250 foot run of UF tripping GFCI with no load

[Art C.]

Stay focused on his question.
How can you help the guy.
Most of know what he should have done but he didn't.

Thanks, and I agree. We all know that conduit is better, and saying so isn't helpful. Troubleshooting tips are somewhat more welcome.

 

[Knuckle Dragger]

Yeah and see how the discussion went sideways. As it does a lot.
Most of the participants got it covered.

I don't have eyes on the problem but these are a few things I would do love it or leave it.

Disconnect everything and meggar systematically. From panel to j box, from j box to switch location from switch location to receptacle location and so on. If all tests pass move on.
Assuming that the circuit is protected on a GFCI circuit breaker.
Disconnect lights on the post from circuit and focus on receptacles individually. Test to see if it holds with a load on it.
If the breaker trips only while on one of the receptacles try to isolate circuit more by separating making sure the wires are clear in the receptacle box, replace receptacle for giggles because it's easy to do so why not.
It isn't all the answers but it something.

 

[winnie]

I would start with a visual inspection of each termination and mapping out the circuit. You might have something simple misfired or an incorrect breaker for the application.

After this, agree with the troubleshooting approach of disconnecting everything and megging the conductors.

Before deciding what to do next, you need to figure out if the problem is in the cable or elsewhere.

Note: if there is a wire insulation fault in the device boxes, the meg detectable fault might come and go as the wires get moved around.

Jon

 

[LarryFine]

This is basic troubleshooting. First, determine whether the breaker is tripping due to fault current or leakage current.

I would see if a non-GFCI trips. If so, temporarily wire a high-wattage incandescent bulb in series at the breaker while checking each receptacle, while you or a helper watches the bulb.

 

[robertd]

>The homeowner bought a roll of UF and had the driveway contractor bury it with loops at each pole location.
How exactly did the driveway contractor bury it? Did in go in a trench backfilled with dirt or did he toss it on the
ground, dump gravel over it and then roll the gravel and put concrete or asphalt over the gravel?

If the outlet opening on the pole is nonstandard and you need to use GFCI outlets you can always mount a standard
box on the pole or next to the pole. Might not be what the owner had in mind but may be the lowest cost fix.

 

[kwired]

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.

I don't know right now, but would after connecting my fault locator to it

 

[kwired]

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

UF is also horrible to pull compared to THWN.

 

[kwired]

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.

He later said the "driveway contractor" buried the cables so probably not a qualified electrician.

 

[junkhound]

original query - can capacitance trip the GFCI on 250 ft of UF?

e.g quick calc shows 85 nF can trip nominal GFCI, 68 nF worst case
Meter on dry 250 ft coil shows 10.6 nF

add to that the capacitance of the hot to soil, say another 40 nF.

not much else in the way of leakage needed to trip (circa 100 kohm). one could calculate resonant impedance also, say 0.3 uH per foot distributed transmission line, and total current to ground from hot could exceed 4 mA.

add in possible .001uF noise filter on a light ballast? Trip.

hmm--- reread OP post, are there TWO hots ( 3 wire and G UFcable?) Then 2 hots from same GFCI in parallel to ground can well exceed the capacitance needed to trip GFCI --- either switch by itself closed - non trip, both = trip.

 

[GoldDigger]

If the two circuits are a true MWBC, fed from opposite bus phases, then the capacitance should be roughly equal from each of the two to the neutral. This minimizes the out-of-balance neutral current that will trip the GFCI.
But if one of the phases is always on, all the way to the end (receptacles) and the other is switched (lights), then whenever the lights are off the unblanced capacitive current may trip the 2-pole GFCI.
But in that case, I would expect the light circuit to also trip the GFCI IF the receptacle circuit is not energized.
What happens when you deenergize the receptacle circuit (downstream of the GFCI), turn the light(s) on, then if ithe GFCI holds energize the receptacle circuit? If the receptacle circuit then holds, one soution is to make the light circuit unswitched, with wireless remote switches at the far end instead.
Can you test by interchanging the conductors within the UF between the receptacle and light circuit? That (and probably the Megger test) will tell you whether there is some assymmetry between the two conductors in the UF.

 

[Dennis Alwon]

If the two circuits are a true MWBC, fed from opposite bus phases, then the capacitance should be roughly equal from each of the two to the neutral. This minimizes the out-of-balance neutral current that will trip the GFCI.
But if one of the phases is always on, all the way to the end (receptacles) and the other is switched (lights), then whenever the lights are off the unblanced capacitive current may trip the 2-pole GFCI.
But in that case, I would expect the light circuit to also trip the GFCI IF the receptacle circuit is not energized.
What happens when you deenergize the receptacle circuit (downstream of the GFCI), turn the light(s) on, then if ithe GFCI holds energize the receptacle circuit? If the receptacle circuit then holds, one soution is to make the light circuit unswitched, with wireless remote switches at the far end instead.
Can you test by interchanging the conductors within the UF between the receptacle and light circuit? That (and probably the Megger test) will tell you whether there is some assymmetry between the two conductors in the UF.

The original poster didn't state it was a dp gfci. My understanding was a sp gfci feeding the light and receptacles. I could be mistaken.

 

[Fred B]

Testing for where issue lays is realy mostly a process of elimination and labor time intensive. With multiple outlets each termination is suspect. Start at shortest point first termination, does GFCI hold, yes/no, yes- move on to next and termination and repeat until it doesnt hold. No - fault is between the 2 points.

The length of the circuit could also be a factor in the tripping. The longer the run the greater the voltage drop and that simple fact would increace the apparent losses that the GFCI is perceiving by the imbalanced load and it will trip. Could try to test by making the circuit shorter by testing at mid point by removing out of circuit the remainder of the circuit. Does it hold? Yes, make the circuit a little longer by reconnecting another section.

Now if your test indicates possible underground fault then locator and an A-frame with thumper will help to locate where to dig.

If you have simple capacitve losses you might also see that by measuring the EGC for voltage or amperage on it while circuit is on. This can be related to devices or simply conductor length. Remember the GFCI is not reading voltage on the ground but only the difference in what goes out must come back on L/N if the balance is outside of the 4-6mA range it will trip.

 

[GoldDigger]

The original poster didn't state it was a dp gfci. My understanding was a sp gfci feeding the light and receptacles. I could be mistaken.

I interpreted his statement that he needed to use "the same GFCI" because of the shared neutral to mean that it was a two-pole GFCI. If the two circuits come from one single pole breaker, then the setup you hypothesize would be legal.
I saw "circuits" and thought "breakers".

 

[winnie]

The original poster didn't state it was a dp gfci. My understanding was a sp gfci feeding the light and receptacles. I could be mistaken.

Huh, shows where our assumptions lead us.

I read "shared neutral ' in the first post as implying an MWBC. But it could also mean the grounded circuit conductor shared between two switch legs of te same single pole circuit.

I expect that apparent ground current caused by capacitive coupling to go down when both legs of an MWBC are energized, as GoldDigger surmises.

I expect that H-G capacitive coupling in UF will be different for different 'hots' because of the way the conductors are laid flat. One 'hot' is closer to the ground wire than the other.

Jon

 

[petersonra]

I interpreted his statement that he needed to use "the same GFCI" because of the shared neutral to mean that it was a two-pole GFCI. If the two circuits come from one single pole breaker, then the setup you hypothesize would be legal.
I saw "circuits" and thought "breakers".

I also assumed a MWBC with a 2 pole GFCI breaker.

But, the advice is the same. Basically run down everything it could possibly be and eliminate them one at a time.