GFI nuisance tripping with fluorescent lights

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Old 11-27-20, 11:14 AM
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GFI nuisance tripping with fluorescent lights

Hi! I have owned my house for about 6 months and have an issue in my garage. What I'm finding is most times when I turn on the garage lights a GFI receptacle on a different circuit trips. The garage has two very large (8' long, I'm guessing?) fluorescent lights, each with 2 bulbs. I have found that - except when it is cool, maybe below 45 degrees - turning on the lights trips the GFI plug. The two lights come on 1-2 seconds apart (always in the same order), and it is always the second one (never the first) that trips the GFI plug. Resetting the GFI plug normally always works (whether the lights are on or off at that point). The garage has a sub-panel (picture attached). The lights are on an AFCI breaker - no idea if that is related. None of the actual circuit breakers ever trip - just the GFI receptacle. I have completely swapped out the GFI receptacle (and checked the wiring) - exact same thing still happens. Help! Thanks - Rob.
 

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11-27-20, 12:06 PM
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Since they are on separate breakers they should not be tripping the GFCI receptacle. I suspect there is something miswired, perhaps the two circuits are sharing a neutral.

I can't read the label very well, does the AFCI say something about a sensor? If you turn off the receptacle breaker will the light still work?

It might also be possible that the lights are old, and are in fact, leaking current to ground.
 
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Old 11-27-20, 12:06 PM
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Since they are on separate breakers they should not be tripping the GFCI receptacle. I suspect there is something miswired, perhaps the two circuits are sharing a neutral.

I can't read the label very well, does the AFCI say something about a sensor? If you turn off the receptacle breaker will the light still work?

It might also be possible that the lights are old, and are in fact, leaking current to ground.
 
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Old 11-27-20, 12:14 PM
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Thanks for the reply! The label says "Front Garage Sensor" - I believe that is an outside light that works on a motion sensor (probably not related). In terms of the lights, if I turn off the AFCI breaker then they do not work (makes sense - since that's the circuit they're on). If I turn off the other circuit (that the GFCI receptacle is on), then yes - they do still work.
 
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Old 11-28-20, 04:59 AM
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I am also leaning towards a "shared neutral".

Sushilover111 google how a GFCI receptacle or breaker works. Understanding how the GFCI operates will also help you evaluate this situation and maybe others down the line.

But it basically functions as follows:
How does a GFCI work? The GFCI will “sense” the difference in the amount of electricity flowing into the circuit to that flowing out, even in amounts of current as small as 4 or 5 milliamps. The GFCI reacts quickly (less than one-tenth of a second) to trip or shut off the circuit.
 
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Old 11-28-20, 10:24 AM
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Since you tried another working GFCI receptacle, I think we can rule the receptacle out. And since you have confirmed that the GFCI receptacle and lights are on different circuits, as Tolyn and AFJES suggested, it sounds like a shared neutral might be the culprit.

From a DIYer perspective, I would start by identifying any boxes with multiple switches or outlets that are fed by both circuits (the light circuit and circuit the GFCI recep is on), and then check if all the neutrals in that box are spliced together.
 
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Old 11-30-20, 10:06 AM
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cartman AFJES - thanks for your help on this! Wondering how I might find out if they share a neutral? To my knowledge, there are not any boxes with multiple switches or outlets that are fed by both circuits.

I've attached a picture of the inside of the box. The yellow insulating casing at the bottom says "garage door outlets," and that is the circuit with the tripping GFCI. I traced the black - and it does go to the 2nd breaker from the top (on the right) - so that looks correct. The neutral for that circuit ties into the very top-most connection on the neutral bar, so it doesn't appear it has anything to do with the AFCI circuit?

FWIW, the AFCI circuit has 2 neutrals tied together - I'm assuming (?) that is correct wiring?
Thanks! Rob.




 
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Old 12-01-20, 06:36 AM
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Did you swap out the GFI with a new unit, or just swap them around with one of the same age and possibly brand?

Just asking because when we bought our house, a lot of the GFI's did nuisance trips. After I started replacing them I realized they were all the same model/brand.

As for the neutrals being wire nutted together, that could just be to save spots in the neutral bar. (And I believe acceptable)
 
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Old 12-01-20, 07:37 AM
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You need to trace the wiring from the load side of the gfci outlet checking for a shared neutral, not between the panel and line side of the gfci outlet.
 
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Old 12-01-20, 08:56 AM
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scoob8000 - I swapped the GFCI receptacle with one that (I assume) is very similar - and from a different circuit in the garage. However when I did that, the tripping followed the circuit (not the receptacle). So I suppose there could be something a tiny bit off with the wiring of that circuit and its just enough to cause this brand of GFCI to trip? Seems like a bit of a long shot?

