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Thread: Shared Neutrals

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    Shared Neutrals

    What section in the code will give me direction on shared neutrals? I'm doing some work in an older building and they have as many as 11 20A/1P breakers on a single neutral conductor, on a 3PH panel. They are trying to aviod rewiring the entire building, but some of this work clearly needs to be redone.

    Is there a section I can reference when stating my case.

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    For code cycles before 2011, article section 225.7 was the closest you will find, it specifically allowed it for outside lighting but, with this being the case and nothing else where prohibitting it it seemed it could be used for other applications as well.

    Roger
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    I stumbled upon 215.4, I think I found what I'm looking for.

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    Quote Originally Posted by rjken1969 View Post
    I stumbled upon 215.4, I think I found what I'm looking for.
    So is this a feeder or a branch circuit. Certainly you can feed a panel with one neutral however there are limits to branch circuits.

    For the 2011 look at 200.4 but these installs were done before then as Roger was mentioning.
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    Quote Originally Posted by roger View Post
    For code cycles before 2011, article section 225.7 was the closest you will find, it specifically allowed it for outside lighting but, with this being the case and nothing else where prohibitting it it seemed it could be used for other applications as well.

    Roger
    I agree. Nothing specifically prohibited a common neutral for MWBC's prior to 2011 NEC.
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    Quote Originally Posted by rjken1969 View Post
    I stumbled upon 215.4, I think I found what I'm looking for.
    Keep in mind 215 is for feeders not branch circuits.

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    Quote Originally Posted by rjken1969 View Post
    What section in the code will give me direction on shared neutrals? I'm doing some work in an older building and they have as many as 11 20A/1P breakers on a single neutral conductor, on a 3PH panel.
    Can you explain how this didn't burn up over time? Was the single neutral oversized?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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    Quote Originally Posted by infinity View Post
    Can you explain how this didn't burn up over time? Was the single neutral oversized?
    Depending on the loads:

    Loads were lucky enough to be reasonably balanced and no significant overheating occurred?

    Circuits were lightly loaded, or maybe little demand most of the time.

    If there would have been 12 circuits instead of 11 and all were identical load, the neutral current should be zero. Assuming they are all in demand simultaneously - which could possibly happen if this is lighting circuits.

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    Quote Originally Posted by kwired View Post
    Depending on the loads:

    Loads were lucky enough to be reasonably balanced and no significant overheating occurred?

    Circuits were lightly loaded, or maybe little demand most of the time.

    If there would have been 12 circuits instead of 11 and all were identical load, the neutral current should be zero. Assuming they are all in demand simultaneously - which could possibly happen if this is lighting circuits.
    Until somebody turns on exactly the right (wrong) combination of 6 of the 12 lighting circuits.

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    I guess that would depend on what type of lighting loads. Do you think that if they all were fluorescent lighting loads the neutral would have burnt up a long time due to triplen harmonics?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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