Understanding Multi-wire Branch Circuits

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Dansos

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I am wondering what the DISADVANTAGES are when running a mulitwire branch circuits in a single phase application.
I have ran 12/3 and 14/3 many times to cover 2 circuits when the distances are very long. The NEC clearly states as long as the wires are "grouped" when entering panel and the 2 circuits are on different phases and can be simultaneously disconnected (Double Pole Breaker), this is a code compliant method of wiring. I am more curious as to the theory behind why it is somewhat frowned upon......

If the NEC is followed in terms of above, what is the potential problem of doing this if any?

I am not too good understanding electrical theory
 
At one time, those two circuits making up a MWBC weren't required to even by side-by-side in the panel. They could be circuits #2 and 15. Take that MWBC to a duplex split receptacle using two single-pole breakers.

You plug in your tester, and turn off one of the breakers. Your tester goes dark. You then pull out the receptacle and bam..... the other circuit nails you.
 
As the others have said there are some issues that need to be considered with MWBC's but over all they are a very efficient wiring method in a number of ways.

Roger
 
Thanks for the replies. So i gather from this if they are installed CORRECTLY, according the NEC, no problems would arise. Thank you for settling this for me.
 
The problem arises that, not so much in residential, but in commercial, if you run a MWBC and need to shut off one circuit to work on it, you have to turn off both circuits. That doesn't work too well in a busy office environment. Shut both cubicles off to work on one.
The problem with residential is with AFCI circuits you have to buy an expensive 2-pole breaker.
 
LarryFine said:
In my opinion, when wired in compliance, there are advantages and no disadvantages.

GFCI and AFCI requirements are not included in this discussion.
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This, but both parts. When the AFCI requirements first hit for bedrooms here, I was called to fix a house that a guy was building for himself. He had MWBC'd a lot of things, including the bedrooms. Well, then the inspector says he has to AFCI all of those. I don't think the 2-pole AFCI's were available back then. I had a helluva time tracing and isolating bedrooms to get off of being MWBC'd.

I think they are a brilliant way to go, and I will still do it whenever I can.

I do, however, agree with the relatively new Code that puts them on a 2-pole breaker. I remember when I first learned how to do a MWBC, and I always put them on 2-pole breakers. I think I was shown that it should be done this way during my apprenticeship. Then, I was doing one on an addition, and remarked trying to figure out how to move breakers so I could get the 2-pole in there. The other guy says I didn't need to do that, but just get them on different poles. He insisted it was to Code. I could NOT believe it. I looked it up. Then I thought "You've got to be kidding me." All the crap in the Code, and THAT isn't?
 
LarryFine said:
In my opinion, when wired in compliance, there are advantages and no disadvantages.

GFCI and AFCI requirements are not included in this discussion.
Click to expand...

roger said:
As the others have said there are some issues that need to be considered with MWBC's but over all they are a very efficient wiring method in a number of ways.

Roger
Click to expand...
I agree.
 
Little Bill said:
The problem arises that, not so much in residential, but in commercial, if you run a MWBC and need to shut off one circuit to work on it, you have to turn off both circuits. That doesn't work too well in a busy office environment. Shut both cubicles off to work on one.
The problem with residential is with AFCI circuits you have to buy an expensive 2-pole breaker.
Click to expand...

Just think what a 277/480V MWBC in an office where you have a 3-pole CB. When you shut that off, or even worse, it trips, an entire floor of lighting can be killed
 
ptonsparky said:
It is a safety issue. Turning off just one breaker of the MWBC allows current flow on the neutral from the other breaker(s) or loads of that circuit. You may think the circuit is safe to work on, but it is not.
Click to expand...

480sparky said:
At one time, those two circuits making up a MWBC weren't required to even by side-by-side in the panel. They could be circuits #2 and 15. Take that MWBC to a duplex split receptacle using two single-pole breakers.

You plug in your tester, and turn off one of the breakers. Your tester goes dark. You then pull out the receptacle and bam..... the other circuit nails you.
Click to expand...
Which is why you better know what you are doing before you touch any wiring. It's not a safety issue, it's a competence issue.
 
Lets summarize. Disadvantages of MWBC's:

1. Requires more than one circuit to be turned off to work on
2. Problematic where an AFCI or GFCI breaker is required or desired (double poles of both are available however)
3. Breakers are much more expensive, particularly in 3 phase application where 3 pole handle ties may not be available for some brands, thus requiring a 3 pole breaker
4. Possibility of shock from neutral while brother in law working on it with one leg still on.
5. Less flexibility if say one circuit was do be changed to L-L instead of L-N.
6. Its not any cheaper with certain cable wiring methods like MC (ever priced 12-4 MC?).

BTW I am not anti MWBC at all...
 
ActionDave said:
Which is why you better know what you are doing before you touch any wiring. It's not a safety issue, it's a competence issue.
Click to expand...

I have to agree. They're really dumbing down the rules to allow more dingbats to work on wiring. When you pull the receptacle out and see two hot wires, your experience should tell you something.

Saving Darwin Award winners from themselves.
 
Just sayin
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