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MWBC + GFCI = Problem?

Tom in Seattle

Well-known member
Joined
Feb 8, 2012
Messages
45
Location
Sammamish, WA
My kitchen remodel turned into a major rewiring project with lots of new circuits. Among these are three 20A MWBCs. I was just reading some notes and now I'm wondering if I'm going to have problems since two of these were planned to have a GFCI receptacle on one branch.

Here's what I have:

1) MWBC that feeds the kitchen island. The red wire is for a receptacle for the microwave drawer and rangetop ignitor. The black wire would then feed a counter receptacle with a GFCI that also protects two other receptacles on the island.

Since the GFCI is downstream of the MWBC will this function correctly? The neutral to the GFCI should only carry the current from the counter receptacles and shouldn't really be affected by the neutral current from the microwave, right?

2) MWBC that feeds a point of use water heater for the 1/2 bath, plus the bath receptacles. This would be wired the same as above, with the bath GFCI downstream of the heater circuit. OK?

3) MWBC for under sink appliances. One leg for the dishwasher and one downstream leg for the disposal, instant heat and water chiller. No GFCI involved.

I'm kinda wishing I'd wired these with individual circuits, but at the time it seemed like less work and expense to go with the MWBCs. Each MWBC will be on a double pole 20A breaker.

All the wiring was inspected and I got the OK to close up the walls. The inspector asked about the MWBC he saw in the bath and asked what else besides the bath receptacles were on the MWBC. He didn't have a problem with it wired as above.

I don't really want to redo these circuits but if I'm going to have problems down the road now is the time to do it since the walls are still open and the new panel isn't in place.

Thoughts?
 
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VHF

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Joined
Oct 27, 2008
Messages
420
Location
NW Wisconsin
Since the GFCI is downstream of the MWBC will this function correctly? The neutral to the GFCI should only carry the current from the counter receptacles and shouldn't really be affected by the neutral current from the microwave, right?
It should work OK. Your theory is correct.

Occasionaly folks have reported having trouble with GFCIs used in conjunction with MWBC--a GFCI on the second leg of the MWBC tripping when a load on the frist leg is used. I'm not sure if this is due to a poor neutral connection somewhere, some kind of stray current, or a defective/cheap GFCI.

Let us know if you have any issues!
 
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MrMark

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Joined
Jan 25, 2010
Messages
4,626
Location
Southern Cal.
You are fine with your understanding. As long as the GFCI is downstream its neutral will only see load current values and not trip unless it sees 5ma leakage like it is supposed to.
 
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