GFCI Single pole breaker feeding sub panel with multi-wire branch circuit

Discussion in 'Electrical Forum discussion & Blog' started by HiQ, Dec 23, 2020.

  1. HiQ

    HiQ Member

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    Jan 4, 2017
    Location:
    Manitoba
    Okay so I got a weird one. Plugged in the wife's block heater on her car and the breaker blew. Went downstairs and realized what is here cannot be right. Looks like a single pole 30A GFCI breaker feeding a 4 circuit sub panel. On that panel is a multi-wire branch circuit feeding the garage outlets.

    The main GFCI breaker trips the second I plug the block heater in with anything else on that MWBC.

    From my understanding, the sub panel would have to be feed by a double pole GFCI breaker for this to work, right? What's the easiest way to fix this at the panel or at the outlets without needing to run new wire? Would the double pole GFCI be a good idea or should I be replacing that with a standard breaker and going to GFCI outlets? Was the setup as it is now ever legal in Canada? The house is from 1979/80, but this was probably done after that.

    Thanks!

    IMG_2680.jpg IMG_2681.JPG IMG_2682.JPG
     
  2. Reach4

    Reach4 Well-Known Member

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    IL
    Pull the cover off of the subpanel. If you measure across the two incoming hots, do you get zero volts, or 240?
     
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  4. HiQ

    HiQ Member

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    Well it's what I thought. Only one hot that he bridged across both bars. Only 120V single phase fed.
     
  5. HiQ

    HiQ Member

    Joined:
    Jan 4, 2017
    Location:
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    So a double pole 30a GFCI breaker is like $317 at HD or $167 at Amazon. That would probably instantly fix my issue if I properly fed this sub panel with two phases and a double pole gfci breaker.

    The cheaper (maybe?) option, but more work is to use a $27 30a double pole non-gfci break to fed this sub panel and then replace the outside outlets with their own gfcis. At least then I can reset the outlets if they blow outside instead of running back inside. I think there are only two, but I'd have to check. Might end up being a lot of work replacing all the outside outlets so $167 for the breaker might be a bargain in the long run.

    Am I way off here or is that the best solution? Thanks!
     
  6. Reach4

    Reach4 Well-Known Member

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    Location:
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    Do you have 4 wires run to the subpanel (hot, hot, neutral ground)? You would need that for a 2-pole breaker to the panel.
     
  7. wwhitney

    wwhitney Well-Known Member

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    Location:
    Berkeley, CA
    2 options:

    (1) if 30A @ 120V is sufficient for the loads in the little 4 space panel, and you can get rid of the MWBC (assuming it's a single 14/3 cable, rewire one side with a separate 14/2 cable, abandon one ungrounded conductor in the 14/3), then just do that. You need to ensure that the conductor size feeding the little 4 space panel (#10 Cu?) isn't too small for the panel lugs, and that the panel lugs have only one conductor per lug (i.e. the one hot is connected to two pigtails that go to the two lugs). Marking the panel "120V only" would be wise.

    (2) Refeed your panel with 10/3 (or larger) from a double pole breaker, and add GFCI protection where required (could be a 15A single pole GFCI and a 15A double pole GFCI)

    Cheers, Wayne
     
  8. HiQ

    HiQ Member

    Joined:
    Jan 4, 2017
    Location:
    Manitoba
    Option 3: Refeed the sub panel with 10/3 from a double pole GFCI breaker and I'm good?

    Option 4: Refeed the sub panel with 10/3 form a double pole standard breaker and then deal with GFCI in the sub panel with different breakers or on the receptacles themselves?

    I'm starting to think 3 is the quickest/easiest fix for now. If it was summer I could do Option 4 and save a bit of money, but it's -30C or colder outside so I'm not rewiring outdoor receptacles.

    So option 3 should work?
     
  9. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
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    Yes, either of those should work, they are versions of my last suggestion, which left open where to put the GFCI. A couple caveats:

    - If it's quite far between the two panels, the GFCIs are better off in the small panel, due to the effects of capacitive coupling between the conductors. [In the extreme case, the GFCI can trip without any current imbalance in the loads.]

    - A related issue is that using a single GFCI breaker for the feeder will mean that any trip will take out both circuits in the small panel.

    So I favor option 4, seems like not really any more work than option 3. Have you confirmed that the small panel lugs are rated for #10?

    Cheers, Wayne
     
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  10. HiQ

    HiQ Member

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    Ahh good call. I kinda forgot about the whole GFCI trip taking out the whole sub panel. It'll be all outside plugs so I guess that's not the end of the world as they're all basically unused.

    The panels are almost touching. The 10/3 run between them is <5'. It is actually currently wired with 10/3, but they just abandoned one of the hot conductors in both panels.

    If this was anytime other than the coldest time of the year I would definitely do option 4. Turns out I'm soft/lazy and will be doing option 3 for now. I can always redo it later if GFCI tripping starts bugging me.

    Thanks!
     
  11. Reach4

    Reach4 Well-Known Member

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    Location:
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    If the power to the outlets drop, figuring out where the ground fault is would be harder with option 3.

    Remember you can use the load terminals on a GFCI outlet to power downstream outlets AFTER any shared neutral. If shared neutral, do not pass power through the GFCI to another outlet.
     
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  12. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Just to be clear, the only extra work for Option 4 is replacing two more breakers. I guess it's also a bit more expensive, as a QO230GFI is probably less than QO230 plus QO115GFI plus QO215GFI.

    Cheers, Wayne
     
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  13. HiQ

    HiQ Member

    Joined:
    Jan 4, 2017
    Location:
    Manitoba
    Wow... My brain today. Not sure why I had it in my head that I had to go swap receptacles to GFCIs. I definitely could have just ran the standard breaker to feed and swapped out the two other sub panel breakers with the GFCI units as you said. Durrr. Definitely time for Christmas break. Thanks guys!
     
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