Help! Wiring question. GFCI keeps tripping!

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  #1  
Old 12-06-10, 05:55 PM
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Help! Wiring question. GFCI keeps tripping!


I had two outlets on the kitchen counter. I had run 12/3 cable with the intent of installing a separate circuit for an UTC microwave in the future.

The time has come, and I busted my stones putting this thing up by myself. But that's besides the point. It's finally up! I ran the wiring and installed an outlet in my cabinet, and it tested fine on my GFCI circuit tester. However, when I plugged the MW in, instead of hearing a satisfying "BEEP" or something, the GFCI tripped immediately.

I tried it again, and it happened again. I used a heavy-duty extension to temporarily plug it in to another GFCI circuit, and the display lights up fine. I then reset the original GFCI, plugged in my drill, and as soon as I pulled the trigger, POP, there it goes again.

Here's how the wiring went:

Outlet Box #1: GFCI -- Live white and black to the LINE (at least, I think, and now I'm wondering as I type this if I might have it backward... but anyway...). I have another 12/3 cable going to Outlet #2. Red wires are pigtailed (MW circuit).

Outlet #2: Black and white from #1 coming in to regular outlet. Red from #1 pigtailed to black from Outlet #3. White from Outlet #3 also connected to #2 outlet.

Outlet #3: Standard hookup in cabinet for MW.

I"m going to grab a bite quickly, and if I don't have a response by the time I'm done, I'll check the wiring on the back of the GFCI. I think I've safely ruled out the MW as the problem.

Thanks in advance!

P.S. Let me add that when I plug the extension cord into Outlets #1 and #2, the MW comes on as well. It's only #3 that fails to power anything.
 

Last edited by Critical Mass; 12-06-10 at 06:00 PM. Reason: Addional info
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  #2  
Old 12-06-10, 06:24 PM
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The downstream LOAD protection of GFCI receptacles is not compatible with a multiwire branch circuit (shared neutral). You have two options.

1) Replace the double-pole 20A breaker currently powering this circuit with a 20A double-pole GFCI breaker. These breakers can be rather expensive depending on the panel brand, so this option may be cost-prohibitive. GFCI protection also is not required for the microwave, so this is probably not the ideal method.

2) Replace each of the standard receptacles on this circuit with a GFCI receptacle and wire the black and white only on the LINE terminals. Each receptacle will protect only itself and nothing downstream.

A couple notes about multiwire circuits in general. The neutral wires must be pigtailed with wirenuts; they cannot depend on the device screws for continuity. As of 2008 code, the breaker must be a double-pole breaker or be handle-tied single-pole breakers with a breaker accessory kit (this guarantees 240V line to line and that both breakers are switched off for service).
 
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Old 12-06-10, 07:12 PM
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Thank you for the info. I was not aware of that.

Is there any way, then, I could rewire it so that outlet #3 (MW) would get juice from #1 instead of #2 so that I could pigtail the neutral BEFORE it goes into the GFCI? I don't need the GFCI for the MW. I'm just trying not to buy two more GFCIs
 
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Old 12-07-10, 10:20 AM
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If you were to rewire it, you could "fork" the circuit at box #1. A 12/3 comes in from the panel, two 12/2s go out. One goes to the microwave, one goes to the remaining countertop receptacles. The 12/2 leg going to the countertop receptacles would be wired from the LOAD side of the GFCI receptacle, and the 12/2 leg going to the microwave would not.

Two more GFCIs should be like $20 though, so it's not a huge price to avoid the hassle of rewiring.
 
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Old 12-07-10, 02:09 PM
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To be honest, since I didn't get a reply from anyone by morning, I went and tried it that way and it worked fine. It wasn't really any trouble at all, since the two countertop receptacles in question are both in between two studs, and the MW receptacle is in the cabinet directly above one of them, so it was only a matter of switching the hot 12/3 AND the GFCI from box #1 to box #2. Once you explained the problem with the GFCI tripping, I didn't see any reason why it wouldn't work this way, and didn't see any reason it would be illegal or dangerous, except that it's a lot of wires in one box. Thanks for your help!
 
  #6  
Old 12-07-10, 06:59 PM
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The GFI was seeing the neutral current imbalance between the red part of the circuit and what was flowing on the black part of the circuit.
 
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