Does a MWBC have to be contained in one cable?

J

JoeNorm

Senior Member
Location
WA
The title asks the question. I have a single 12-2 running to an outbuilding. I want to make it a MWBC and put on a 2 pole breaker. Can I run another 12-2 and use the two cables or do I have to abandon the first run and just use a 3 wire cable?
 
It must be within the same cable.

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, 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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As long as not in a ferrous wiring method, I believe this is allowed. Still a good idea to keep all conductors of the circuit in close proximity to one another.

See 300.3(B)3
 
You can split a circuit between multiple cables if you are using non-ferrous wiring methods. You want to minimize the 'loop area' between cables, and you want any penetration of metallic boxes to go through the same holes.

If these are cables buried outside, then they need to buried 'in close proximity in the same trench'; if you are digging up an old run of 12-2, it might be easier to simply install a new cable then to carefully dig the old one up.
 
JoeNorm said:
The title asks the question. I have a single 12-2 running to an outbuilding. I want to make it a MWBC and put on a 2 pole breaker. Can I run another 12-2 and use the two cables or do I have to abandon the first run and just use a 3 wire cable?
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Why not just keep the two neutrals separate? What are you trying t accomplish?
 
If you ran two different 12-2 cables from one DP breaker and kept everything separate on each leg, and had no 240 volt loads, is it even a MWBC?
 
JoeNorm said:
The title asks the question. I have a single 12-2 running to an outbuilding. I want to make it a MWBC and put on a 2 pole breaker. Can I run another 12-2 and use the two cables or do I have to abandon the first run and just use a 3 wire cable?
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IMHO... Running two separate 12/2 cables would put the neutrals in parallel. That would be a violation.
 
If you're going to run a new cable, use it for the MWBC and use the old wires for lighting or something.

While you're at it, the new cable could be larger for more ampacity.
 
Scenario
2 cables 12/2 going to outbuilding. In the panel of origin splice the 2 neutrals and add 1’ white 12 awg conductor to neutral bar.
Land each ungrounded conductor on a 2 pole breaker.
You have a mwbc supplying outbuilding.
Magnetic fields cancel each other out in each cable .
No different than 12/3 to j box and separating with 2x 12/2s
 
darekelec said:
Scenario
2 cables 12/2 going to outbuilding. In the panel of origin splice the 2 neutrals and add 1’ white 12 awg conductor to neutral bar.
Land each ungrounded conductor on a 2 pole breaker.
You have a mwbc supplying outbuilding.
Magnetic fields cancel each other out in each cable .
No different than 12/3 to j box and separating with 2x 12/2s
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What's happening with the two whites (which cannot be paralleled) in the outbuilding?
 
One goes left to supply lights with black conductor
One goes right with red conductor to supply receptacles.

No paralleling
2 pole switch is required as a disconnect nearest point of entry to outbuilding
 
Gotcha.

But, is it still a MWBC and not two circuits when without a shared neutral?

The 2p breakers alone don't assure that.
 
2 neutrals are connected to 1’ of white 12 awg conductor in the panel of origination. So the circuit starts as mwbc .
 
2- 12/2's don't make it a MWBC, and doing this as a feed to a separate structure might be an issue related to what sounds like an attempt to meet the allowance for 2 circuits run to a separate structure as long as it is a MWBC.
darekelec said:
2 neutrals are connected to 1’ of white 12 awg conductor in the panel of origination. So the circuit starts as mwbc .
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Really reaching, Splitting at the other structure is way different from splitting just past the breaker. All conductors of the MWBC shall be contained within the same cable

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.
Described use by OP doesn't fit any of the Special conditions listed in 225.30(A)

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.

300.3(B) Conductors of the Same Circuit. (MWBC as provided in 225.30)
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).
 
darekelec said:
2 neutrals are connected to 1’ of white 12 awg conductor in the panel of origination. So the circuit starts as mwbc .
Click to expand...
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.
 
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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Splicing on a short pigtail for the two neutrals makes it a 3-wire circuit instead of two 2-wire circuits. The 3-wire portion of the circuit meets the definition of a MWBC.
 
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