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Two circuits - Single neutral & GFCI


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Hi guys,

Long time lurker and first time post here..

First off let me introduce myself. I am a Technologist with an engineering background and education. I do fabrication (racecars and restorations) for fun.

I'm working on my kitchen which was last remodeled back in the early to mid 50's.

There are a lot of outlets in the kitchen and most of them are worn out so I was going to replace them. In the process I found that there are two circuits. One 15 amp and one 20 amp. All wiring is good solid core 12g. The top of the outlets was wired to the 15 amp and the bottom to the 20 amp. No GFCI anywhere. Well, long story short, I altered the wiring to only bring the 15 amp to the outlets and I wired in a GFCI outlet for that entire leg. Everything is honky dory. The 20 amp leg I am using solely for the Micrwave, garbage disposal and refrigerator.

I went to install a 20 amp GFCI into that second circuit and suddenly the 15 amp GFCI now trips whenever I plug my circuit tester into it. I don't even need to push the button. With the GFCI out of the 2o amp circuit - everything is fine and both circuits work perfectly.

Now - according to my unprofessional knowledge, and from what I have seen in my breaker boxes (yes I have two - which may have just answered my own question) the neutrals are all tied to a single bus in each box. The 15 amp circuit in one box and the 20 amp in the second box. Both coming off of the same inbound leg as best I can tell.

What I suspect is happening is that there is a single neutral that has been shared (hidden inside a wall somewhere) and that even though electrically it seems to make sense and work, the GFCI senses a ault and trips as soon as there is any current flow.

So my question is -

Should I run a common neutral connection between the neutral busses in both boxes?

Give up on the notion of installing a GFCI in the 20 amp circuit?

Note that the basement in this house is totally finished and ripping out ceilings and walls is not an option.

I welcome your thoughts!

Thanks!!!

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GFCI's won't work on a multi-wire circuit. If it isn't a multi-wire, then try another GFCI on that 20A circuit. I've had some that were defective right out of the box.

You're dealing with ungrounded circuits, right?

Marc

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You have 2 hot wires going to each outlet. You've removed the 20 amp jumpers at all the countertop outlets, right? That same 20 amp circuit still goes to the outlets but with no terminations and now supplies power to the dishwasher, disposer and fridge? I would pull all that wiring out and check all connections. It sounds like there is a fault.

In my area, kitchen GFCI's are 20 amp. If your wiring is all #12 copper, I don't know why you've installed a 15 amp GFCI.

I would invest in a couple of fish tapes and pull some new wire in behind the cabinets.

Don't mess with the subpanel neutrals. Get an electrician to check that out, or post some 800 X 600 pixel pics here, and we can help sort it out.

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If done properly, GFCI's definitely work on MWBC's.

No. They don't. (EDIT - at least not for a pass-through application such as the original poster is talking about.) A GFCI works by detecting an imbalance in the current between two conductors. With a multi-wire branch circuit, there will - by definition - be an imbalance every time one side of the circuit is used. If the GFCI is working properly, it will trip as soon as you use current on either side of the circuit.

To make this work, he'd have to put a GFCI device on every single countertop receptacle outlet and wire them with pigtails. With 50's era boxes, that would be a sketchy proposition.

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If done properly, GFCI's definitely work on MWBC's.

No. They don't. A GFCI works by detecting an imbalance in the current between two conductors. With a multi-wire branch circuit, there will - by definition - be an imbalance every time one side of the circuit is used. If the GFCI is working properly, it will trip as soon as you use current on either side of the circuit.

I forgot to mention that my comment applies only to GFCI's breakers installed on a multi-wire circuit.

If it's a GFCI receptacle we're speaking of then it will work on multi-wire circuits because the imbalance that the device looks for is between the two currents within the receptacle itself.

Marc

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If done properly, GFCI's definitely work on MWBC's.

No. They don't. A GFCI works by detecting an imbalance in the current between two conductors. With a multi-wire branch circuit, there will - by definition - be an imbalance every time one side of the circuit is used. If the GFCI is working properly, it will trip as soon as you use current on either side of the circuit.

I forgot to mention that my comment applies only to GFCI's breakers installed on a multi-wire circuit.

If it's a GFCI receptacle we're speaking of then it will work on multi-wire circuits because the imbalance that the device looks for is between the two currents within the receptacle itself.

Only if he wants to use pigtails and put a GFCI receptacle at each and every location where a receptacle serves the countertop. If you try to feed the circuit *through* the GFCI, it won't work.

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  • 1 year later...

Hello, first time poster here.

I would like ask for some clarification from the "Jims" related to the shared nuetral and the gfis.

Quote: Originally posted by Jim Port

The neutral of the MWBC just needs to be spliced ahead of the first GFI. You

would then feed xx-2 to any of the downstream receptacles.

Exactly, Jim. Have done it hundreds of times.

I am currenlty rewiring at my rental property (whole house) and have two separate 12/2 circuits feeding the kitchen (small) through the unfinished basement below for the two 20 amp appliance circuits. Each circuit has a gfci receptacle with a duplex receptacle on the load side of the gfci (4 countertop outlet locations total).

I have come across a great deal on some 12/3 and ask the following:

Can I feed the two 12/2 circuits from a junction box mounted underneath the kitchen in the basement with the junction being fed by the 12/3? (of course replacing the two 20 amp breakers with a single double pole). The run from the junction box to the panel would be the length of the house and I could save money and physical space feeding to the outdoor panel; not to mention repurpose the 12/2.

If I run the 12/3 to the junction and "split" (pigtail) the nuetral at that location and continue on with two 12/2 runs, would I still need to put gfci receptacles at each location and rewire the current ones off the "load" side of the other gfcis?

I hope my questions make sense and thanks in advance for any input!!

Dan

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. . . Can I feed the two 12/2 circuits from a junction box mounted underneath the kitchen in the basement with the junction being fed by the 12/3? (of course replacing the two 20 amp breakers with a single double pole). The run from the junction box to the panel would be the length of the house and I could save money and physical space feeding to the outdoor panel; not to mention repurpose the 12/2.

If there's a prohibition against this, I'm not aware of it. Jim Port might be aware of something that I'm missing. Depending on how old the 12/2 is, you might not want to repurpose it.

If I run the 12/3 to the junction and "split" (pigtail) the nuetral at that location and continue on with two 12/2 runs, would I still need to put gfci receptacles at each location and rewire the current ones off the "load" side of the other gfcis?

I don't understand the last part of the question. After the multi-wire circuit is split into two 12/2 runs, you can treat the 12/2 runs just as you would any other.

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Thanks for the input. The 12/2 is 5 years old or less. If I split the run and treat it as two 12/2 runs does it still have to be on a double pole breaker or can I use the same two 20amp breakers (opposite phase of course)?

Yes, of course. As long as there

is a common neutral, you need the tie on the two breaker.

I agree with Morgan. Get yourself a 'Johnny on the spot' electrician.

Marc

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