MWBC ( families )

[zooby]

scenario-- blk, red, blu & wh and another red( or any phase) & wh in a conduit off to wherever........ What actually occurs if the two neutrals are mixed up ? anything?
AND I know its different now with "common ties" but theoretically could two or even three families share a neutral ? a huge one of course. Thanks for any insight--- Doug

 

[infinity]

So one is a full set of Blk, Red, Blu, Wht and the other hot has a different neutral? If the neutrals go directly back to the neutral bar nothing will happen. Now if two hots of the same phase share a neutral then you'll have a problem.

 

[Ohm2]

scenario-- blk, red, blu & wh and another red( or any phase) & wh in a conduit off to wherever........ What actually occurs if the two neutrals are mixed up ? anything?
AND I know it’s different now with "common ties" but theoretically could two or even three families share a neutral ? a huge one of course. Thanks for any insight--- Doug

To name a couple things that could happen if you mix neutrals: 1. Imbalance and overloading 2. Incorrect Voltage
Not sure I understand the three families comment. But yes a neutral can be shared between phase conductors. They call this a multi wire branch circuit. But there are rules: common tie handle ( which you mentioned), balanced loads and etc.. I’ll let others respond.

 

[zooby]

To name a couple things that could happen if you mix neutrals: 1. Imbalance and overloading 2. Incorrect Voltage
Not sure I understand the three families comment. But yes a neutral can be shared between phase conductors. They call this a multi wire branch circuit. But there are rules: common tie handle ( which you mentioned), balanced loads and etc.. I’ll let others respond.

i guess the "family" would be the three phases and a neut. or a "full boat" slang that is likely regional i suppose. Thanks

 

[jaggedben]

What will actually happen is neutral current will take all paths back to the source, i.e. will split among the various neutrals.

If each circuits wires are properly grouped in the panel but cross-wired out at boxes or devices, someone working in the panel could be duped into thinking that a particular neutral is de-energized when they turn off a circuit, but it won't be. This is all the more so when there's an MWBC.

Also any GFCI circuit breakers will trip.

Also if the neutrals are of different ampacities then possibly the smaller one could get overloaded.

 

[jaggedben]

Btw there is a weird permission in the code for a certain number of feeders to share a common neutral, which I've never seen used. But there's no such permission for branch circuits.

 

[winnie]

A system where multiple hots possibly on the same phase share a suitably sized neutral is called a 'common neutral' branch circuit or feeder.

Common neutral branch circuits used to be explicitly described in the code section for outside branch lighting circuits. See, for example, 225.7(B) in the 2002 code. At the time, common neutral circuits were not prohibited for ordinary branch circuits, and this lead to long threads about them being legal. Some people would say 'code is permissive, so since they are not prohibited they are allowed', others would say 'code explicitly allows them in one place and not in the other, so by implication they are not allowed'. AFAIK they are explicitly prohibited now.

IMHO a common neutral circuit with a suitably sized neutral (say 2 12ga 'full boats' on 20A breakers sharing a common 10ga neutral) would work just fine, but is no longer permitted and is not code legal.

-Jonathan

 

[LarryFine]

What the OP asked about is perfectly fine. The two neutrals are interchangeable, as long as they travel the same pathway and terminate the same place. Two lines on the same phase must not share one neutral.

(Yes, I mean grounded conductor.)

 

[zooby]

So one is a full set of Blk, Red, Blu, Wht and the other hot has a different neutral? If the neutrals go directly back to the neutral bar nothing will happen. Now if two hots of the same phase share a neutral then you'll have a problem.

If each circuits wires are properly grouped in the panel but cross-wired out at boxes or devices, someone working in the panel could be duped into thinking that a particular neutral is de-energized when they turn off a circuit, but it won't be. This is all the more so when there's an MWBC.

I will try not to bore you all but..... I was asked to try and relocate recept circuits (9) into a gen backed panel. Took time to keep the phasing the same , coming from the gen panel. The numbering would be different but I would be able to use common ties. FYI this is in our Vet Hospital so complete access and shutting things off is tough. Got my new circuits to the panel (yes splices in the panel---all the circs I was working with leave out the bottom to slab and up at various locations in block walls) Unplugged EVERYTHING in these areas...Labs, Treatment, Radiology... so I could try to confirm neutrals for the ones that are not gfi's. Did my hook ups and there it was...the dreaded odd voltage issues. My time was running out so put it back like was. Much as anything it was a blow to the ego. Depending on hard ceiling hatch access I may still be able to give them most of what they want. I do question that all the make ups stayed true on the original install ( like jaggedben mentioned) but anyway, thats my story here.

 

[don_resqcapt19]

Btw there is a weird permission in the code for a certain number of feeders to share a common neutral, which I've never seen used. But there's no such permission for branch circuits.

There was one very specific common neutral permission prior to the 2023 code. 225.7(B).
However prior to the 2011 code, there was no rule that actually prohibited the use of "common neutrals". There were just two specific permissions to use a common neutral, and many read that as a prohibition on using them for other applications, but a permission to do something does not prohibit you from doing something else and that is way 200.4 was added to the code.