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Thread: AFCI required on MWBC?

  1. #191
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    In that case, there's your AFCI workaround: get a large rotary phase converter and run all your circuits from a 3 phase panel. :-)

    Seriously, with a GE or BR panel, there are AFCIs without GF detection, so you could run a 3-phase MWBC for 120V loads with 3 single pole AFCI breakers and a handle tie. [I'm assuming that GE or BR make a 3-phase panel that accepts those breakers.]

    Cheers, Wayne

  2. #192
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    Quote Originally Posted by jaggedben View Post
    Which of the following require arc-fault protection in dwellings? (Check all that apply.)
    • a 120/240 3-wire MWBC
    • a 120/208 3-wire MWBC
    • a 120/208 4-wire MWBC*
    My provisional position is that no MWBCs require AFCI. We've already had the discussion regarding an MWBC protected by a single multi-pole breaker. So that leaves the case of an MWBC protected by multiple single-pole breakers.

    At first glance, such an MWBC consists of multiple 120V Branch Circuits, as the circuit consisting of one single pole breaker, one ungrounded conductor, and one neutral could be a Branch Circuit. However, when I think about the word "protecting" in the definition of Branch Circuit, I realize that the neutral is not protected by just one single pole breaker. It takes all of the single pole breakers to protect that neutral; if any one of them fails to trip on overcurrent, then an overcurrent on the associated ungrounded conductor would overload that neutral.

    That's a bit of a dilemma, since there is no single physical device protecting the circuit, as the definition of Branch Circuit assumes. The easiest solution is to consider the multiple handle-tied single pole breakers collectively as one overcurrent device for the purposes of applying the definition of Branch Circuit to an MWBC. (*) So that puts us back into the same discussion as an MWBC protected by a single multi-pole breaker.

    Cheers, Wayne

    (*) A similar strategy is required for twin breakers: even though a twin breaker is one physical device, it should be considered two overcurrent devices for the purposes of applying the definition of Branch Circuit. Otherwise there would only be one associated Branch Circuit.

  3. #193
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    Quote Originally Posted by jaggedben View Post
    Wouldn't I need a 3-pole AFCI for the 4-wire, and doesn't that not exist?
    For the sake of discussion, a set of 3 single pole GE AFCI handle-tied breakers, in my opinion, will actually work, as none of the GE AFCI breakers require that the 4-wire MWBC neutral be connected to any of them per GE installation instructions. The GE AFCI breaker neutral pigtail IS connected to the panelboard neutral bus.

    Remember, the MWBC minimum requirement is to simultaneously disconnect the ungrounded conductors, . . . overcurrent protection is not part of that minimum requirement.

    As for the Eaton BR AFCI, and the recently shared 2019 installation instructions, the LOAD neutral MUST land on the BR AFCI breaker load neutral terminal. There are no printed instructions for pigtailing the MWBC neutral to a pair or three single pole BR AFCI, and there are two-pole BR AFCI sold by Eaton. Eaton is silent about three pole AFCI.
    Another Al in Minnesota

  4. #194
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    Quote Originally Posted by wwhitney View Post
    It takes all of the single pole breakers to protect that neutral; if any one of them fails to trip on overcurrent, then an overcurrent on the associated ungrounded conductor would overload that neutral.
    Let me see if I can write what you are saying in this sentence in the quote.

    You are saying that an overcurrent (say a 25 Amp current) passing through a working single pole 20 Amp breaker, and returning to the source on the branch circuit neutral (the full 25 Amps) is something that SHALL NOT ever occur under any condition? Does this mean your thinking about the word "protect" means that the protection must be instantaneous? Theoretically instantaneous?
    Another Al in Minnesota

  5. #195
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    Quote Originally Posted by wwhitney View Post
    if any one of them fails to trip on overcurrent, then an overcurrent on the associated ungrounded conductor would overload that neutral.
    It's possible to speculate anything into a failure.

    Speculating that the unbalanced current,(returning on the shared neutral from an overload on one of the individual circuits of the MWBC that didn't trip ) combined with the unbalanced current returning from a secondary overload on one of the remaining one or two OCPD's of the MWBC that didn't trip, will overload the shared neutral, is somewhat far fetched.


    In that scenario, I would think the ungrounded conductors would be taking more of a hit than the neutral conductor.



    JAP>

  6. #196
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    Quote Originally Posted by wwhitney View Post
    It takes all of the single pole breakers to protect that neutral; if any one of them fails to trip on overcurrent, then an overcurrent on the associated ungrounded conductor would overload that neutral.
    Actually, the overloaded phase that lost it's OCP is what is overloading the neutral. The higher the amperage on the other 2 phases the lower the shared neutral current would be.

    If all 3 ungrounded conductors failed to trip the OCP, the neutral might never see an overloaded condition, it depends on the loading.

  7. #197
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    Quote Originally Posted by oldsparky52 View Post
    Actually, the overloaded phase that lost it's OCP is what is overloading the neutral. The higher the amperage on the other 2 phases the lower the shared neutral current would be.

    If all 3 ungrounded conductors failed to trip the OCP, the neutral might never see an overloaded condition, it depends on the loading.
    to throw a wrench into this - at some point in the circuit most MWBC downstream are only a 2 wire circuit both H and N see the total load unless shorted to ground.

    Just saying.

  8. #198
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    Quote Originally Posted by Sierrasparky View Post
    to throw a wrench into this - at some point in the circuit most MWBC downstream are only a 2 wire circuit both H and N see the total load unless shorted to ground.

    Just saying.
    You are talking about the load of just that one 120v circuit. The other 2 circuits (in the above example) don't affect the down stream neutral.

  9. #199
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    Quote Originally Posted by Sierrasparky View Post
    to throw a wrench into this - at some point in the circuit most MWBC downstream are only a 2 wire circuit both H and N see the total load unless shorted to ground.

    Just saying.
    That would be past the point of the "shared" neutral of a MWBC.

    I would think the current of the remaining 1 or 2 circuits would have to be at zero for the shared neutral to receive the full return current of the overloaded circuit.

    JAP>

  10. #200
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    Quote Originally Posted by jap View Post
    That would be past the point of the "shared" neutral of a MWBC.

    I would think the current of the remaining 1 or 2 circuits would have to be at zero for the shared neutral to receive the full return current of the overloaded circuit.

    JAP>
    At least one of them at/near 0.

    A=30
    B=15
    C=15
    N=15

    A=30
    B=15
    C=0
    N=26

    A=30
    B=30
    C=0
    N=30

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