Legal? Safe?

I don't see a problem with this as long as neutral uses a gauge high enough to support both branches. Am I missing something here?


Here's an illustration:

enter image description here

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    What's pictured in the question is a classic MWBC or multi-wire branch circuit. The requirements are that it is fed from a double pole breaker, so that circuit 1 and 2 are on opposite phases and have common disconnect. – Tyson Apr 21 '17 at 1:20
  • @Tyson The circuit breakers on the diagram are not feeding opposite phases, but the same one. – andrey Apr 21 '17 at 2:37
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    If you're in the US, and the circuit breakers are stacked on top of each other like in the diagram, then the breakers should be on separate legs of the service. Multi-wire branch-circuits do not require a common trip device (double pole breaker), but in current code they do require handle ties between the breakers (common disconnect). In days past, these handle ties were not always used (maybe not required, I'm not sure when that code was added). – Tester101 Apr 21 '17 at 11:00
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    Check the voltage between the two ungrounded (hot) conductors, to determine if they're on the same leg. If you get 0 volts, then they're on the same leg. 240 volts, and they're on separate legs. – Tester101 Apr 21 '17 at 11:01
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    @isherwood it's possible either the work is in conduit, or they had a non-mandatory reason to significantly derate the hots, owing to for instance long distance. For instance they had a 400' run of 10/3 feeding two 15A circuits. They might be intentionally on the same pole because of a rule saying you can't have more than 120V between conductors. Improbable but it could happen. – Harper - Reinstate Monica Apr 21 '17 at 17:55

To those who know all about MWBCs, this particular case is bizarre, interesting, and falls in the "you learn something new every day" category. Please read carefully and not leap to conclusions. Thank you.

OP's comment The circuit breakers on the diagram are not feeding opposite phases, but the same one. means this is a shared neutral which is not a MWBC.

The neutral is carrying twice the return current!

And this (a non-MWBC) is an area often messed up by electricians, so it is generally frowned upon quite a lot.

There are old circuits out there like this. This is is "leeeeeegal" per NEC... look at 215.4(A), or 225.7(B). However these apply to feeders, and outdoor lighting, respectively. You must upsize the neutral to handle the current for all the circuits which are on the same phase.

However, as a practical matter, any authority able to do so is going to nix it. Prepare to have your installation fail inspection, and your protestations of being allowed in Code fall on deaf ears.

Any installation like this has some rules.

Pigtail neutrals

You must pigtail all the neutrals, except where a pair of wires branches off from the group and doesn't come back. That is because removing a device on one circuit (e.g. a receptacle) mustn't break the neutral path for any other circuit.

Common maintenance shut-off

You will also need a common maintenance shut-off, powering down all circuits which share the neutral. If your circuits come out of a fuse panel, a shut-off switch will suffice. Common trip is not required; i.e. all the circuits don't need to shut off together if one overloads and blows a fuse or breaker. However with breakers, those are the shut-off, so they must be handle-tied to shut off together. Rather than chase around looking for handle-ties, most people just use a 2-pole or 3-pole breaker. They will have the side-effect of common trip; again that's not required.

GFCI and AFCI will be impossible

Increasingly, GFCI or AFCI protection is being required - most rooms in a house must be one or the other. GFCIs are simply not built for shared-neutral circuits.

It is possible to use a GFCI or AFCI on a multi-wire branch circuit, which is a special case of a shared-neutral circuit, in which each circuit is intentionally on a different phase. In this case the neutral carries only differential current, and can be the same size as the other conductors.

Generally a multi-wire branch circuit is the only kind of shared-neutral circuit an authority will allow you to have.

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    Wow harper, I have never seen an electrician mess up a simple multi wire branch circuit! MWBC are common and not frowned upon and do not need the neutral to be upsized. Handle ties started being required years ago this prevents the breakers from being moved to the same phase later and de energises both circuits in case maintenance is being done and the neutral is opened a hazard is not present. Where in the code is it require neutrals to be pigtailed on a MWBC. – Ed Beal Apr 21 '17 at 13:35
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    I pig tail all my junctions so I have never been called on a MWBC but did find 300.13.b requires MWBC the grounded conductor to be pig tailed so you are correct there. – Ed Beal Apr 21 '17 at 13:51
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    @EdBeal I've seen more MWBCs messed up by bona-fide electricians than actually done properly, but that's mostly one guy. Neither OP nor I are referring to MWBC. MWBC is a special case of shared neutral, we are talking about the general case of shared neutrals which are not necessarily on opposing poles. Yeah, that's a thing. – Harper - Reinstate Monica Apr 21 '17 at 16:21
  • @Harper Why would a common maintenance shut-off be necessary for branches sharing a neutral? – andrey Apr 21 '17 at 17:24
  • @andreyg A maintainer normally uses the shutoff switch to de-energize the circuit for maintenance or repair. Maintainers have gotten killed when they did not realize they had only shut off part of the circuit (ora ircuit defect cross-energized things). EdBeal is correct that almost any listed shut-off mechanism for circuit breakers is going to force you to put the legs of the circuit on opposite poles, forcing you into an MWBC comfiguration. That is by design to prevent people from accidentally creating what you're trying to create on purpose – Harper - Reinstate Monica Apr 21 '17 at 17:41

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