GFCI intermittently trips in wet weather

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Old 02-20-18, 06:49 AM
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Cool GFCI intermittently trips in wet weather

Hi all - I have an intermittent problem with an outdoor GFCI circuit.
  • 120V circuit, 20amp, main panel mounted GFCI breaker. Line runs to an outdoor circuit. Circuit has an outlet near the back door. About 7 years ago we added an extender box to the outlet near the back door, then rigid conduit to extend the circuit to a small shed about 50 feet away. A portion of the conduit is above ground, along back of house, then it goes underground to the shed, coming up on the side to a j-box, then into the shed to an outlet.
  • There are two external j-boxes with regular weather resistant outlets installed on the portion that is above ground - the outlets are all relatively new I replaced them all last year. ALL j-boxes have rubber seals and the outlet boxes have domed clear weatherproof covers. All of the wiring has been checked multiple times to make sure we have good, clean, solid ground connections.
  • The only loads on the circuit are (a) outdoor 100 watt low voltage lights, (b) small 20 watt outdoor light on outside of shed and (s) a 40 watt fluorescent lamp in the shed. All operate fine.
Problem - whenever we get wet weather such as a heavy rain storm, the GFCI will trip off and will not let me reset it, typically until the weather dries out for a day or so. Then I can reset the breaker and all is fine, sometimes for several months until we have another bout of wet weather. Totally random, sometimes it stays on even when it rains. It seems that very heavy rain or a long period of rain (couple of days) will trip it. It never trips any other time, only when we get some serious wet weather. I've checked, double checked and triple checked all my grounds. I thought about installing a ground bar outside the shed, and running a copper wire to the outlet in the shed (why? I don't know, I tried everything else).
It's not a big deal, but an annoyance. Clearly the wet weather is doing something to cause the GFCI to trip, but since everything is sealed, grounded, etc. and the circuit works fine 90% of the time, I am at a loss in terms of trouble shooting this one. Any ideas on what else I should check?

Thanks in advance......
Ross
 
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Old 02-20-18, 07:18 AM
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I would move the GFCI outlet inside the house and have a regular outlet outside running off the protection side of the indoor GFCI outlet. GFCI outlets are very sensitive to any imbalance of the current flowing from hot to neutral.
 
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Old 02-20-18, 09:01 AM
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There are no GFCI outlets outside. The GFCI is a main breaker GFCI mounted in the main cabinet in my basement. All of the outdoor receptacles are standard weather resistant outlets under plastic weather covers.
 
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Old 02-20-18, 09:19 AM
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What kind of cable did you run in the rigid underground?
 
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Old 02-20-18, 09:52 AM
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Standard 14 gauge plastic coated wire. Hmmm, is it possible the plastic has cracked in one or more places and when enough moisture is around it "leaks" and creates a ground fault?? Hadn't thought of that....
 
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Old 02-20-18, 11:07 AM
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Standard 14 gauge plastic coated wire.
120V circuit, 20amp, main panel mounted GFCI breaker.
You need 12ga wire/cable on a 20A breaker. While that's not your GFI issue, it's certainly an issue if that's true.

Also, is it UF cable? (typically gray). Standard NM-B (Romex) cable is not suitable for underground or outside. This could be an issue if the insulation is degraded. Some insulation could have been damaged during installation with a shovel or rock... enough to expose the conductors to the ground, but only allow a small current if the ground is wet. Unfortunately, it's hard to find an issue like that without specialized equipment (Megger meter)
 
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Old 02-20-18, 11:14 AM
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Canít run that in conduit under ground,thatís probably where your problem is especially when it happens after a heavy rain.
Geo
 
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Old 02-20-18, 12:31 PM
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Sorry, misspoke. It is 12 ga wire. Inside EMT metal conduit. It's not buried romex (grey outdoor romex). But I thought it is okay to run the regular plastic coated wire outdoor as long as it's in EMT or PVC conduit. I understand that if the wiring cover got nicked or scratched during the pull through the EMT, then it may allow enough leakage to trip the GFCI. If I have to, I suppose I can simply re-pull new outdoor wire through the EMT to the shed.
 
