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  1. #1
    Tim Barnett's Avatar
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    Default Good sub panel explanation link

    Hey gents...had a curious client today wanting more info on why sub panels need to have separated neutrals and grounds and isolated neutrals. Not argumentative, just curious. I tried to explain in basic terms, but it didn't really soak in I don't think.
    Does anybody have a good article or web link they'd be willing to share here? They are not a real mechanical/handy person, so simple is good.

    Thanks in advance!

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  2. #2
    Robert Meier's Avatar
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    Default Re: Good sub panel explanation link

    Here's a graphic from Mike Holt that depicts objectionable current which might be helpful.




  3. #3
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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Robert Meier View Post
    Here's a graphic from Mike Holt that depicts objectionable current which might be helpful.
    Ooh, ooh - Let me say it before JP does.

    I don't see where it says "sub" panel anywhere on that graphic?!

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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    Default Re: Good sub panel explanation link

    The graphic doesn't address the safety issue that arises if the neutral connection is interrupted when there is no separate ground connection and the neutral is used for grounding purposes. This leads to voltage being present on the "ground" connections.

    Occam's eraser: The philosophical principle that even the simplest solution is bound to have something wrong with it.

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    Default Re: Good sub panel explanation link

    Quote Originally Posted by John Arnold View Post
    Ooh, ooh - Let me say it before JP does.

    I don't see where it says "sub" panel anywhere on that graphic?!
    'Cause it does not say "sub panel" anywhere in the NEC.


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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Derek Guridi View Post
    'Cause it does not say "sub panel" anywhere in the NEC.

    But it Does in the NEC Handbook !


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    Default Re: Good sub panel explanation link

    Quote Originally Posted by ken horak View Post
    But it Does in the NEC Handbook !
    Oh, no! Not a controversy about the term "sub panel"!? Say it ain't so!

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
    www.ArnoldHomeInspections.com

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    Default Re: Good sub panel explanation link

    Quote Originally Posted by ken horak View Post
    But it Does in the NEC Handbook !
    Let me know if you ever find a jurisdiction that adopts the NECHB.


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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Derek Guridi View Post
    Let me know if you ever find a jurisdiction that adopts the NECHB.
    The NEC Handbook IS The NEC. It just has commentary that is intended to assist users in understanding and applying the NEC

    LOOK on the inside front cover of the handbook.
    From the 2011:
    This twelfth edition of the National Electrical Code Handbook contains the complete mandatory text of the 2011 edition of NFPA 70, National Electrical Code, and the nonmandatory annex material. Commentary is provided in this handbook to explain the reasoning behind the Codes requirement.

    I challenge you to find a jurisdiction the adopts the nEC and does not have at least one NEC Handbook in their office

    Besides the conversation has nothing to do with adoption of a code


  10. #10
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    Default Re: Good sub panel explanation link

    Quote Originally Posted by ken horak View Post
    The NEC Handbook IS The NEC.
    Incorrect.

    The NEC Handbook CONTAINS the NEC ... *as well as* ...

    ... commentary that is intended to assist users in understanding and applying the NEC
    LOOK on the inside front cover of the handbook.
    From the 2011:
    This twelfth edition of the National Electrical Code Handbook contains the complete mandatory text of the 2011 edition of NFPA 70, National Electrical Code, and the nonmandatory annex material.
    See? That is what I said "contains", not "is" the NEC.

    Jerry Peck, Construction / Litigation Consultant
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  11. #11
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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Jerry Peck View Post
    Incorrect.

    The NEC Handbook CONTAINS the NEC ... *as well as* ...



    See? That is what I said "contains", not "is" the NEC.

    Ok -I stand corrected. the Handbook contains the NEC and commentary.


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    Default Re: Good sub panel explanation link

    Yes, as the graphic illustrates, it is possible to find current present on the conduit between panels. However, this current path will require an electrical "open" somewhere along the conduit and box junctions. If the conduit is properly grounded, as it is intended to be, and connections are of good integrity, then the current path (conduit) will be at ground potential.

    The possibility of electrical opens at the conduit connectors, is why the code was changed to not use the conduit as an earth ground. For that reason (possibility of opens), the NEC considers the conduit to be just that, a container, not a conductor.


  13. #13
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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Jimmy Roberts View Post
    Yes, as the graphic illustrates, it is possible to find current present on the conduit between panels. However, this current path will require an electrical "open" somewhere along the conduit and box junctions. If the conduit is properly grounded, as it is intended to be, and connections are of good integrity, then the current path (conduit) will be at ground potential.

    The possibility of electrical opens at the conduit connectors, is why the code was changed to not use the conduit as an earth ground. For that reason (possibility of opens), the NEC considers the conduit to be just that, a container, not a conductor.
    Hi Jim,

    I do not beleive that this is accurate. When did the NEC change and no longer allows metallic conduit not to be used as an EGC?

    Corey


  14. #14
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    Default Re: Good sub panel explanation link

    Quote Originally Posted by Jimmy Roberts View Post
    Yes, as the graphic illustrates, it is possible to find current present on the conduit between panels. However, this current path will require an electrical "open" somewhere along the conduit and box junctions. If the conduit is properly grounded, as it is intended to be, and connections are of good integrity, then the current path (conduit) will be at ground potential.

    The possibility of electrical opens at the conduit connectors, is why the code was changed to not use the conduit as an earth ground. For that reason (possibility of opens), the NEC considers the conduit to be just that, a container, not a conductor.
    Quote Originally Posted by Corey Friedman View Post
    Hi Jim,

    I do not beleive that this is accurate. When did the NEC change and no longer allows metallic conduit not to be used as an EGC?

    Corey
    Corey,

    I suspect that Jimmy is referring to conduit is not a conductor in relation to the drawing, which is showing the conduit, the enclosure, etc., as neutral conductors, and conduit is not approved for use as a neutral conductor, only for unintended current such as would be on an equipment grounding conductor.

    Anyway, that's how I took it - maybe I interpreted what he said incorrectly?

    Jerry Peck, Construction / Litigation Consultant
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  15. #15
    Robert Meier's Avatar
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    Default Re: Good sub panel explanation link

    In the graphic the conduit is in parallel with the neutral conductor so depending on the resistance of the conduit versus the resistance of the neutral conductor the conduit would carry some of the neutral current. If the conduit and neutral resistances were the same then the conduit would carry 1/2 of the neutral current.


  16. #16
    Doctor Haus's Avatar
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    Default Re: Good sub panel explanation link

    Some utility company's websites have good information big on graphics and simple non-technical but accurate explanations for the utilities basic residential customers. Some companies put out some information for students for school projects or 'fun for kids' also.


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