of it. Per the SCCR tables from AB, you can ALMOST list that panel at 65kA SCCR. The problem is, you are apparently attempting to use that last MPCB as a "feeder" breaker, and it is not UL listed for that purpose. The 140M devices are only listed as "Branch" protection, meaning it is the last protective device before the end use unit, i.e. a motor. You are "feeding" another panel, so the device in 7549 must be UL 489 listed as a "Feeder" and per the NEC and UL, that means it must have fixed trips, i.e. a "Molded Case Circuit Breaker" (MCCB).

    There is a version of those little IEC devices that IS listed under UL 489 as an MCCB that looks essentially the same, albeit the next frame size up.
    It would be a 140U-D6D3-C25 (with 25A trips) and is listed at 100kAIC. So with that change, you could apply an SCCR of 65kA on that panel.

    The 5kA listing of the hoist panel is probably going to be a problem for someone though. For that to be connected, whomever makes that connection will have to show an AFC of 5kA or less at it's terminals, and that is extremely unlikely on a 480V system. It's a problem people have in allowing vendors to sell them panels without making this issue part of the purchase specifications. Some OEMs will take advantage of it because anyone can get a 5kA "courtesy" listing of a panel without testing or paying attention to the component selection, meaning they can buy mis-matched components as cheaply as they can find them. The problem is passed down to the installer, and IMHO that's not right, because it's EXTREMELY difficult to deal with in the field.
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  6. #6
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    Quote Originally Posted by jim dungar View Post
    Your panel rating is independent of the sub-panel's rating, unless your contract says otherwise. It is up to the installer to follow NEC 110.10 - a common methods is to add enough impedance (e.g. long lengths of small conductors) to lower the available Short Circuit Amps.

    Has anyone calculated what fault current is likely to exist at both panels?
    I should have been more clear, the sub panel is still part of our original machine. It just happens to be a prebuilt part of a hpist assembly that we mount onto our machinery. So does your statement about still hold ?

  7. #7
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    Quote Originally Posted by milmat1 View Post
    I should have been more clear, the sub panel is still part of our original machine. It just happens to be a prebuilt part of a hpist assembly that we mount onto our machinery. So does your statement about still hold ?
    I would still say that it does not affect the rating of your panel, but that you may not be allowed to install it in the machine in the first place because of its own rating.

    If it is part of a (UL listed?) assembly, I am not sure whether Jraef's concern about a feeder breaker still applies though.

  8. #8
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    I can change the hoist feeder breaker to a 489 breaker, no problem there. If I do that, and then I rate my 7549 panel at 65ka. Now the hoist panel is at 5ka SCCR.
    If I calculate the AFC to the hoist panel (via the 489 MCCB), and it is much higher than 5ka. Then that panel becomes the problem. Are you starting to see my dilemma here, I might have to rate my 7549 panel at 5ka, just so I remain compliant with feeding the 5ka hoist.

    I know this is very confusing and I appreciate everyone trying to help..

  9. #9
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    Quote Originally Posted by milmat1 View Post
    I can change the hoist feeder breaker to a 489 breaker, no problem there. If I do that, and then I rate my 7549 panel at 65ka. Now the hoist panel is at 5ka SCCR.
    If I calculate the AFC to the hoist panel (via the 489 MCCB), and it is much higher than 5ka. Then that panel becomes the problem. Are you starting to see my dilemma here, I might have to rate my 7549 panel at 5ka, just so I remain compliant with feeding the 5ka hoist.

    I know this is very confusing and I appreciate everyone trying to help..
    Based on Jraef's earlier comments, making your panel 5kA SCCR would just be kicking the can back down the road to the customer at install time for your equipment and may very well, in practice, make your equipment unusable.
    Changing your panel SCCR rating to 5kA is not what would make the sub panel legal. It would be configuring the installation environment to be less than 5kA available fault current that would do that.

    The whole assembly would technically conform to code and UL requirements, but likely be impossible (or at least very difficult and expensive to install. Is that the kind of behavior that would give you a competitive advantage? I suspect not.

  10. #10
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    Quote Originally Posted by GoldDigger View Post
    Based on Jraef's earlier comments, making your panel 5kA SCCR would just be kicking the can back down the road to the customer at install time for your equipment and may very well, in practice, make your equipment unusable.
    Changing your panel SCCR rating to 5kA is not what would make the sub panel legal. It would be configuring the installation environment to be less than 5kA available fault current that would do that.

    The whole assembly would technically conform to code and UL requirements, but likely be impossible (or at least very difficult and expensive to install. Is that the kind of behavior that would give you a competitive advantage? I suspect not.
    I think this might be the silver bullet..

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