GFCI and 'shared' neutral troubleshooting

[jmtrup01]

I was upgrading a receptacle to a GFCI receptacle in a bathroom (Circuit 16) but ran into some issues. The receptacle was tripping as soon as energized. I traced the problem to a nearby light switch. The box contains two switches ? a single switch for a back porch light on Circuit 16 and the final 3-way switch for a 4-way light in the backyard, which is on Circuit 14. When tracing this out, I noticed that as the power from 14 goes into the first 3-way switch, the LINE neutral is just abandoned in the j-box, uncapped. As the LOAD from the last 3-way goes back to the light (from 14), the neutral is pulled from the convenient circuit 16 neutral in the shared box. By disconnecting this neutral, the GFCI receptacle and back porch light work as expected. To get the back yard lights to operate again, I will go back and run a new neutral from a circuit 14 source to the light.

Now a new problem?when the hall lights are turned on ? which are two fluorescent fixtures, 3-way switched, on circuit 12 ? the GFCI receptacle trips, killing the lights while other circuit 12 devices are reported to work just fine. I have not been back out to troubleshoot yet, but I assume these lights are also borrowing a neutral from circuit 16.

My questions:

1. The GFCI was installed with the two LINE wires into the LINE position, and the remaining 2 wires into the LOAD position. Are there any alternate wiring configurations that may work with the existing shared neutral configuration?
2. Is my solution to run a new neutral from circuit 14 to the circuit 14 back yard lights appropriate?
3. Any thoughts/suggestions about the circuit 12 hall lights also sharing a circuit 16 neutral?

 

[Dennis Alwon]

Are you a general contractor or a licensed electrician?

 

[kwired]

Not using the "feed thru" of the GFCI will solve your tripping problems.

That said you are likely uncovering other problems that probably do need some attention.

 

[jaylectricity]

Forum rules aside, there's a chance you have shared neutrals between the bath GFCI circuit and the lighting circuit.

That's all I am allowed to say.

 

[jmtrup01]

Dennis - yes; no.

I forgot to mention that I chose this recep because it would also protect the homeowner's other bathroom. If I do as kwired suggested I will need to install a GFCI in his other bath, and I imagine I am only pushing the issue further up the circuit.

 

[GoldDigger]

Dennis - yes; no.

I forgot to mention that I chose this recep because it would also protect the homeowner's other bathroom. If I do as kwired suggested I will need to install a GFCI in his other bath, and I imagine I am only pushing the issue further up the circuit.

You are not pushing the issue further up the circuit if there are no feed through loads from the GFCI you install in the second bathroom. (Hopefully the circuit feeds only bathroom outlets.)
If the neutral issue is between the 1st bathroom and the 2nd, you may not be able to fix it "right" without pulling more wires in the walls. Or it may be as simple as somebody tying one two many neutral wires into the same wirenut in the 2nd bathroom.