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Old 02-20-18, 12:44 PM
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More...... my best guess without checking is that we used THHN. And yup, it's not outdoor rated. So, if I buy 50 feet or so of THWN, re-pull through the conduit, that might solve the problem. Jesus, what a pain in the arse, I hate pulling wire..... How do you guys who pull wire all the time ease the process? I've see sprays, etc. What is the best approach? Thanks all, I think you've helped me pinpoint the likely culprit...... R
 
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Old 02-20-18, 01:05 PM
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First thing is to replace the EMT as itís not rated for underground use, PVC is your best bet with THHW.
Sorry for the bad news.
Geo
 
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Old 02-20-18, 01:11 PM
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Virtually all THHN is dual-rated THWN, so I'm not convinced that's your problem. A skinned wire during installation could certainly be the problem. If the EMT runs underground it might have collapsed and cut the wire - EMT is not for underground use. I'd also be willing to accept it's just a matter of condensation and high humidity causing the occasional GFCI nuisance trip. It only takes a tiny droplet of water forming in the right place to kick it off -- entirely possible in an unconditioned outbuilding during a heavy rain.

Pulling wire is best done as a two man job unless you have a high quality spool rack, one guides the wires in and applies wire lube while the other smoothly pulls the tape or twine. You can get pulling lube in the electrical aisle of the hardware or bigbox - it will make a big difference.
 
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Old 02-20-18, 02:59 PM
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Ugh, yeah I was fearing that as this thread developed..... (sigh) Okay, well, gotta do it right. Will dig up the old EMT (thankfully it's only about 50 feet, and not terribly deep maybe 18 inches) and will install PVC with new THHN/THWN as recommended. Not the end of the world, just a PITA and a long weekend of digging and back-filling. And since I am replacing the conduit the pulling will be much easier, as I can feed it through sections before I glue them together (thumbs up!). This has been very helpful, and I suspect this is the problem - condensation forming in the metal EMT and a couple of areas where the insulation has been scraped or nicked = GFCI trip. Thanks all, will report back in a few weeks after I do the dirty work and we get a good rain storm. Cheers R
 
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Old 02-20-18, 05:22 PM
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Being a 120V 20A circuit that is on a GFCI breaker the cover/bury depth needed is only 12 inches.
 
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Old 02-21-18, 04:26 AM
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... much easier, as I can feed it through sections before I glue them together.


I'm not sure but I think I've read here that is not allowed, something about the glue?
 
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Old 02-21-18, 11:28 AM
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That PVC or any conduit must be completed before conductors are pulled in.
Geo
 
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Old 02-21-18, 01:08 PM
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You can run a twine through the conduit as you install it, but the wires should not be pulled in until the pipe is fully assembled and the glue is dry.
 
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Old 02-21-18, 01:35 PM
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I suspect this is the problem - condensation forming in the metal EMT and a couple of areas where the insulation has been scraped or nicked
Exactly what I have experienced a while back at friend's house. The GFCI trips after the rain.
Many months later, I have discovered that there was a small nick on the wire in one of the weather proof junction box. It was full of dead ants and it was leaking current through dead ants to the grounded junction box only when the humidity level is very high.
It was discovered accidentally when I plugged something to the outlet, which moved the wire and creating complete short to the ground.


While the code states conduits have to be fully assembled before pulling wire, I'd say it is ok to do so for small job like yours. The reason behind this code is to make sure the conduit is installed correctly to allow pulling wire inside.

There are THHN/THWN with 3 wires twisted together. This might allow easier installation.
https://www.homedepot.com/p/Southwir...8301/206205896

PVC conduits are very cheap and easy to work with. Make sure to use proper PVC conduit.
I have seen some people using DWV PVC for conduit, which is illegal. Have no idea why when PVC conduits are much cheaper.
 
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Old 02-22-18, 12:01 PM
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FYI, the PVC has to be the gray stuff for electrical wiring. Using the white PVC is a code violation.

Our neighbor almost got tagged for using white PVC. He figured out his mistake before the inspection and spray painted the PVC gray. It passed, to my amazement.
 
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