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Old 01-06-2012, 07:56 AM   #1
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Wiring outlets in a MWBC


I plan to run outlets around my garage using 2 circuits on a MWBC to save on wire. I want 2-gang boxes around the garage, each with 2 receptacles. Each receptacle would be on its own side of the MWBC.

Is this an acceptable method to wire them, or must I pigtail all the neutral wires so everything still has a neutral if I disconnect an outlet?

Thanks.
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Old 01-06-2012, 08:09 AM   #2
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Wiring outlets in a MWBC


Quote:
Originally Posted by secutanudu View Post
...to save on wire.
You won't save that much wire.

In the even that you do any of this...
join your feed through's under wirenuts then pigtail to the devices.

hth

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Old 01-06-2012, 08:14 AM   #3
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Wiring outlets in a MWBC


DONT wire it like that, you can potentially have smoke coming out of stuff plugged into the outlets downstream if you service the outlets improperly.
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Old 01-06-2012, 08:15 AM   #4
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Wiring outlets in a MWBC


Pigtailing the neutrals is a requirement.
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Old 01-06-2012, 08:44 AM   #5
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Wiring outlets in a MWBC


I thought it might be, but why exactly? I understand if your neutral gets disconnected at the panel, your devices turn to about 240v, but I don't see why that would happen in this setup.

I am not arguing, I will pigtail all neutrals, just trying to understand why.

Also, I don't need to pigtail the hots, right?
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Old 01-06-2012, 08:53 AM   #6
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Wiring outlets in a MWBC


Same thing can happen if you disconnect the neutral form the black circuit and the red circuit is still energized. Code required neutral to be pigtailed on MWBC.
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Old 01-06-2012, 08:53 AM   #7
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Wiring outlets in a MWBC


Quote:
Originally Posted by secutanudu View Post
I understand if your neutral gets disconnected at the panel, your devices turn to about 240v, but I don't see why that would happen in this setup.
If you're not doing this with conduit...
How about buying that double neutral romex?
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Old 01-06-2012, 08:56 AM   #8
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Wiring outlets in a MWBC


Garages require GFCI protection, so 12/3 can only be used to the first set of receptacles which would be GFCI. If you use 12/3 everywhere, you will need GFCIs at every receptacle location or a double pole GFCI breaker. You then have negated any cost savings in wire.
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Old 01-06-2012, 08:58 AM   #9
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Wiring outlets in a MWBC


Quote:
Originally Posted by secutanudu
I thought it might be, but why exactly? I understand if your neutral gets disconnected at the panel, your devices turn to about 240v, but I don't see why that would happen in this setup.

I am not arguing, I will pigtail all neutrals, just trying to understand why.

Also, I don't need to pigtail the hots, right?
Well if something is plugged in downstream and the neutral is broken at some point it can put 120v on the neutral up until the place where its broken. X by 2 and you have 240 running on that neutral crossing phases.
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Old 01-06-2012, 09:01 AM   #10
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Wiring outlets in a MWBC


I forgot about the fact that I'd need GFCIs in the garage. A GFCI double-pole breaker would certainly be more expensive than getting an extra run of 12/2.

Double-neutral romex? Never even heard of it.
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Old 01-06-2012, 09:07 AM   #11
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Wiring outlets in a MWBC


There are drawings out there in how to do this properly. The mikeholt forums have good info, there are also previos posts regarding mwbc on here, just do a search.

Problem with mwbc and gfci's, is that you can ed up with a nuisance trip.
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Old 01-06-2012, 09:23 AM   #12
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Wiring outlets in a MWBC


Quote:
Originally Posted by secutanudu View Post
I thought it might be, but why exactly? I understand if your neutral gets disconnected at the panel, your devices turn to about 240v, but I don't see why that would happen in this setup.

I am not arguing, I will pigtail all neutrals, just trying to understand why.

Also, I don't need to pigtail the hots, right?

The reason the neutral connection can’t be dependent on the device is if one of the ‘hot’ legs is de-energized, but the other isn’t, the one that’s still ‘live’ is still using the neutral (current is flowing if a load is on), and is a shock hazard. I’ve been bitten before by one of these (I know…stupid accident). To me the shock was more intense than getting shocked by the line side. This shouldn’t come into play anymore with the requirement that all MWBC’s have a disconnect that de-energizes all ungrounded conductors of the circuit, but they kept this requirement in the code for safety.

No…you don’t have to pigtail the ‘hots’ if you don’t want to.

As mentioned, you have the problem of GFI-ing the recepts when using 12/3.

Double-neutral romex= 12-2-2 w/g…a cable made with 2 colors for ‘hots’ (usually black and red) and 2 whites (one has a stripe to be able to match it to the corresponding ‘hot’)
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Old 01-06-2012, 09:44 AM   #13
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Wiring outlets in a MWBC


So if I use 12-2-2, can I just wire it as 2 independent circuits and not treat it as a MWBC? Maybe this is cheaper than 2 runs of 12/2 w/g because I am running one less ground conductor.

Or maybe it's more expensive because it's non-standard.

In any case, if each circuit has its own neutral back to the panel, GFCIs should operate as normal, right? Would a setup like this still require a double-pole breaker (or handle ties)?
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Old 01-06-2012, 09:59 AM   #14
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Wiring outlets in a MWBC


I don’t know if 12-2-2 is any cheaper or easily available….I think no on both accounts.

When using 12-2-2, GFCI’s are set-up and operate as normal, as the cable contains 2 independent circuits. 2-pole breakers are not required, because they are 2 independent circuits, though you can if you want to.
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Old 01-06-2012, 10:13 AM   #15
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Wiring outlets in a MWBC


You will discover 12-2-2 is hard to find and much more expensive than two runs of 12/2. I really don't think you are going to save on wire by using the MWBC approach, and IMHO, unless you have a really difficult pull between the first outlet box and the panel, I wouldn't even consider it.

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