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Thread: MWBC Question Again

  1. #11
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    Judging by some of the replies posted here it's looking more and more like the CMP decided to make 210.4(B) a "one size fits all" code section and I'm also guessing it was done for safety reasons. However, in my scenario there's one MWBC running to one 2-gang JB where the neutral will be spliced to 2 pig tails, each of which get landed on 2 separate duplex receptacles. From the standpoint of safety I can't understand the requirement to use a 2-pole breaker to disconnect ALL un-grounded conductors yet allowing s/p breakers to shut off each circuit separately if I ran an extra neutral wire. There are no JB's in between - just 1 conduit run to one JB which is less than 50' away and within sight of the breaker panel (if that means anything). The only difference would be is if you are working live in that JB. Then there's 240V present so a total disconnect would make sense. But, then you would have to ask "Why are you working it live?"

    BTW, I would also like to point out that we can't use the word IF in a scenario. That small word can make any scenario work in your favor. For example : IF a bank employee left the front door open and IF that same bank employee left the bank vault door open and IF I were so inclined I could walk into that bank and take all the $$ I could handle (IF I were so inclined)

  2. #12
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    Quote Originally Posted by goldstar View Post
    However, in my scenario there's one MWBC running to one 2-gang JB where the neutral will be spliced to 2 pig tails, each of which get landed on 2 separate duplex receptacles. From the standpoint of safety I can't understand the requirement to use a 2-pole breaker to disconnect ALL un-grounded conductors yet allowing s/p breakers to shut off each circuit separately if I ran an extra neutral wire.
    Because with a MWBC and no handle ties shutting off only 1 of the breakers does not shut off all of the power associated with that circuit.
    That circuit consists of 2 ungrounded conductors and 1 grounded conductor.

    It boils down to Off should mean Off when shutting off circuits, and, that includes the neutral associated with those circuits.

    In a MWBC without handle ties in your case, if someone shut off one of the circuits and something was plugged into the other, there would be 120 volts on the return neutral in the panel.

    Therefore it did not disconnect all of the power associated with that circuit like it would if seperate neutrals were pulled for each circuit.

    JAP>

  3. #13
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    And the word if does matter.

    That's the only way we ward off 90% off construction accidents to begin with.

    If the current gets too high the breaker trips
    If you step on the brake the car better stop
    If you mom yells at you to do something you'd better get to it.
    If the heat melts the glass at the sprinkler head the water better come out.
    If I flush the toilet the water better go down the drain.

    and so on....


    JAP>

  4. #14
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    Quote Originally Posted by jap View Post
    Because with a MWBC and no handle ties shutting off only 1 of the breakers does not shut off all of the power associated with that circuit.
    That circuit consists of 2 ungrounded conductors and 1 grounded conductor.

    It boils down to Off should mean Off when shutting off circuits, and, that includes the neutral associated with those circuits.

    In a MWBC without handle ties in your case, if someone shut off one of the circuits and something was plugged into the other, there would be 120 volts on the return neutral in the panel.

    Therefore it did not disconnect all of the power associated with that circuit like it would if seperate neutrals were pulled for each circuit.

    JAP>
    Sorry, I guess I'm just not thinking out of the box here. I understand your explaination completely if it were installed say for a kitchen with multiple outlets. Let me try one more time. You have a 10' length of conduit with only one 2-gang JB at the end of it and the other end connected at the breaker panel. You can see the breaker panel - you can walk over 10' and shut a breaker off if you wish. What difference would it make at the 2-gang JB if I had a common neutral or if I ran 2 neutrals ? In the case of a MWBC I'm required to use a 2-pole breaker and disconnect power to ALL un-grounded conductors but in the case of 2 separate circuits (of opposing phases) if I install separate neutrals I can use s/p breakers. I just don't see the logic in the Code requirement in my specific instance.

  5. #15
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    Quote Originally Posted by jap View Post
    And the word if does matter.

    That's the only way we ward off 90% off construction accidents to begin with.

    JAP>
    If the current gets too high the breaker trips
    It's not IF the current gets too high - Its WHEN the current gets too high
    However, you can ask WHAT IF the current gets too high and the breaker doesn't trip ........?


    If you step on the brake the car better stop
    It's not IF you step on the brake - It's WHEN you step on the brake
    Again, you can ask WHAT IF I step on the brake and the car doesn't stop.....?


