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Thread: Sharing neutrals in lighting circuits

  1. #1
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    Sharing neutrals in lighting circuits

    Do to harmonics in lighting circuits feeding flourescent lighting, the code requires dedicated neutrals for each lighting circuit. I spoke with a company owner yesterday, and he said that with the new electronic ballasts, that we can go back to sharing neutrals for flourescent lighting as long as the lights have electronic ballasts. I disagree with him. What do you think?

  2. #2
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    The code never required dedicated neutrals for each lighting circuit, that is a design choice.

    You may end up counting the neutral as a current carrying conductor if the major portion of the load is non-linear.

  3. #3
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    Wavector, there are many rumors about harmonics, this is another one.

    Read 310.15)(B)(4) for what the NEC actually says concerning the neutral.

    Roger

  4. #4
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    I knew about the counting of the neutral as a current carrying conductor when connected to nonlinear loads.

    I thought that was the point of have a dedicated neutral for flourescent lighting, the fact that the resulting overcurrent was nonlinear. The fluctuation of harmonics overloaded shared neutrals in circuits that feed flourescent lighting and caused problems (i.e. heat overloaded neutrals due to nonlinear shaped overcurrent). I also thought that is why electrical manufacturers derated the neutral in some of their MC cable assemblies.

  5. #5
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    I don't believe anyone is saying that a dedicated neutral on fluorescent lighting is a bad idea, just not a code requirement.

    Mark

  6. #6
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    Still unsure of what the first poster means by a "dedicated" neutral. Does he want one per breaker or one per three breakers on all three phases? There's a difference.

    I generally require one neutral per "three phase grouping."

  7. #7
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    Quote Originally Posted by Ragin Cajun
    Still unsure of what the first poster means by a "dedicated" neutral. Does he want one per breaker or one per three breakers on all three phases? There's a difference.

    I generally require one neutral per "three phase grouping."
    A dedicated neutral, in this instance, would be one neutral dedicated to each circuit.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  8. #8
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    Quote Originally Posted by Ragin Cajun
    Still unsure of what the first poster means by a "dedicated" neutral. Does he want one per breaker or one per three breakers on all three phases? There's a difference.

    I generally require one neutral per "three phase grouping."
    Ragin Cajun-

    Be sure to check out 210.4 Multiwire Branch Circuits. Unless you generally specify 3-pole breakers, this wouldn't be a safe practice. When you break a neutral under load, part of it goes to line voltage. It's not fun working in a large industrial space where all the lights share one neutral circuit.

    JMHO.

  9. #9
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    Quote Originally Posted by DGrant041
    Quote Originally Posted by Ragin Cajun
    Still unsure of what the first poster means by a "dedicated" neutral. Does he want one per breaker or one per three breakers on all three phases? There's a difference.

    I generally require one neutral per "three phase grouping."
    Ragin Cajun-

    Be sure to check out 210.4 Multiwire Branch Circuits. Unless you generally specify 3-pole breakers, this wouldn't be a safe practice. When you break a neutral under load, part of it goes to line voltage. It's not fun working in a large industrial space where all the lights share one neutral circuit.

    JMHO.
    How does opening the nuetral have anything to do with what Mr. Cajun was saying? And why would you have to specify three pole breakers? (not a code requirement). You don't even need a handle tie across the phases, much less a common trip breaker.
    Ryan Jackson, Salt Lake City
    Inspector, Instructor

  10. #10
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    I would also ask what qualified individual would be opening any splice or lifting any wire from a terminal without knowing it was underload?


    Roger

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