The interesting thing about the neutrals being wired together is there are a ton of spots open on the neutral bar, and both AFCI circuits (one on the opposite side) are wired the same way (wit the neutrals nutted together).

Thanks - Rob.
 
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Old 12-01-20, 09:43 AM
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The interesting thing about the neutrals being wired together is there are a ton of spots open on the neutral bar, and both AFCI circuits (one on the opposite side) are wired the same way (wit the neutrals nutted together).
When we are saying there could be a shared neutral issue, we're not talking about the panel, where all the neutrals land on the neutral bar. Please re-read @pattenp's post as well as @AFJES' post.
 
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Old 12-01-20, 12:26 PM
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pattenp cartman I see, sorry - did not understand exactly what that post meant. The GFCI receptacle is in a box on a wall with sheetrock (so not trivial to trace the load side). How about if I did this: open the GFCI receptacle, disconnect the load side, then test to see if I still have the issue? If I still have it then I think (?) that would rule out a load side shared neutral. But if I don't have it, then that's definitely the issue and I can do more searching. Thanks again for your help!
 
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Old 12-01-20, 02:50 PM
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@sushilover111 - read AFJES' post on how GFCI works. It's looking for a current differential. Assuming your GFCI outlets (since you tried two different outlets in the same spot) aren't the issue, there is some wiring interconnection between the two circuits involved, probably a shared neutral. If the two circuits were completely independent, then using your garage light circuit wouldn't be tripping your GFCI on a different circuit.

From my DIY'er perspective (and if the pros here say different, I'd go with whatever they suggest), here's how I'd approach it.

1. Figure out everything (outlets, switches, fixtures) on the load side of the GFCI outlet. You can turn the GFCI outlet off and see which devices no longer work.

[You can skip steps 2 and 3, but if you have a lot of devices on the load side of the GFCI and they are spread out, this will help narrow things down. But read them anyway to understand the logic I'm using here.]

2. Figure out all devices on the garage light circuit. Turn off that breaker and see which devices no longer work.

3. Make a rough map of the wiring. You don't need to take down drywall to figure out how the wiring was run. Generally wires are run in a logical manner to conserve wire. Once you identify all the devices on the circuit, odds are decent that the closest device to the panel containing the breaker is the first device on the circuit.

4. Wherever devices on the load side of GFCI outlet and devices on garage light's breaker are in proximity, I would look at the device's wiring and the wiring in the box containing the device. Probably in one of these boxes, you will find some interconnection between these two circuits. Again, a shared neutral seems like the most likely culprit. As you saw, all the circuits' neutrals come together at the panel, but that's the only place each circuits' neutrals should come together. If they come together at any point before that, that's what we are referring to as a "shared neutral". Shared neutrals are a common cause of tripping GFCI's because GFCI sees imbalance in current flows because some of the current flowing in on the hot isn't coming back on the neutral for that circuit - it's hopping onto the neutral for a different circuit, which it can only do if there is a shared neutral. [If you skipped steps 2 and 3, then look at wiring of all devices identified in step 1].
 
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Old 12-01-20, 07:36 PM
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Ok, I executed the steps - I understand how a GFCI works and I mapped out all my electrical devices on all my circuits - made a diagram. I looked carefully at the wiring - the wire running to the light looks pretty clean and that doesn't surprise me given the wiring is only 10 years old and the inspector in our town is pretty tight.

I then executed the test I described earlier: on the GFCI that trips, I removed the hot and neutral LOAD wires. So the GFCI receptacle stayed live, but everything downstream of that had no power. I then turned the lights on, and that GFCI did not trip - however a completely different GFCI - on yet a different circuit (a 3rd circuit) tripped. At that point my head exploded and it was difficult to troubleshoot.

I repeated the test with the same results, and I am 100% sure this new GFCI is on a different circuit than the GFCI that had been tripping and a different circuit than the fluorescent lights.

Any ideas?!? Thanks - Rob.
 
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Old 12-01-20, 10:45 PM
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Seems to me you just proved that the wiring at some device on the load side of GFCI #1 is connected to the circuit that the garage lights are on. Probably a shared neutral.

So when the load side wires are disconnected from GFCI #1, and you turn the garage lights on, there is no path for the return current via GFCI #1 and thus it does not trip.

Are you sure that GFCI #2 is on a different circuit? You turned off breakers for the garage lights and GFCI #1, and GFCI #2 still works? If so, it's likely another shared neutral situation.
 
 

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