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Thread: How does the neutral wire prevent MWBCs from operating at 240v?

  1. #1
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    How does the neutral wire prevent MWBCs from operating at 240v?

    I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.

  2. #2
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    It wouldn't be 240 across both (actually it wouldn't be 240 on either but that is nit picking) it would be high on one side and low on the other, see the illustrations below.



    Here is a true neutral



    And a neutral carrying the unbalanced current.



    Roger
    Moderator

  3. #3
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    Transformers that supply 120/240v circuits have 240v windings with a center tap. When the neutral is connected, it connects to the center tap of the transformer, creating two 120v circuits and the multi-wire operates as a 120/240v circuit. Without that connection, no 120v circuit is created by the transformer. So only 240 volts can be supplied by the transformer.

  4. #4
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    Quote Originally Posted by jselesk2 View Post
    I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.
    If i'm reading you right it's because a 120V load uses one side of the cycle , not both

    ~RJ~

  5. #5
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    The resistance on the neutral is much much much lower then the resistance in either 120 volt load. if the resistance starts rising, like due to a poor connection, or becomes infinite, due to a cut or lifted wire, then those 120 volt loads are effectively wired in series instead of parallel, and will see 240 volts across the pair of appliances....
    Electricians do it until it Hertz!

  6. #6
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    you still have 240v across the two high legs. the neutral has no effect on that relationship.
    Bob

  7. #7
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    Quote Originally Posted by jselesk2 View Post
    I am aware that if the neutral wire becomes open on an MWBC, this will result in a 240v circuit across the two phases of the circuit. I am curious as to why this does not happen under normal operation of a MWBC in which the two phases share a neutral wire. Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.
    I know what you are asking, I asked the same question when I first joined here. I have a little better understanding now than I did then, but for me it's still just easier to accept that it works because that's the way it works.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  8. #8
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    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  9. #9
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    Here is nice video that may help you better understand.
    https://www.youtube.com/watch?v=eVamt9IdQd8

  10. #10
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    Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

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