MWBC

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dvcraven052256

Member
Location
St. Louis, missouri
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Electrical Engineer
I was thinking of providing a neutral for each 120V branch circuit in lieu handle ties or 2-pole breakers. I am not sure which method best in terms of cost. Your thought are most welcome.

Thank you
 
Yes I understand the concept. But MWBR require to disconnect all ungrounded conductors via handle ties or 2-pole breaker. I was thinking if I ran a neutral for each circuit then it would no long be a MWBC.
 
dvcraven052256 said:
Yes I understand the concept. But MWBR require to disconnect all ungrounded conductors via handle ties or 2-pole breaker. I was thinking if I ran a neutral for each circuit then it would no long be a MWBC.
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Correct, it would not be a MWBC.
 
Not many places (in residential anyways) to use a MWBC unless you buy a 2-pole AFCI or use two AFCI, with a handle tie, that don't have the GF portion in them.
 
infinity said:
Handle ties are cheap. Running a 4th conductor will be more expensive.
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Most times; but there are breakers brands that I can't get a handle tie for even though the mfg. list them, or they don't even list them for a particular breaker brand.
 
MWBCs also are more efficient in terms of waste energy, especially for heavier duty circuits like kitchens or single phase air conditioners.
 
Little Bill said:
Not many places (in residential anyways) to use a MWBC unless you buy a 2-pole AFCI or use two AFCI, with a handle tie, that don't have the GF portion in them.
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I believe using two single-pole breakers with a handle tie is no longer allowed for MWBC, even non-AFCI. If that's correct, I doubt it would be for AFCI.
 
So much easier to have a combo GFCI/AFCI breaker than dealing with MWBC. There was a time it made sense, but no longer.
 
RustyShackleford said:
I believe using two single-pole breakers with a handle tie is no longer allowed for MWBC, even non-AFCI. If that's correct, I doubt it would be for AFCI.
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Not true at all. When AFCI first came out they had a form of GFCI in them. That prevented you from using two single pole AFCI because it would detect neutral current going where it shouldn't. Now, some mfg have taken the GFCI portion out of the AFCI and you can use two single pole AFCI. As for the regular single pole breaker, a handle tie has always been allowed, and still is.
 
tthh said:
So much easier to have a combo GFCI/AFCI breaker than dealing with MWBC. There was a time it made sense, but no longer.
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Occupants wont identify xFCI breaker test-buttons that typically fail while energized.

xFCI breakers are worse for occupants, accessing remote panels to reset them. Tight tolerances of modern panel doors often break finger nails, requiring tools to open.

While AFCI breakers may be unavoidable in new construction, MWBC wont stop GFCI’s at first openings, near point of use.

The problem is panel flippers, who persist avoiding inside wiring, MWBC’s, and remain ignorant of occupant needs.
 
There's another one odd issue with MWBCs. I've heard of people backfeeding their panel (through an interlocked generator inlet, of course) from a 120vac inverter, and tying the 120vac hot wire to both phases in the panel. Which can overload the shared neutral, since the hots aren't in opposite phase. Ditto if a homeowner who knows "enough to get in trouble" adds a circuit or something, and connects both hots to the same phase.
 
RustyShackleford said:
Ditto if a homeowner who knows "enough to get in trouble" adds a circuit or something, and connects both hots to the same phase.
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We can't protect against stupid!

A properly wired MWBC is not unsafe, it's actually a good thing. It's still allowed in the code, the only reason it's not used much in dwellings anymore is all the GFCI and AFCI requirements.

The code did dumb it down, requiring the handle ties on single pole breakers.
 
One often forgotten aspect of MWBCs is what happens if the neutral is broken/lost. The two hots become a 240V circuit with the load resistances dividing the 240V unevenly between them. Mike Holt has a video on this and on there he touts “Murphy’s Law of MWBCs”, claiming that when that happens, the highest voltage will go to the most expensive device in the circuit, every time. So if your MWBC is feeding a DW and Disp., and the neutral gets disconnected, it will ALWAYS fry the DW…
 
Jraef said:
One often forgotten aspect of MWBCs is what happens if the neutral is broken/lost. The two hots become a 240V circuit with the load resistances dividing the 240V unevenly between them. Mike Holt has a video on this and on there he touts “Murphy’s Law of MWBCs”, claiming that when that happens, the highest voltage will go to the most expensive device in the circuit, every time. So if your MWBC is feeding a DW and Disp., and the neutral gets disconnected, it will ALWAYS fry the DW…
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Not until you switch on the disposal! 😜
 
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