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Old 11-29-2009, 12:12 PM   #1
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Switch triggering GFCI breaker on a DIFFERENT circuit


I just installed a new light for our kitchen table. The circuit this light and switch are on are on one circuit. When we flip the light on, it causes the GFCI outlet breaker on a completely different circuit (on the opposite side of the panel, even) to flip. This occurs only if our low powered microwave is plugged in to that circuit (but not even running). This suggests that the light puts too much current in the circuit, but its NOT on that circuit. Also, we can reset the GFCI and it stays on, even with light on AND microwave running.

Help! Don't have any idea where I would even start looking except in the breaker box (but the circuits, as I noted aren't even near each other). Weird.

thanks for your help.

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Old 11-29-2009, 12:42 PM   #2
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Switch triggering GFCI breaker on a DIFFERENT circuit


It is common for a kitchen to have what is called a "Multi-Wire Branch Circuit".

With this two different circuit breakers control the 3 wires run in one electric cable. The white "neutral" wire is shared by both circuits in this cable.

1. These are very dangerous to work on if you are not familiar with how they work.

2. Always turn off the MAIN electric power to the entire house before working on anything electrical, then you are sure ALL the power is off.

3. GFCI's need to be wired a certain way when incorporated with these circuits. If not wired correctly, problems like you are having can happen.

4. I would suggest hiring an electrician for this as this could be dangerous to work on, the existing wiring might not be installed properly, and an electrician would be able to tell what was right / what was not right.

5. If you want to work on this, learn all about "Multi-Wire Branch Circuits", GFCI's, and FULLY understand the wiring in your house. How it is wired. Be able to tell if something is not wired correctly and be able to fix the problem.

Be safe when working on any wiring and turn off the main breaker please!

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Old 11-29-2009, 12:50 PM   #3
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Switch triggering GFCI breaker on a DIFFERENT circuit


Thanks for that idea. I wired the kitchen myself and didn't create any multi-branch circuits. We've been operating the kitchen circuit for years without any problems. I had wired the switch for the ceiling light a few years ago and just ran it wild into the attic (capped off of course) until we found a light we liked and knew exactly where we wanted to put it. I just installed the light and now we have the problem. The other light switches on that same circuit and in the same box do not cause the GFCI to flip.

Any other suggestions?

[Yes, I wired the whole house, so I'll definitely be safe and turn off the main breaker before even opening the panel. I'm a mom with two kids and want to stick around for awhile!]
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Old 11-29-2009, 02:23 PM   #4
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Switch triggering GFCI breaker on a DIFFERENT circuit


Ok, good. Glad there is no MWBC, that makes things safer to work on.

The way a GFCI works is it measures the electricity going out one wire, then makes sure the same amount returns on the other. Like a bank teller balancing the books!

So it is possible you tied into a neutral from the GFCI, but the hot from the breaker controlling the circuit. Then the GFCI would have a fit and trip!

I would try disconnecting the hot AND neutral from the GFCI circuit in the panel, then see if your light still works...
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Old 11-29-2009, 06:36 PM   #5
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Switch triggering GFCI breaker on a DIFFERENT circuit


Quote:
Originally Posted by stillmarried View Post
When we flip the light on, it causes the GFCI outlet breaker on a completely different circuit (on the opposite side of the panel, even) to flip.
1

This occurs only if our low powered microwave is plugged in to that circuit (but not even running).
2

we can reset the GFCI and it stays on, even with light on AND microwave running.
3
1
Incand. light?

2
If a grounded microwave, check for leakage current in the ground pin.

3
So you are looking for some transient event, or the GFCI is running close to its 5 mA limit.

I guess you could try unplugging the microwave, turning on the light and then plugging in the microwave.

There might also be a high resistance connection in the neutral busbar.

Last edited by Yoyizit; 11-29-2009 at 06:39 PM.
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Old 11-29-2009, 10:14 PM   #6
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Switch triggering GFCI breaker on a DIFFERENT circuit


You have a poltergeist in your house!! J/K

I would try replacing the GFI unit and see if that makes any difference. Some are more sensitive than others.

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