Does a MWBC have to be contained in one cable?

jaggedben said:
However the NEC does define the term 'Branch Circuit' clearly, and clearly defines an MWBC as a branch circuit (not only in the name but in the definition). And a branch circuit is the entire circuit from the OCPD to the loads, and it is one branch circuit, not multiple ones, because of how its beginning and end(s) are stipulated. So for any branch circuit that is partly an MWBC, the whole branch circuit is an MWBC.
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Will disagree here. The definition is:

2023 NEC said:
Branch Circuit, Multiwire. (Multiwire Branch Circuit) A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a neutral conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral conductor of the system. (CMP—2)
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If you want to say that because the start of the definition is "a branch circuit" the definition looks at the branch circuit in its entirety, rather that at any given point in the run of conductors, then the example at hand (a circuit that starts out with a neutral pigtail in the panel that connects to two separate neutrals) does not consist of "a neutral conductor" plus ungrounded conductors. It has two neutral conductors. So under your logic, none of the branch circuit would be an MWBC.

However, the more usual interpretation is that the notion of MWBC is not unitary for the whole circuit, and that part of a brach circuit may be an MWBC, while another part may not be. Certainly that is the only way for which it makes sense to apply a rule like 300.13(B). And 210.4(A) speaks to this as well ("A multiwire circuit shall be permitted to be considered as multiple circuits.")

Cheers, Wayne
 
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wwhitney said:
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If you want to say that because the start of the definition is "a branch circuit" the definition looks at the branch circuit in its entirety, rather that at any given point in the run of conductors, then the example at hand (a circuit that starts out with a neutral pigtail in the panel that connects to two separate neutrals) does not consist of "a neutral conductor" plus ungrounded conductors. It has two neutral conductors. So under your logic, none of the branch circuit would be an MWBC.
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I could probably be persuaded that if the pigtail is entirely within the panel where the circuit(s?) originate(s) then that 'doesn't count' towards making the two cables an MWBC. But if the shared neutral travels as far as through an offset nipple to an adjacent enclosure, I'm gonna stand by my reasoning. Because once you've gone that far then requiring any further distance is arbitrary. The OP wasn't entirely precise about where two 12/2s might be joined up so I think the point remains apropos.

As far as the definition of an MWBC applying to the entire branch circuit, I stand by that, especially for the purposes of 225.30.

However, the more usual interpretation is that the notion of MWBC is not unitary for the whole circuit, ...
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I'm not persuaded that's the 'usual interpretation' because so far as I've directly experienced it hasn't practically matter when it comes to complying with any of the requirements for MWBCs. I'd wager most people have never really thought about it and there's no real consensus. The OP's original scenario, especially supposing the 'pigtail' exits the panel, is an interesting test case.
 
jaggedben said:
I'm not persuaded that's the 'usual interpretation' because so far as I've directly experienced it hasn't practically matter when it comes to complying with any of the requirements for MWBCs.
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Again, it obviously matters for 300.13(B). Such a requirement only makes sense where the neutral is associated with more than one ungrounded conductor. So it does not make sense to extend it to a 2-wire circuit supplied by an MWBC.

I agree that the definition of MWBC would be clearer if it started off "A portion of a branch circuit . . ." Will stand by my "usual interpretation" comment, as your post is the first time in many years reading this forum that I've heard of that interpretation. Versus countless times someone has said something equivalent to "once you split it into separate 2-wire circuits, it's no longer an MWBC."

Cheers, Wayne
 
wwhitney said:
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I agree that the definition of MWBC would be clearer if it started off "A portion of a branch circuit . . ." Will stand by my "usual interpretation" comment, as your post is the first time in many years reading this forum that I've heard of that interpretation. Versus countless times someone has said something equivalent to "once you split it into separate 2-wire circuits, it's no longer an MWBC." ....
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Seems to me that I've seen more than one thread with the discussion "is an MWBC one circuit or more than one?", and then it's pointed out that 210.4 says it can be more than one and 225.30 says it can be one. So I, likewise, stand by my position on whether there is a usual interpretation. In fact I see no fundamental problem with treating the MWBC as 'unitary' for the purposes of 225.30 and not for 300.13(B).
 
jaggedben said:
Seems to me that I've seen more than one thread with the discussion "is an MWBC one circuit or more than one?", and then it's pointed out that 210.4 says it can be more than one and 225.30 says it can be one. So I, likewise, stand by my position on whether there is a usual interpretation. In fact I see no fundamental problem with treating the MWBC as 'unitary' for the purposes of 225.30 and not for 300.13(B).
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Yes MWBC is one circuit for some aspects but also is multiple circuits for other aspects.

Three wire MWBC to a kitchen can certainly fulfil the requirement for two SABC's yet it is also a single branch circuit in other aspects.
 
kwired said:
Yes MWBC is one circuit for some aspects but also is multiple circuits for other aspects.

Three wire MWBC to a kitchen can certainly fulfil the requirement for two SABC's yet it is also a single branch circuit in other aspects.
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Agreed, but only the portion between the panel and the dividing point is the MWBC; everything beyond that point is individual circuits.
 
LarryFine said:
Agreed, but only the portion between the panel and the dividing point is the MWBC; everything beyond that point is individual circuits.
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As I've pointed out, the first part of that is not how branch circuits and MWBCs are defined in article 100. An MWBC is a branch circuit, and a branch circuit is the whole thing.

The part after the semi-colon is fine using the dictionary definition of circuit which need not correspond to a code defined type of circuit.
 
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