Multi-wire Circuits

Northernboy

GSE, you need to go pack to pounding nails or whatever you really do for a living. I have never pulled a neutral to a 3 phase motor. If your talking about a 3 phase panel, fine, but that's a feeder not a branch circuit. A branch circuit is from the final OCPD to the outlet. I think we all know that the original question was referring to 2 hots and a neutral and whether that was single phase or not. You go pull a neutral to the motor and show everyone how smart you are.

Speedy Petey

Easy Northern. I think this is getting out of hand simply due to different terms being used.

A service with three hots and a grounded conductor is a "3-phase" service. Depending on which type of three phase service it is there is either two or three transformers fed from two or three primaries.

A service with two hots and a grounded conductor is a single-phase service. This uses ONE transformer fed from ONE primary.

You ABSOLUTELY CAN have a 3-phase MWBC. WHO said anything about a motor??? You mean you never worked in a commercial setting where you fed three lighting circuits with three hots and one neutral??? Be it 277v or 120v.

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Magnettica

LOL!

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Magnettica

Correct, a 120/240 3-wire service uses 1 phase of a 3-phase distrubution system.

Yes, you can have a phase-to-phase short because one of the phases from the original 3-phase system has been transformed and the secondary coil has a center tap that provides a common point for both 120 volt phases.

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Northernboy

Nice to finally have a voice of reason out there Petey. Yes, I agree with you on the lights. Never thought of that one.

Magnettica

I kind of disagree on the lights.

Won't harmonic currents from electronic ballasts overheat the neutral?

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MechanicalDVR

Well if you can have a phase to phase short you have two phases, so it really isn't single phase then is it?

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Northernboy

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 shall originate from the same panelboard or similar distribution equipment.

FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents.

Take it for what it's worth

Speedy Petey

An oversized neutral is sometimes used or spec'd, but it is not code required nor always necessary.

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Speedy Petey

Again, semantics.

A question to anyone who has every done commercial troubleshooting work:

You have a three-phase motor. You lose one leg and it is running on just two legs (or phases) and tripping the starter.
What is this scenario referred to as?

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Magnettica

SINGLE PHASING

(as in 2 poles, A & B)

The motor will heat up because one less winding is being used thus tripping the heater in the MS.

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Speedy Petey

EXACTLY!
It is commonly referred to as "single phasing", running on two "legs".

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mickeyco

I'm expecting crap but I'm saying it anyway. I'm familiar with the "single phasing" term, happened at a company I worked at, they had to have a bunch of specialty motors re-wound. However if you only lost one leg you'd be dual-phasing or 2 phasing or whatever you want to call, you'd have to lose 2 legs to really be "single phasing".

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Magnettica

And if I could add.... the motor starter heater trips because amperage increases on the loss of one phase, or single-phasing.

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Northernboy

If you lost 2 legs you would have no current flow at all, so you wouldn't even be single phasing. With 3 "legs" you have 3 "phases" and 3 current paths (circuits). One circuit would be between A and B, the second circuit would be between B and C, the third circuit would be between C and A.
If you now lost the C leg, you lose 2 phases. You lose B and C, and you lose C and A. The only complete circuit you have left is A and B. That would be 1 circuit, 1 phase, thus the term "single-phasing".

Speedy Petey

Exactly.

My point was not that this is a legal and technical term. It is a generic slang term for running a 3-phase motor on only two "phases", which would be the same as running it on a single phase service.

What Northern said is true. With no neutral it is impossible to literally single phase (leg, whatever) a 3-phase motor. You need at least two for it to do anything.

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fridaymean

I am glad someone brought this thread back to reality.

MechanicalDVR

Lighten up I was just busting.

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Mark Twenhafel

Thanks to everyone. Lot's more info than I had bargained for. NorthernBoy, I liked the drawing. If you would post one up for delta circuits, too, that would be great.

I had never considered a 12/3 pulled to a half-hot receptacle to be a MWBC. I can see the benefit of having both ungrounded conductors to a half-hot being on tied breakers, to prevent half of the duplex from being hot after only one breaker is thrown. But, do most people here agree that such a situation is a MWBC? To me, it is two separate branch circuits. Whether or not it is determines whether or not a tied breaker is required, I think.

After all the back-and-forth about legs vs. phases, there seem to be only two or three cases of MWBC's:
(1) 3-phase service, with an equipment ground being pulled. An industrial motor would be an example.
(2) 240 volt service, i.e., two legs off the same phase, with a neutral and a ground being pulled. An example of this would be a new-style 4-terminal electric range receptacle.
(3) 3-conductor romex being pulled to a half-hot receptacle (I'm still not convinced that this is a MWBC. It still seems like two separate circuits, to me.)

Any thoughts? And, thanks for all that came before.
Mark

Celtic

The NEC gives this definition of a MWBC:

Believe it or not!

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