250 foot run of UF tripping GFCI with no load

[Art C.]

Contractor buried 250' of UF going to three new lamp posts when they repaved the driveway. Each post has a lamp and a convenience receptacle, on different line conductors but sharing a neutral. Because of the shared neutral, the lights and receptacles must share the same GFCI, but we wired them to different switches downstream from the GFCI (customer wants to be able to switch Christmas lights and such).

The lights work fine. The receptacle side trips the GFCI immediately upon being energized, even without a load attached. Is it really possible that there is enough capacitive coupling between the conductors in that long run to fool the GFCI into thinking there is a ground fault? And if that is the case, why isn't the conductor with the lights doing the same thing?

Would appreciate any advice, including further tests to run. Never seen anything like this before, even with long-ish runs of UF. Moving GFCI protection out to the pole locations isn't an option because the poles don't have standard size openings.

 

[Todd0x1]

Might be something simple like nicked insulation or a pinched wire. This seems like a pretty bad design to have all that on one GFCI. Once you factor the leakage in that much cable you don't have much headroom for your loads. I would expect the holiday lighting to be constantly tripping the GFCI.

 

[petersonra]

disconnect UF at GFCI.

disconnect lamp fixtures.

disconnect outlets.

perform insulation (megger) test of UF.

if it passes, reconnect each outlet separately and see if the gfci trips.

my guess is someone nicked the UF somewhere or one of the outlets has an issue like some dirt got inside it.

 

[roger]

I know PVC has drastically increased in price but I would have used it at least under the driveway portion of the run, if there is in fact some damage to the conductors it will be hard to fix.

 

[petersonra]

I know PVC has drastically increased in price but I would have used it at least under the driveway portion of the run, if there is in fact some damage to the conductors it will be hard to fix.

depends on how deep they buried it.

 

[roger]

depends on how deep they buried it.

It wouldn't matter how deep it is under concrete or asphalt, it will be hard to fix.

 

[petersonra]

It wouldn't matter how deep it is under concrete or asphalt, it will be hard to fix.

if it is bad under the concrete or blacktop it might be easier to bore under the driveway and install a new section than to try and fix it.

not an especially difficult task for someone with the right equipment if you have a blank check.

if you go to the trouble to bore under the driveway it would make sense to install some kind of protection for the UF.

I think the minimum burial depth would be 12". That is not real deep.

 

[roger]

if it is bad under the concrete or blacktop it might be easier to bore under the driveway and install a new section than to try and fix it.

Which proves my point that regardless of what PVC is going for now it would have been cheaper to go with it in the beginning.

 

[petersonra]

Which proves my point that regardless of what PVC is going for now it would have been cheaper to go with it in the beginning.

can't go back in time and do it right. have to figure out how to fix it.

but fixing it starts with figuring out what is actually wrong and none of us knows.

no one here knows if there is a bad section under the driveway or not. it might be perfectly fine.

 

[suemarkp]

Are there multiple runs of UF cable from the switchbox out to the lights with multiple switched hots in different cables and the neutral in only one cable, or is it like a 10-3 cable with a two switched hots (one for lights, one for receptacles) and one neutral?

Perhaps pull out each receptacle outlet from the post and let them hang in free air. Then energize the receptacle circuit and see if it holds. If so, perhaps one of the neutral screws on a receptacle was touching the metal receptacle box, or maybe a bare ground was touching something on the receptacle. If it doesn't hold, the cable could be damaged or there is just too much capacitive loss. You could disconnect the downstream connection to the next pole to see if the problem is at or before the 1st light pole, or at the 2nd or 3rd.

You could remove the GFCI, energize the circuit, and see how much current is flowing on the hot, neutral, and ground. This could differentiate from a full short, to a nick, versus capacitive coupling to the ground.

 

[ptonsparky]

Call someone with a fault finder and a megger.
I sold mine but they said I could borrow it anytime. Where do I need to be?

I just spent three days in Colorado buying three lugs of peaches so another side trip means little.

 

[roger]

can't go back in time and do it right. have to figure out how to fix it.

but fixing it starts with figuring out what is actually wrong and none of us knows.

no one here knows if there is a bad section under the driveway or not. it might be perfectly fine.

Right and that is the reason I said "I would have". I know it's not a solution to the present problem but maybe a precautionary approach in the future. I would have probably put an empty spare too, comes with years of experience

 

[Art C.]

Are there multiple runs of UF cable from the switchbox out to the lights with multiple switched hots in different cables and the neutral in only one cable, or is it like a 10-3 cable with a two switched hots (one for lights, one for receptacles) and one neutral?

It's the latter, multiple switched hots in the same cable. The homeowner bought a roll of UF and had the driveway contractor bury it with loops at each pole location. I agree PVC would have been a better choice but it's too late now.

Based on what has been said here I'm leaning towards the conductor having been damaged, because the lamps are on the same GFCI and they're working fine. I was hoping to avoid taking the circuit apart at every post but it's looking more and more like that's what has to be done.

 

[petersonra]

Right and that is the reason I said "I would have". I know it's not a solution to the present problem but maybe a precautionary approach in the future. I would have probably put an empty spare too, comes with years of experience

If one goes to the trouble of running PVC why put UF inside of it? Just run some THWN conductors.

 

[ptonsparky]

It's the latter, multiple switched hots in the same cable. The homeowner bought a roll of UF and had the driveway contractor bury it with loops at each pole location. I agree PVC would have been a better choice but it's too late now.

Based on what has been said here I'm leaning towards the conductor having been damaged, because the lamps are on the same GFCI and they're working fine. I was hoping to avoid taking the circuit apart at every post but it's looking more and more like that's what has to be done.

Don't feel too bad for the HO, he skimped enough on installation that he can pay now.

 

[roger]

If one goes to the trouble of running PVC why put UF inside of it? Just run some THWN conductors.

Exactly what I would have done, you're catching on.

 

[Knuckle Dragger]

Right and that is the reason I said "I would have". I know it's not a solution to the present problem but maybe a precautionary approach in the future. I would have probably put an empty spare too, comes with years of experience

I'm trying not to be disrespectful but this happens so often.
You're the mod right?
Stay focused on his question.
How can you help the guy.
Most of know what he should have done but he didn't.

 

[petersonra]

I'm trying not to be disrespectful but this happens so often.
You're the mod right?
Stay focused on his question.
How can you help the guy.
Most of know what he should have done but he didn't.

According to the original post "contractor" installed the UF. We don't know if he's associated with the installation or not. He may have just been left with the mess to clean up.

Realistically if this is still under warranty I would get whoever installed it back to fix it. It's also possible this was done by an unlicensed electrician as a cost saving measure. Who knows. With these cases we never seem to get the full story.

 

[Knuckle Dragger]

Very true

 

[roger]

I'm trying not to be disrespectful but this happens so often.
You're the mod right?
Stay focused on his question.
How can you help the guy.
Most of know what he should have done but he didn't.

Huh huh, and sometimes the best answer is keeping the mistake from happening again, so that is how I helped the guy. With that said, what would you do?