Definition mwbc

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Srv52761

Srv52761

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lowa
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Energy Manager
Once a mwbc, always a mwbc?... Or can I magically create another circuir?

Detached garage.
By code, you need a 20 amp circuit dedicated to convenience receptacle(s), and you need a circuit for lights and stuff.
You could have more, but you must have those two.

You may only run one 20 amp circuit , or you must run a sub pane!.
Which means you must run a sub panel or a mwbc (which counts as one).
You can run uf 12" underground if it is gfci protected.

So, if I run the 12-3 to a 4" box in the house, pigtail the neutral to two gfci receptacles, then run two 12-2 uf, one from each receptacle, from there underground in the same trench to the detached garage, is it now considered two circuits or still one mwbc? I am puzzled....
 
Good Question -- I would say it is a multiwire branch circuit to the jb and then 2 circuits to the garage. Why would anyone do that?
 
Srv52761 said:
So, if I run the 12-3 to a 4" box in the house, pigtail the neutral to two gfci receptacles, then run two 12-2 uf, one from each receptacle, from there underground in the same trench to the detached garage, is it now considered two circuits or still one mwbc? I am puzzled....
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That would be a violation since you're only allowed for the garage to be supplied by one circuit (or 2 or 3 circuits if a MWBC) and 2-12/2's is two circuits.
 
infinity said:
That would be a violation since you're only allowed for the garage to be supplied by one circuit (or 2 or 3 circuits if a MWBC) and 2-12/2's is two circuits.
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I think that was his question. Since it started as a multiwire branch circuit he was wondering if the circuit would stay a mwbc even though he changed the wiring method.
 
Dennis Alwon said:
I think that was his question. Since it started as a multiwire branch circuit he was wondering if the circuit would stay a mwbc even though he changed the wiring method.
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Yes, the supply to the garage is two separate circuits which is not permitted. The fact that it begins at the panel as a MWBC doesn't matter.
 
Agree with all of the above. It needs to arrive at the garage as a MWBC, with a single neutral.
 
Thank you. That means if going with a mwbc you must go at least 18” in conduit or 24” direct buried, or 6” rigid.
You can’t do the 12” because you can’t gfci protect in the house.
Is this correct?
 
Good question on the burial depth. The 12" minimum is for branch circuits of 120 volts. Is this case the MWBC is considered one circuit and it would be 240 volts.
 
Srv52761 said:
But it has to be gfci protected to be 12" .
Does a 240 gfci breaker protect both legs of a mwbc equally?
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Yes.

All three circuit conductors go through the sense coil. Any leakage from any of the conductors gets detected unless it somehow returns on one of the other conductors, just like a 2 wire GFCI.

Jon
 
I have never seen that as an issue but I can see why the burial depth would or could be an issue. As I said in my first post- good question.
 
Dennis Alwon said:
I have never seen that as an issue but I can see why the burial depth would or could be an issue. As I said in my first post- good question.
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In this case the 240 volt circuit would negate your ability to use the reduced burial depth of 12" with GFCI portection.
 
So, the 12” depth direct burial is not allowed to a garage, as a garage needs two circuits. A garage would need to be 6” in rigid, 18” in conduit, or 24” direct burial.
You could do 12” direct burial for a shed or workshop, though, just not a mwbc.

P.S. how many get frustrated with auto correct when typing electrical acronyms? It obviously doesn’t like terms like gfci, mwbc, afci, egc, and such.
I spent an inordinate amount of time switching all those terms back to what I had.
Rant over
 
Srv52761 said:
So, the 12” depth direct burial is not allowed to a garage, as a garage needs two circuits. A garage would need to be 6” in rigid, 18” in conduit, or 24” direct burial.
You could do 12” direct burial for a shed or workshop, though, just not a mwbc.
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That's correct. You cannot use the 12" burial depth for a 240 volt MWBC.
 
Could you run a GFCI protected 120V feeder at the shallow depth, and then have a panel at the garage to give the required circuits?

Jon
 
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