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Presently I have a GFCI (120V) located on an outlet in my utility room of the house. From this outlet on the LOAD side, I have a #12 3 wire cable running in Schedule 40 conduit underground 150 feet to a pole outside in my yard. Things worked Ok for a while but now the GFCI trips all the time.

I unplugged the spot lights from the outside outlet box (protected by code with weatherproof plastic cover box), but the GFCI still trips. I am wondering if I should connect the Ground wire to a ground rod at the pole due to the long cable run which may have enough resistance to make the GFCI trip? I have replaced the GFCI inside the house, same result, checked the cable, no shorts or high resistance shorts.

Can't figure this out !

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    What type of cable was used for the run? – ThreePhaseEel Apr 26 '18 at 0:11
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    I suspect you have a ground fault, I’ve seen GFCI’s preform fine at greater distances. GFCI’s don’t trip via resistance, they trip because of imbalances. – Tyson Apr 26 '18 at 1:12
  • I have seen that also, and if the ground fault is not in the spotlights then perhaps it is in something plugged into the outside outlet. It doesn't seem likely that ground rods can solve this. But we have a lot of smart people on this SE who might have experience they can share as answers. – SDsolar Apr 26 '18 at 1:31
  • Also, does the GFCI trip if the outdoor cable is unhooked at the house-end? – ThreePhaseEel Apr 26 '18 at 1:45
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    The first question is does it trip with everything disconnected? With only an underground cable an no load? – Tyson Apr 26 '18 at 1:45
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Do not connect the ground wire to a ground rod at the pole! This is against the code and is very dangerous.

For underground wiring, you want to use UF-B cable. This is rated for direct burial or use in conduit. You should have used scehdule 80 for the conduit as it is thicker and stronger. Schedule 40 PVC conduit can break easier and may be what happened. Depending on how deep you buried it, the conduit could have broke and damaged the cable.

If you used standard romex cable underground, that could be causing a ground fault because the paper insulation in the cable can wick water. Although the paper is not touching the copper of the line and neutral in romex, if water is in the conduit, the cable could have deteriorated and could be shorting to ground slightly.

A big clue is that everything was working for a while and then it started to trip. Look for what has changed since it started to trip. Did you drive something heavy over the area the conduit is buried? Was there recent flooding?

But to reiterate my first comment, DO NOT GROUND THE GFCI TO A SEPARATE GROUND ROD! The circuit breakers will not properly trip during a short circuit condition in this situation.

  • I used standard romex cable. Schedule 40 Electrical use PVC conduit is allowed here in MA by code. It is buried by code 18" deep in sand with electrical warning tape buried above the conduit, and the conduit is buried per code in sand to within 4" of the surface. As I mentioned in original statement , the spotlights are unplugged. There is no other circuit/outlet on this line inside or outside the house.Nothing has caused damage to the conduit in it's entire run from house to outlet box. The Conduit is glued together, no way for water to enter the conduit. – Richard79 Apr 27 '18 at 3:49
  • @Richard79 water gets in. Anything underground is wet. – Tyson Apr 28 '18 at 0:18
  • Richard79, NM-B (romex) cable is not designed for underground use. As Tyson stated, anything underground will get wet. This is a standard rule of thumb. In the northeast, you will get frost heaving and that will break the PVC seals. The use of NM-B may be the problem. Water may have wicked into the cable and degraded the insulation. I understand that you tested it, but I'm assuming you used a standard multimeter rather than a ground test meter or dielectric withstand tester. A standard multimeter will not be able to measure the resistances that could be indicative of a ground fault. – EEKeefe Apr 30 '18 at 15:21
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Problem found ! First, I checked the resistance between all the three conductors. In order for a GFCI to trip, it takes only 5mA, so that would equate to about 24K ohms. There were no high resistance shorts. The problem turned out to be one or both LB's that were in the ground. I did not have any sweep elbows, and thought that gasket-ed LB's would be OK. Not so. After heavy rains or Spring thaws,water seeped into the LB's and would cause the GFCI to trip. When dry, no problem. I have since rectified this problem with sweep elbows. Thanks for all the suggestions guys, much appreciated.

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Maybe cable get rotten or moisture got in the outside outlet. If a GFCI trips, it trips because there is a current leak. You can try going at inside GFCI box, disconnect the cable going outside and see if still trip, if it doesn't you have a problem on outside wiring, if it keeps tripping you have to replace the GFCI.

  • The GFCI in the house is OK. I cannot tell if the cable has deteriorated inside the conduit along the run, however, at the point where I brought the line to the outside outlet, the cable look OK within the conduit. Being a run of over 100 feet in 1" conduit it would be tough to tie on another replacement run and pull it through the conduit because of an LB outside the house before the run heads up back. A lot of digging. So I am considering removing the GFCI inside the house and placing it at the outside outlet. – Richard79 Apr 30 '18 at 3:49

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