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Thread: MWBC Question Again

  1. #1
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    MWBC Question Again

    I'm trying to understand the difference in the CMP's thinking between 210.4(B) and 210.7 (2014 NEC). Under 210.4 if I run a MWBC to a 2-gang JB and install two separate receptacles, this section infers that I have to use a 2-pole CB to disconnect all un-grounded conductors. Yet under 210.7, if I run two separate 120V branch circuits to a 2-gang JB and install two separate receptacles I am not required to use 2-pole CB's unless I snap off the tabs on the receptacles and install opposing circuits on the same yoke.

    If the thinking was to disconnect all un-grounded conductors for safety reasons in the case of 210.4(B) why wouldn't the same logic be applied to 210.7?

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    The common disconnecting means for two circuits on one device has been around for decades, the MWBC disconnecting requirement is newer. The prior has to do with working on a single device and having both circuits de-energized. The latter has to do with ensuring the neutral is not energized if only one circuit is shut off.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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    Quote Originally Posted by infinity View Post
    The common disconnecting means for two circuits on one device has been around for decades, the MWBC disconnecting requirement is newer. The prior has to do with working on a single device and having both circuits de-energized. The latter has to do with ensuring the neutral is not energized if only one circuit is shut off.
    So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?

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    Quote Originally Posted by goldstar View Post
    So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?
    Two individual SP breakers without handle tie is fine.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by goldstar View Post
    So, if I run two separate 120V circuits into a 2-gang JB and land the circuit wires on two separate receptacles, can I then use s/p breakers ? Or, am I required to disconnect all power to the JB ?
    As Jumper stated two single poles is fine since you do not have a MWBC. There is no requirement to disconnect all of the power to a box.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  6. #6
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    You run into this alot with switch boxes where 3 switches may be on one circuit and then you may have a 3 way fed from another circuit
    They say I shot a man named Gray and took his wife to Italy
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    Quote Originally Posted by infinity View Post
    As Jumper stated two single poles is fine since you do not have a MWBC. There is no requirement to disconnect all of the power to a box.
    I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?

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    Quote Originally Posted by goldstar View Post
    I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?
    If wired correctly perhaps.

    But,

    If someone were to jumper the neutral from the 1st receptacle to the 2nd where the removal of the 1st receptacle would disconnect the neutral to the 2nd receptacle, if something was plugged into the 2nd receptacle, you stand the chance of throwing 240 volts across the 120v load plugged into the 2nd receptacle in the MWBC scenario.

    That would not happen if the 2 receptacles were fed from 2 separate circuits with 2 separate neutrals.

    Jap>

  9. #9
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    The common trip or handle tie requirement when sharing a neutral is not just about securing all power to an outlet (not just receptacle) box, but also for working with the neutral at any intermediate boxes along the way. If you turn off just one breaker to de-energize the specific circuit you are working with, you could be exposed to full line voltage on the "neutral" when you open it at an intermediate junction box.
    If the circuit is an uninterrupted home run, then the primary concern is at the outlet box. But there could still be a risk of injury when working with the neutral at the panel where it originates.

    PS: If you turn off one side of an MWBC, there is no risk of putting double voltage on a connected load even if you interrupt the common neutral!

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    Quote Originally Posted by goldstar View Post
    I understand that and I can buy into the logic behind the Code section. However, from the standpoint of safety, I don't see the logic if you have 2 separate circuits when you have separate neutrals. If you run an MWBC into a 2-gang box and are installing 2 duplex receptacles, you're splicing the neutrals under a wire nut and terminating the tails onto each receptacle. What's the difference, from the standpoint of safety ?
    If the two circuits are from a MWBC then de-energizing one circuit will not remove the load from the neutral for the other circuit and this can create a shock hazard if the neutral is opened. If there are two separate neutrals then when you de-energized one circuit you can safely open the neutral from that circuit.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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