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Thread: Tap Rules, Feeders, and MLO Panels

  1. #1
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    Tap Rules, Feeders, and MLO Panels

    You see, I had this nifty thread all edited and ready to go - then, I realized my question was pointless because feeders of identical ampacity are not considered a tap under 240.21.

    I am posting it anyway just to be on the safe side.

    -----------------------------

    I am trying to get my head around 225.33 - maximum number of discos and 240.21(B)(5) - taps of unlimited length.

    Given that these panels are all in unattached separate structures, do I understand correctly that A is ok, but B is a violation because it is a tap of unlimited length and must terminate in a single breaker?

    Code:
    A.              Main Panel                 6 circuit MLO Panel            6 circuit MLO Panel  
      ----MB-----Feeder Breaker-----------------------Lugs--------------------------Lugs
    
    
    B.        Main Panel                      6 circuit MLO Panel 
       ----MB-----Feeder Breaker---------------------Lugs
                                          |                                    6 circuit MLO Panel  
                                          |-----------------------------------------Lugs
    
    Last edited by NeoAmish; 07-05-12 at 03:52 PM. Reason: Formatting

  2. #2
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    Sorry, but your graphic is just not conveying your situation very clearly. I am not certain I understand what is connected to what. But I'll take a stab at it.

    In both situations, you appear to have a main panel that has its own main breaker. It also has a feeder breaker that provides power to two other, separate buildings. I get that part.

    In situation A, you have an MLO panel with 6 breakers. That is OK. But how is power getting to the third panel? Does it come from the main panel, on the same feeder breaker? If so, then I think that is OK. Does it come from one of the 6 breakers on the second panel? If so, then I think that is also OK. In either case, it seems to me that situation A is OK.

    In situation B, it seems that power to the third panel is tapped from the feeder that serves the second panel. Now, if I understand that the feeders are all the same size, then this is also OK. It is not a tap, as you correctly pointed out. On the other hand, if the feeder to the third panel has an ampacity that is less than the rating of the upstream feeder breaker, then you are into the tap rules. In that case, that feeder would have to terminate in a single breaker.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    Charlie, my apologies for the lousy ascii art. In scenario A, the third panel is fed by feedthrough lugs on the second panel. Other than that, you interpreted it correctly.


    This whole mental Chinese Fire Drill was brought on by an upcoming project where I need to decide whether to feed the panel #3 from the panel #2 - or, to tap/splice the feeder before it enters the second structure.

    Actually, I have never seen a 6 circuit panel with feedthrough lugs, but one probably exists somewhere. The existing panel in the second structure does not have feedthrough lugs, but it does have space for another DP breaker (which would be a waste since the breaker would be used for its screw clamps only). I might be able to retrofit some clamps to the lugs, but that is getting a bit nuts.

    So, it comes down to time and materials cost between:

    A. Bringing the new feeder inside to panel #2.

    or:

    B. Mounting a JB on the exterior wall and tapping inside it (plus related PVC work).

  4. #4
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    I can't advise you on relative costs. But if the wire sizes are such that the upstream breaker is sufficient to protect them, then both your scenarios are compliant.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  5. #5
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    As far as it affecting time, I am leaning toward option B.

    Option A would involve sleeving through some thick, hard concrete or running a long distance vertically to find a softer point of entry.

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