Does a MWBC have to be contained in one cable?

This is like arguing that all two-wire circuits are MWBCs because the service has a shared neutral.
 
Part between structures I would call a part of mwbc because of the 1’ grounded conductor pigtail in panel.

Etruscan shrew is the smallest mammal and I’m sure lions, hippos and other mammals have difficulty accepting it but the smallest mammal still meets the definition of a mammal.
 
darekelec said:
Part between structures I would call a part of mwbc because of the 1’ grounded conductor pigtail in panel.

Etruscan shrew is the smallest mammal and I’m sure lions, hippos and other mammals have difficulty accepting it but the smallest mammal still meets the definition of a mammal.
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The multi-wire circuit ends when the circuit become (2) 2-wire circuits. The separate structure would need to be supplied by 1 grounded conductor and 2 un-grounded conductors.
 
curt swartz said:
The multi-wire circuit ends when the circuit become (2) 2-wire circuits. The separate structure would need to be supplied by 1 grounded conductor and 2 un-grounded conductors.
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I agree. It doesn't matter if the splice is 6" from the neutral bar or 60' away in a junction box. The portion of the circuit that is 3-wires is the MWBC part.
 
don_resqcapt19 said:
How is that any different from the two neutrals being landed on the neutral bar. I do not see that as any part of a MWBC.
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Since you have at least some portion that has a common conductor, it at very least will kick in the handle tie rule? Doesn't matter if it only goes 1 foot or 50 feet before splitting into two two wire components.

Add: maybe should read on before posting🥵
 
kwired said:
Since you have at least some portion that has a common conductor, it at very least will kick in the handle tie rule? Doesn't matter if it only goes 1 foot or 50 feet before splitting into two two wire components.

Add: maybe should read on before posting🥵
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225.30 Number of Supplies.
A building or other structure that is served by a branch circuit or feeder on the load side of a service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (F). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.
(A)-(F) not applicable here.

The building shall be supplied by Only One Branch circuit. Supply Can be a MWBC. Once you get to the other building it can be divided, don't see the wording as saying you can divide before you get to the building. The "feeder or branch circuit" is not the breaker but the wiring between the buildings.

See definition of a MWBC:
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.

I see the term "a neutral conductor", That to me says it is a singular neutral. Combine that with 225.30 I read that as only one neutral between the two structures.
 
Fred B said:
225.30 Number of Supplies.
A building or other structure that is served by a branch circuit or feeder on the load side of a service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (F). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.
(A)-(F) not applicable here.

The building shall be supplied by Only One Branch circuit. Supply Can be a MWBC. Once you get to the other building it can be divided, don't see the wording as saying you can divide before you get to the building. The "feeder or branch circuit" is not the breaker but the wiring between the buildings.

See definition of a MWBC:
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.

I see the term "a neutral conductor", That to me says it is a singular neutral. Combine that with 225.30 I read that as only one neutral between the two structures.
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Correct. Now run two 12-2 UF cables to that building, use two blacks as ungrounded conductors, and one of the whites as the grounded conductor and keep the cables buried in close proximity to one another on the way there and you still have an appropriate MWBC, and a spare white conductor.
 
kwired said:
Correct. Now run two 12-2 UF cables to that building, use two blacks as ungrounded conductors, and one of the whites as the grounded conductor and keep the cables buried in close proximity to one another on the way there and you still have an appropriate MWBC, and a spare white conductor.
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No, the neutral must be run within the same cable assembly or conduit with the ungrounded conductors.

300.3(B) Conductors of the Same Circuit.
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, conduit body, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

All conductors of the same circuit or MWBC and the grounded conductor shall be contained within the same cable. Can't parallel small conductors (Less than a 1/0) 310.10(G)

If need to dig up to bury a second cable (non compliant) anyway just put in the 3 wire. It would then without question meet the code for a MWBC.
 
Fred B said:
No, the neutral must be run within the same cable assembly or conduit with the ungrounded conductors.

300.3(B) Conductors of the Same Circuit.
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, conduit body, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).
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OK, but 300.3(B)(3) applies to non-ferrous wiring method. Last I checked, UF cable has no iron in it. So just like with NM cable, it is allowed to separate the circuit conductors across multiple UF cables.

Cheers, Wayne
 
Fred B said:
No, the neutral must be run within the same cable assembly or conduit with the ungrounded conductors.

300.3(B) Conductors of the Same Circuit.
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, conduit body, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

All conductors of the same circuit or MWBC and the grounded conductor shall be contained within the same cable. Can't parallel small conductors (Less than a 1/0) 310.10(G)

If need to dig up to bury a second cable (non compliant) anyway just put in the 3 wire. It would then without question meet the code for a MWBC.
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Right; however there is an exception to the above requirement for non-ferrous wiring methods. Two plastic UF cables run side by side in the same trench, and through the same holes in metal enclosures, IMHO meet the exception.
 
winnie said:
Right; however there is an exception to the above requirement for non-ferrous wiring methods.
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AKA 300.3(B)(3).

Of course, just because it's allowed doesn't make it a good idea; in the OP's scenario, it would still be better to install a new 12/3 and abandon the 12/2 than install a new 12/2 and utilize the new 12/2.

Cheers, Wayne
 
wwhitney said:
OK, but 300.3(B)(3) applies to non-ferrous wiring method. Last I checked, UF cable has no iron in it. So just like with NM cable, it is allowed to separate the circuit conductors across multiple UF cables.

Cheers, Wayne
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Correct. For years I have run two 14-2 cables between switch boxes that needed power in both boxes plus a set of three way travelers between them. I tend to keep them together throughout the run as it will reduce EMF's but NEC doesn't even require that. Or two 14-2 to a bathroom exhaust unit that had three loads like fan, light, night light.

14-2 plus a 14-3 has one more conductor than needed, and they take up space, especially in some exhaust fan connection boxes. Using a white as a switched leg rather than the supply to a switch is possibly questionable, but any real electrician should still understand what they have there. I don't wire things to make them simpler for the DYI guys that may follow me someday.
 
I'd sorta been ignoring this thread because my initial intuition was that reply #2 was correct. But looking again I think that reply is incomplete, and multiple cables/conduits are allowed. And BTW, ferrous vs. non ferrous is almost entirely irrelevant.

300.3 uses the term 'circuit' which the NEC does not define. If you want to split an MWBC into two parts by branching out a separate neutral with each hot, you can do that, if you like, including in ferrous wiring methods. It's done all the time and considered 300.3 compliant as long as you run a neutral to the load with each hot. It's fine because the two-wire sections can each be a dictionary-definition 'circuit' and 300.3 does not use a term like 'branch circuit' which would sweep up all conductors in the MWBC.

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.

So, if you start a branch circuit as an MWBC, say at one building, and then branch out the hots, each with their own neutral, into two separate cables or raceways (ferrous or non-ferrous), and run those separate cables or raceways to a separate building, you are still running a single branch circuit to the outbuilding as clearly allowed in 225.30. As long as you don't join the neutrals at the outbuilding (illegal parallel conductors) or fail to run a neutral with each hot, there's no violation.

In light of posts #18 and #38 it's moot for the OP. But let's say two 12/2s - or two ferrous conduits with two conductors each - had already been run, and buried, there's an argument to make that it's all compliant. Perhaps not entirely moot for the OP, actually, since by my argument they can pigtail the 12/2 and 12/3 all into a single MWBC at or near the panel it's supplied from, and still be compliant.
 
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