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  1. #1
    Join Date
    Jun 2004
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    Oakhurst, CA (near Yosemite Natl Park)
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    Default GFCI wiring on shared circuits?

    Hi all:

    I'm finishing up some wiring done by a previous electrician who used 12/3 all over the place as a home run for two separate circuits. Under this configuration the two circuits share a neutral, and it's no problem as long as you put them on opposite buses in the panel.

    Here's the problem I'm having: what if one or both of those circuits must be GFCI protected? I can't do that with a shared neutral because a GFCI monitors the neutral current. If I wire a GFCI on one of these circuits normally, it will trip as soon as the other circuit sharing the neutral is used. I don't think I can use GFCI breakers either. Any ideas?

    Aaron

  2. #2
    Join Date
    Jun 2004
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    Meadow Vista, CA
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    Default Re: GFCI wiring on shared circuits?

    Aaron, try putting a GFCI on one leg of the 12-3 and then test the outlets downstream on both the black and the red leg. The currrent on the other leg shouldn't trip the GFCI 'cause that current doesn't flow through the GFCI.

  3. #3
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    Default Re: GFCI wiring on shared circuits?

    Mark:

    If I'm not mistaken, a GFCI monitors the hot and neutral current to be sure they are within a certain tolerance (4-6mA I think). When the red leg is used, (GFCI is wired on black leg), current flows back on the neutral side of the GFCI with no current flowing on the hot side. GFCI trips.

    I connected neutral from panel to "LINE" side and neutral to next plugs to "LOAD" side. I think any other wiring approach will compromise the GFCI's effectiveness, although I am going to try a couple other ideas on Monday.

  4. #4
    Join Date
    Jun 2004
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    Default Re: GFCI wiring on shared circuits?

    Iwould use a GFCI at each wet location and connect both incoming and outgoing wires to the line side. That way the appliance plugged into the receptacle is protected.

  5. #5
    Join Date
    Jun 2004
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    581

    Default Re: GFCI wiring on shared circuits?

    You can use a 120/240 GFCI breaker.

  6. #6
    Join Date
    Jan 2006
    Location
    Southern NH
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    56

    Default Re: GFCI wiring on shared circuits?

    The neutral can only be "shared" to the point you split the two circuits. The neutral can run to the GFCI receptacle - and connect to the LINE side of it, and then the remaining receptacles "downstream", that you wish to protect GFCI, must be connected from the LOAD side of that first receptacle.

    Brett

  7. #7
    Join Date
    Feb 2005
    Location
    North Carolina
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    2,240

    Default Re: GFCI wiring on shared circuits?

    Quote Originally Posted by Mark G
    Iwould use a GFCI at each wet location and connect both incoming and outgoing wires to the line side. That way the appliance plugged into the receptacle is protected.

    I agree with Mark with the exception of the woud "WET"

    Use receptacles and line in and Line out at each place that "REQUIRES" GFCI protection.
    Seeking to be the best and the safest in the electrical trade.

  8. #8
    Join Date
    Jun 2004
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    Default Re: GFCI wiring on shared circuits?

    Absolutely. Sometimes the words get in my way. One thing (of many) I'm learning on this forum is to use the words as defined in the code book. A receptacle is an outlet but an outlet is not necessarily a receptacle. It goes better when we all speak the same language, eh? (Oh, no; am I starting the bilingual argument again?)

  9. #9
    Join Date
    Jun 2004
    Location
    Oakhurst, CA (near Yosemite Natl Park)
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    Default Re: GFCI wiring on shared circuits?

    Mike:

    This might be one to work in your basement. I'd really like to get to the bottom of it and understand what's going on. Bear with me.

    Scenario: Wire GFCI by connecting black to LINE hot and white to LINE neutral, with the red serving another circuit. If you pigtail the neutral on the LINE side, you will be OK, but you will need to install a GFCI receptacle in every outlet where GFCI protection is required. (The LOAD side therefore becomes as useless as wings on a pig.) If instead you connect the neutral heading downstream to the LOAD side, the GFCI will trip the instant current from the other (red) circuit is used, because it sees the neutral current go by with no current on the black to balance it. Right or wrong?

    If this analysis is correct, it provides basis for my preference of running two 12/2 home runs when GFCI requirements are involved. This is faster and cheaper than installing a $13 GFCI in every outlet.

    -Aaron II

    Btw this is an old thread that Brett bumped to the top.
    www.telianconstruction.com
    Criticism comes easier than craftsmanship. - Zeuxis, 400 B.C.

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