Would this MWBC be allowed by NEC?

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jeff48356

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Livonia, Michigan
Suppose I wire a garage using an MWBC. I put the lights on one circuit, and the receptacle outlets on another. I run a PVC conduit underground with 4 conductors: Black, white, green are all #12, and the red is #14 to feed the lights. As long as the white and green are #12, would the red be allowed to be #14? I would connect the black and red to 20A and 15A breakers, respectively, in the basement. I don't see a point in running all #12AWG to lights when #14 would be plenty sufficient. Or do all conductors in an MWBC need to be the same size?
 
I think you can even use #14 for the portions of the white run that only serve the lights, switching to #12 at the point the neutral run from the two circuits join up for their return trip to the panel.
 
charlie b said:
I think you can even use #14 for the portions of the white run that only serve the lights, switching to #12 at the point the neutral run from the two circuits join up for their return trip to the panel.
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Yes, that's what I meant. The MWBC and the PVC conduit would terminate at a junction box in the garage, where the #12 receptacle branch and the #14 lighting branch would each get their power using runs of Romex.
 
jumper said:
As Bob and Charlie said-legal.

You need a handle tie for the breakers.
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I thought the only time a handle-tie was needed is if both circuits serve the same yoke, or wiring device. In this case, they would first go to a junction box, then be split off into their respective branch circuits, with a common neutral at that point. Did the Code change recently to require this?
 
jeff48356 said:
I thought the only time a handle-tie was needed is if both circuits serve the same yoke, or wiring device. In this case, they would first go to a junction box, then be split off into their respective branch circuits, with a common neutral at that point. Did the Code change recently to require this?
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This was added in 2008.

210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall
be permitted as multiwire circuits. A multiwire circuit shall be
permitted to be considered as multiple circuits. All conductors
of a multiwire branch circuit shall originate from the same
panelboard or similar distribution equipment.

(B) Disconnecting Means. Each multiwire branch circuit
shall be provided with a means that will simultaneously
disconnect all ungrounded conductors at the point where
the branch circuit originates.
 
jumper said:
This was added in 2008.


(B) Disconnecting Means. Each multiwire branch circuit
shall be provided with a means that will simultaneously
disconnect all ungrounded conductors at the point where
the branch circuit originates.
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OK, thanks. I guess a MWBC is out then; I'll wire them separately. I wouldn't want all of the lights to go out if something tripped a breaker for one of the wall plugs. This seems to be a senseless rule change that only causes inconvenience and doesn't really add any safety to the system, since the only point that both circuits are present is in the first junction box. I don't see how this would be any less safe than if two separate circuits with two separate neutrals were run.
 
Note carefully that s handle tie does not create s common trip.
If one of the two circuits trip, the other will not.
However to reset that one breaker you may have to toggle both off and back on again.
And if you are working on one side of the circuit, you will have to turn off both.
 
jeff48356 said:
OK, thanks. I guess a MWBC is out then; I'll wire them separately. I wouldn't want all of the lights to go out if something tripped a breaker for one of the wall plugs. This seems to be a senseless rule change that only causes inconvenience and doesn't really add any safety to the system, since the only point that both circuits are present is in the first junction box. I don't see how this would be any less safe than if two separate circuits with two separate neutrals were run.
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If the light switch is on and someone comes in after you and turns off the breaker for the wall plugs and starts working on them, he can be zapped by 110V from the neutral if he disconnects it at that j-box or in the panel. Heck, he's even encouraged to keep the breaker and the switch on for the lights so he can see what he's doing. ;)

Is a handle tie safer than relying on you to group the wiring as an MBWC, plus relying on the guy who comes after to check for that and understand it? Arguably it is.
 
jeff48356 said:
OK, thanks. I guess a MWBC is out then; I'll wire them separately. I wouldn't want all of the lights to go out if something tripped a breaker for one of the wall plugs. This seems to be a senseless rule change that only causes inconvenience and doesn't really add any safety to the system, since the only point that both circuits are present is in the first junction box. I don't see how this would be any less safe than if two separate circuits with two separate neutrals were run.
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It is. Too many people are freaked out by them though. I don't see any chance of going back to the way it was. I actually believe MWBC will either be regulated out of existence or designed out because of the handle tie rule, which is really too bad.
 
If this is a detached garage he can only run one circuit out to it.

So it has to be one MWBC or one 2-wire circuit.

The only other option is a feeder to a panel in the garage.
 
For the life of me I cannot understand why you would run 14 gauge instead of 12 for one conductor. How much money do you think you are saving? I would rather have a larger wire for the "in case" situation. Anyway, to each his own...:D
 
Dennis Alwon said:
For the life of me I cannot understand why you would run 14 gauge instead of 12 for one conductor. How much money do you think you are saving? I would rather have a larger wire for the "in case" situation. Anyway, to each his own...:D
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I was wondering that as well but I have to assume that in the garage itself he used, or wants to use 14/2 NM for the lighting. Likely out of habit more than anything else.
 
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