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I just installed a gfci outlet, and its tripping when a 3way switch on a separate breaker is turned on

Some background:

I have two separate circuit breakers, when they are turned off, the following correctly happens:

Breaker #1) Shuts off most everything in Garage (No power to GFCI outlet)

Breaker #2) Shuts off most everything in Living Room (No power to 3-way lightswitches, or the single outlet they control)

The problem scenario:

If I have breakers #1 and #2 turned on at the same time, and have something plugged into the single outlet (controlled by 3way switch on breaker #2), the GFCI trips on breaker #1.

Any thoughts on what may be occurring or next steps to diagnose the issue?

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    Are the breakers handle tied or next to each other? They may be a multi wire branch circuit sharing a neutral or the neutrals crossed. – Ed Beal May 14 at 22:47
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    Is there any place where these two circuits share a junction box, other than at the panel feeding them of course? – ThreePhaseEel May 14 at 22:49
  • @EdBeal They are on the same side of the panel, separated by one other breaker. (not tied together) – heyitsmyusername May 14 at 22:50
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    So on the same leg it could be a crossed neutral causing the problem. – Ed Beal May 14 at 22:52
  • Can crossed neutrals be fixed at the junction box? Or would I need to remove the GFCI and live with how its wired? – heyitsmyusername May 14 at 22:54
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Crossed neutrals can be fixed depending on where they are crossed.

Start in your main panel. You will need to open the panel. WARNING, there is lethal voltage in there, if you aren't sure of what you are doing in the panel, hire a professional.

Look at both the breakers in question. Follow the hot wire from each breaker and find its corresponding neutral. Where the hot leaves the panel a neutral should enter. Does each breaker have its own neutral? If yes we can move on. If no, you have a common neutral and you should call a professional to fix it or start a new question because you have something that needs to be fixed.

Since we have two neutrals we now need to figure out where they got tied together. You will need to go to every box that either of these circuits passes thru one at a time.

Shut off the circuit whose outlets are closest to your breaker panel (call it circuit 1). Go to each box, you will need to open it up and check for voltage on all the hots going in and out of the box. If you find a hot circuit, confirm that it is circuit 2 by turning it off and checking again. Once we find the hot circuit look at the neutrals in the box they are probably all tied together. That is your problem.

With both circuits off, you will need to figure out which neutrals go to which circuit, and separate them. To be clear when you are done, all neutrals need to have a path back to your main panel.

Good Luck

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