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  1. #1
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    Default MWBC Three circuit breakers?

    Here is a new one on me. Two separate three wire circuits share one breaker. Other than a safety issue with the three breakers not being tied together, I don't see an issue; am I missing something?

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  2. #2
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    Default Re: MWBC Three circuit breakers?

    Break, Break!!!! Radio check Ya'll run out of ink arguing with Lisa?

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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Vern Heiler View Post
    Break, Break!!!! Radio check Ya'll run out of ink arguing with Lisa?
    Vern I can't make out the pic.
    How did you see 3? Do you mean a red, a black and the white from one conductor are each on an adjacent breaker?

    The two that are on the same bus will not be going to the same fixture. There is no potential between them. So one of those two goes off to another fixture or it should not be there. The other one could be operating a 240 volt device with the third wire.

    John Kogel, RHI, BC HI Lic #47455
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by John Kogel View Post
    Vern I can't make out the pic.
    How did you see 3? Do you mean a red, a black and the white from one conductor are each on an adjacent breaker?

    The two that are on the same bus will not be going to the same fixture. There is no potential between them. So one of those two goes off to another fixture or it should not be there. The other one could be operating a 240 volt device with the third wire.
    I've never seen this done before and it is hard to describe. One set of 3 wires leaves the panel at the top, the other set of three wires leave the panel at the bottom. Three circuit breakers in a row have the center breaker supplying one leg of both the top and bottom set of wires. As far as loading I don't see a problem but need another set of eyes.

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  5. #5
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    Default Re: MWBC Three circuit breakers?

    Your drawing shows 3 breakers and 2 loads, and, not, those are not MWBC.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    Your drawing shows 3 breakers and 2 loads, and, not, those are not MWBC.
    Drawing is correct; what is it (they) if not MWBC's?

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  7. #7
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Vern Heiler View Post
    Drawing is correct; what is it (they) if not MWBC's?
    Wrong.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    Wrong.
    It has apparently worked for some years, how do I make a case that it is wrong?

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  9. #9
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    Default Re: MWBC Three circuit breakers?

    Come on Jerry, I gotta bag this thing up and send it. You got anything?

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  10. #10
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Vern Heiler View Post
    It has apparently worked for some years, how do I make a case that it is wrong?
    Electric chairs worked well for years too ... then they didn't.

    Okay, not a good example ...

    Jerry Peck, Construction / Litigation Consultant
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  11. #11
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    Electric chairs worked well for years too ... then they didn't.

    Okay, not a good example ...
    Seriously, if the shared circuit breaker is in the center of the three breakers, I don't see a problem with over loading of any wires. I know this is not the MWBC we see all the time, but why wouldn't they have called it a three wire circuit if something like this was not anticipated?l

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  12. #12
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    Default Re: MWBC Three circuit breakers?

    MWBC - see drawing

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    Jerry Peck, Construction / Litigation Consultant
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    MWBC - see drawing
    Is that your drawing? That was terrible. Is supplying one leg of two different 3 wire circuits any different than one leg supplying multiple 2 wire circuits?

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    I know it feels wrong, but the technician in me can't find a reason it shouldn't be done.

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  14. #14
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    Default Re: MWBC Three circuit breakers?

    Vern,

    Your N connections are at the center of the loads with 240 volt connection across each of the loads.

    If you disregard the N connections then there are 2 - 240 volt loads sharing a common connection, which could overload that common connection and wire.

    Jerry Peck, Construction / Litigation Consultant
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  15. #15
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    Vern,

    Your N connections are at the center of the loads with 240 volt connection across each of the loads.

    If you disregard the N connections then there are 2 - 240 volt loads sharing a common connection, which could overload that common connection and wire.
    Drawing is correct. You can re-draw with load split as is found with dishwasher/garbage disposer in MWBC all the time. On kindle so typing is slow

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  16. #16
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Vern Heiler View Post
    Drawing is correct. You can re-draw with load split as is found with dishwasher/garbage disposer in MWBC all the time. On kindle so typing is slow
    Then what you have is still ... Wrong!

    2 MWBC requires 4 breakers, not 3.

    It may be working, but you've seen the discussions here about wiring up MWBC off the same phase leg (which means they are not MWBC) and the neutral current doubles?

    That same thing is happening on that hot which is common to both.

    Doesn't matter if it is a neutral or a hot, put twice as much current on it and it heats up ... when wires heat up they have a tendency to start fires - that's what is wrong with it.

    Jerry Peck, Construction / Litigation Consultant
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  17. #17
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jerry Peck View Post
    Then what you have is still ... Wrong!

    2 MWBC requires 4 breakers, not 3.

    It may be working, but you've seen the discussions here about wiring up MWBC off the same phase leg (which means they are not MWBC) and the neutral current doubles?

    That same thing is happening on that hot which is common to both.

    Doesn't matter if it is a neutral or a hot, put twice as much current on it and it heats up ... when wires heat up they have a tendency to start fires - that's what is wrong with it.
    This is not the same condition as discussed in past threads. The common hot is protected from over current by the breaker; in past discussions the neutral was passing current from two breakers which added together will over heat the conductor.

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  18. #18
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    Default Re: MWBC Three circuit breakers?

    Even on a larger monitor I can't see enough detail, but get the impression that 2 b/r/w cables were just spliced in the panel and placed on one half of two tandems on opposite legs of the panel. The red just look like they were spliced to blue leads at least once. I can't see where the other blue comes from or the blacks associated with the reds.

    All answers based on unamended National Electrical codes.

  19. #19
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    Default Re: MWBC Three circuit breakers?

    Quote Originally Posted by Jim Port View Post
    Even on a larger monitor I can't see enough detail, but get the impression that 2 b/r/w cables were just spliced in the panel and placed on one half of two tandems on opposite legs of the panel. The red just look like they were spliced to blue leads at least once. I can't see where the other blue comes from or the blacks associated with the reds.
    Two reds are spliced together along with a blue and are powered by the center breaker. Black from the top wire is spliced to a blue which is powered by the bottom breaker. Black from the bottom wire is powered by the top breaker.

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