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12-3 with ground everywhere in this house

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  • 12-3 with ground everywhere in this house

    I am a licensed electrician and I am about to finish a job where the house was wired 20 plus years ago with all 12-3 w/g for the 120 volt circuits. In essence, they have two circuits in one wire but share the neutral and ground.

    I do not have my final inspection yet or a meter so I cannot test it but I think I could have a problem with my GFI circuits in the kitchen/garage/outside/bath. I never ran into this before but two circuits sharing the neutral may trip my GFI's or am I brain dead from overworking in the Katrina flood zone.

    I searched my online NEC and could not find anything on sharing a neutral.

    Have you guys ever ran into this?

  • #2
    You will run into problems if you try to feed other plugs off the GFCI using 12/3. GFCI's only look for a current differential downstream which means out the face of the plug or on its load terminals. the GFCI does not look for current differential on its line connections. In your case you will need to use a GFCI plug in all locations that require GFCI protection, I don't think you will be able to protect other plugs off the load terminals of 1 GFCI or us a GFCI breaker to protect a line because as soon as the other hot wire in the 12/3 uses the neutral to return power the GFCI will see it as a load imbalance and will trip


    • #3
      Everywhere everywhere, or just the home runs are 12-3?

      If they're running 12-3 everywhere, just do some selective ditching. Just because a conductor is there doesn't mean you have to use it.


      • #4

        as with single phase services (110-220v) two hots can share the same neutral and ground. you won't run into any problems as long as you use a separate gfci outlet at every location, and feed the gfci on the line side with a hot, neutral and use the ground on the green screw. you will not be able to downstream this way though. if you intend to use gfi breakers, you are probably out of luck. something along the way always seems trips the breakers.


        • #5
          also be aware that if this is getting inspected and must be up to the new code sharing a neutral will kick those arc fault breakers too and will be nearly impossible to troubleshoot good luck