    If you mom yells at you to do something you'd better get to it.
    WHEN

    If the heat melts the glass at the sprinkler head the water better come out.
    WHEN

    If I flush the toilet the water better go down the drain.
    WHEN the toilet is flushed the water WILL go down

    I know we're playing with semantics here but I think there are many parts of the NEC that create "what if" scenarios, the possibilities of which could be million to one shots.

  6. #16
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    Quote Originally Posted by goldstar View Post
    Sorry, I guess I'm just not thinking out of the box here. I understand your explaination completely if it were installed say for a kitchen with multiple outlets. Let me try one more time. You have a 10' length of conduit with only one 2-gang JB at the end of it and the other end connected at the breaker panel. You can see the breaker panel - you can walk over 10' and shut a breaker off if you wish. What difference would it make at the 2-gang JB if I had a common neutral or if I ran 2 neutrals ? In the case of a MWBC I'm required to use a 2-pole breaker and disconnect power to ALL un-grounded conductors but in the case of 2 separate circuits (of opposing phases) if I install separate neutrals I can use s/p breakers. I just don't see the logic in the Code requirement in my specific instance.
    If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral. The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence). If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  7. #17
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    Quote Originally Posted by infinity View Post
    If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral. The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence). If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.
    I understand your explanation completely. I'm not asking the CMP to re-write this Code section. Just looking for some clarification.
    If you have a two wire circuit (hot/neutral) and you open the neutral you have a shock potential from the neutral.
    Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?
    The part you're missing is that for a MWBC you are sharing the neutral, if you shut off one of the two circuits and open the neutral you can get shocked because the other circuit on that shared neutral is still energized (the same as the 2-wire circuit in the first sentence).
    Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?
    If you have a two wire circuit (hot/neutral) and you shut off the hot the neutral has no shock hazard.
    Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?

    Now, if you tell me that this Code section is designed to prevent any stupid HO's or otherwise unqualified personnel from getting shocked - then I'll buy into that explanation and reasoning completely.

  8. #18
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    Quote Originally Posted by goldstar View Post
    I understand your explanation completely. I'm not asking the CMP to re-write this Code section. Just looking for some clarification.
    Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?

    Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?
    Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?

    Now, if you tell me that this Code section is designed to prevent any stupid HO's or otherwise unqualified personnel from getting shocked - then I'll buy into that explanation and reasoning completely.
    The MWBC handle-tie rule exists because if the neutral is opened at some point upstream of a load that is on a leg that is not turned off, it can shock you or arc flash. This may not be likely if you are working at a receptacle on the tail end of conduit run, but that is not the only place you're ever going to be working on an MWBC. You need to understand the danger that the code rule is trying to address, not keep asking how it relates to an example that may not apply.

    There are two different code rules for two different dangers that are not the same thing.

  9. #19
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    Quote Originally Posted by goldstar View Post


    WHEN the toilet is flushed the water WILL go down
    Now I know for sure your living in a fantasy world if you believe there's no question that will happen every time.

    Jap>

  10. #20
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    Quote Originally Posted by goldstar View Post
    Please explain how the neutral can become open and where and to whom the shock hazard might be if I have a 10' length of conduit with a receptacle installed at the load end ?

    Again, please explain how the neutral becomes open. It is wire nutted together with two pig tails to each of 2 receptacles. Where is the shock hazard and to whom ?

    Why should this be any different than the precautions taken with a MWBC if you have 2 circuits with separate neutrals in one 2-gang JB ?
    The code has no way of knowing whether any given installation consists of a conduit run supplying one 2-gang box or a multi-outlet run with many boxes and/or outlets. The concern with a shared neutral is that someone could believe they've de-energized a complete circuit when flipping one breaker.

    Suppose you turn off a breaker with the intent of relocating that circuit's neutral to another neutral bus terminal or even another neutral bus. If that neutral is shared by another hot wire with a load, it becomes energized the moment it's loosened from the bus. The same thing could happen in any box.

    It's an intentional or even accidental opening of a shared neutral that is of concern, which, again, could happen if someone mistakenly thinks they've de-energized a complete circuit by opening a single breaker. A handle tie is for manual operation, while a common-trip mechanism is for automatic operation.

    Yes, an intact neutral theoretically poses no threat, but a neutral pathway could be interrupted without realizing another circuit is still dependent on it. The same concern does not exist when each hot wire has its own corresponding neutral because opening one breaker does indeed de-energize an entire circuit.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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