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Thread: AFCI required on MWBC?

  1. #21
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    Quote Originally Posted by Sparks4All View Post
    Back in the day, the MWBC circuits did not need to be on the same breaker so they were technically two different circuits. You can still do this but a handle tie is required. Usually everyone just installs a 2-pole breaker.
    Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

    Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


    JAP>

  2. #22
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    Quote Originally Posted by Sparks4All View Post
    Back in the day, the MWBC circuits did not need to be on the same breaker so they were technically two different circuits. You can still do this but a handle tie is required. Usually everyone just installs a 2-pole breaker.
    Quote Originally Posted by jap View Post
    Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

    Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


    JAP>
    Sparks is saying that at one time you could run a 3 wire, shared neutral, MWBC and use any two circuits in the panel that weren't one the same leg or phase. That is no longer a viable option because of the handle-tie requirement.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  3. #23
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    Quote Originally Posted by jap View Post
    Back in the day, there was not a rule that stated every circuit that shared a common neutral had to be "simultaneously disconnected" that is.

    Without 2 or more "Ungrounded Conductors that have voltage between them" or "Ungrounded Conductors from 2 different phases" you have no MWBC.


    JAP>
    So the NEC is 'incorrectly' speaking of a split phase as "2 different phases??" Or are we supposed to be wiring our homes on 3-phase power?

  4. #24
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    Quote Originally Posted by PaulMmn View Post
    So the NEC is 'incorrectly' speaking of a split phase as "2 different phases??" Or are we supposed to be wiring our homes on 3-phase power?
    The "different phases" requirement is for a multi-phase system, the "Ungrounded Conductors that have voltage between them" is from a single winding center tapped system.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  5. #25
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    Quote Originally Posted by infinity View Post
    A MWBC is two circuits when applying the requirements of 210.12 for AFCI protection. It can be considered a single circuit elsewhere in the NEC such as 225.30.
    210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." Which also means it is not required to be considered as multiple circuits and may generally be considered as one circuit.

    So what text says that for purposes of 210.12, a multiwire circuit must be considered as multiple circuits?

    Cheers, Wayne

  6. #26
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    We love to see how complicated we can make a simple question, don't we?

    For the sake of the OP's question, a residential MWBC (where a single load is not supplied by both ungrounded conductors) is two 120v circuits.

    The only reason "1 phase vs 2 phases" comes up here is because of the tendency of some to call every ungrounded conductor a phase.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  7. #27
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    Quote Originally Posted by wwhitney View Post
    210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." Which also means it is not required to be considered as multiple circuits and may generally be considered as one circuit.

    So what text says that for purposes of 210.12, a multiwire circuit must be considered as multiple circuits?

    Cheers, Wayne
    210.12(A) says all 120 volt circuits, if you run MWBC for two 120 volt circuits then it requires AFCI protection.

    210.12(A) Dwelling Units. All 120-volt, single-phase, 15- and
    20-ampere branch circuits supplying outlets or devices in-
    stalled in dwelling unit kitchens, family rooms, dining
    rooms, living rooms, parlors, libraries, dens, bedrooms,
    sunrooms, recreation rooms, closets, hallways, laundry ar-
    eas, or similar rooms or areas shall be protected by any of
    the means described in 210.12(A)(1) through (6):
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  8. #28
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    Darn, maybe I should give up.

    Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.

    As a solar installer I've been able to almost entirely ignore arc-fault stuff until recently we got into doing battery backup and now I'm dealing with rewiring house loads and it's starting to come up at inspections. I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.

  9. #29
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    Quote Originally Posted by jaggedben View Post
    Darn, maybe I should give up.

    Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.

    As a solar installer I've been able to almost entirely ignore arc-fault stuff until recently we got into doing battery backup and now I'm dealing with rewiring house loads and it's starting to come up at inspections. I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.

    The bold part is specific to where you earn your living. If you're in a place that follows the NEC to the letter then you will need AFCI protection on a MWBC that is providing 2-120 volt circuits.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  10. #30
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    Quote Originally Posted by jaggedben View Post
    Darn, maybe I should give up.
    Please don't.

    Forget the logic and arguments. Anyone else want to help me out with their AHJ experience on AFCIs? I've got jap saying all the AHJs agree with him. Others saying AFCIs required. Is this regional? This thread so far seems to suggest there's no consensus.
    There may not be a consensus to be had. Here in VA, we're fortunate enough to still on the '14 NEC, with the option to use the '11 until September. Only bedroom outlets require AFCI protection.

    I could use some practical advice as to whether I can expect any consistency from one AHJ to the next. I already know that not all AHJs in my area enforce AFCI generally to begin with, and now I'm wondering if I can expect any consistency on MWBCs specifically.
    You're answering your own question; or rather, you're saying that there is no single answer where you are. When and where AFCI protection is required for you is inconsistent.

    The most direct answer we can give you is that having a MWBC does not have any effect on whether AFCI protection is required in any given installation, new work or otherwise.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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