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Discussion Starter · #1 · (Edited)
Hi all. I am stumped.

Yesterday I did a few minor updates on a home that was wired in the 80s.

I will try to be brief.

There is a (surface mount) receptacle on an outside wall of a home for a deck area, the deck is 30 ft x 30 ft and other than 30 feet away the deck it 100% closed in and water tight, under no circumstances can any snow or rain come anywhere near the receptacle, nor do they use this receptacle to plug in any power tools or anything else for use "off" of the deck.
Out of the top of the PVC receptacle box is 1/2 inch PVC conduit up to a water proof switch that feeds four light in the deck area. Coming out of the bottom of the surface mounted PVC receptacle box there are two, 12/2 NMWU wires that feed two sheds (via under the ground) in the back yard. Each shed has one receptacle and one light. Neither are GFCI protected. So I decided to replace the receptacle on the deck with a GFCI receptacle to protect the two sheds.
The bottom line? Once the GFCI receptacle was installed, it trips. Every time.... When I disconnect the two 12/2 NMWU wires that feed the sheds it doesn't trip, The GFCI operates fine as a receptacle and as a feed for the lights, but again, hook up the two 12/2 NMWU wires for the sheds,,,,,it trips.

I have carefully inspected the visible part of the 12/2's under the deck (before they go underground) and they are fine.
I inspected the 12/2 s from the entry points in the shed to where they enter a junction box, the wires are fine. I disconnected everything from the junction box except the incoming feed and capped off the wires and after doing so, still the GFCI on the deck trips.

There must be a short or something wrong under the ground between the deck and the sheds, no?
Also. I tried connecting the 12/2 NMWU's independently, one at a time, and once each one is connected by itself, the GFCI trips. The only difference is when I connect one of them to the GFCI it trips instantly, the other trips after about 3 seconds.

I am sorry this is such a long drawn out explanation but I wanted to give as much info as I could. It has to be a fault in the 12/2 under the ground, no?

Any ideas or suggestions will be much appreciated.

P.S. Everything works fine if the deck receptacle is not a GFCI.
With the deck receptacle standard, not GFCI, I temporarily, installed a GFCI receptacle in one of the sheds, and it works fine. (just some added info)
 

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Hackenschmidt
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Summary -

  • no issue with the underground cables disconnected
  • GFCI trips with either underground cable connected at the deck, disconnected and capped at shed

So it sounds to me like you have isolated the problem to the underground cables. (You could test the GFCI to make sure everything's OK but that seems like a longshot.) So it looks like both underground cables have a ground fault. Since it happens unloaded I think it's got to be hot to ground fault. You could megger the wires to confirm.

I am thinking you know this but you're just trying to be careful before you bring the bad news, you don't want to dig up those cables and find out you were mistaken. Another test, which would not require a megger, would be to run temp cables on the ground and power the sheds expecting that everything works without those underground wires connected.
 

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Discussion Starter · #3 ·
Splatz, Hi.

Thanks, I did run temp wires above ground and all worked fine.
I am thinking megger but since everything works fine when the under ground wires are bypassed, is there any point.?

Keeping in mind everything works fine without the GFCI. Up to and including I temporarily installed a GFCI inside one of the sheds, (no GFCI on deck) and it worked fine.
 

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Sure there are no underground splices/junction to another location ?

Keep in mind, a GFCI will trip with no load, and a ground neutral short.
 

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Hackenschmidt
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Splatz, Hi.

Thanks, I did run temp wires above ground and all worked fine.
I am thinking megger but since everything works fine when the under ground wires are bypassed, is there any point.?

Keeping in mind everything works fine without the GFCI. Up to and including I temporarily installed a GFCI inside one of the sheds, (no GFCI on deck) and it worked fine.
The only point to the megger would be double checking, if you run OK substituting temp wires, I'd be satisfied you've proved the underground cables bad.

It isn't surprising that everything works without the GFCI, the ground resistance won't pass enough current to trip the breaker and clear the fault. But, the ground resistance is low enough to trip the GFCI.

I personally wouldn't be interested in leaving the ground fault in place, if it isn't dangerous right now, maybe it is tomorrow, or when wet, or etc. I'd disconnect it and offer to fix it, if they insist, they can have someone else come reconnect it.
 

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We see this all the time. Someone has a UF cable running out to a light then the rainy season starts and the GFCI starts tripping.

Doesn't seem surprising that the cable is damaged.

PS what is NMWU? Is that supposed to be UF?

Sent from my Pixel 4 XL using Tapatalk
 

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I would borrow a meter (if you don't have one) and ohm the wires that go underground. You might be able to show the customer the wiring is compromised and remove all doubt from your mind.
 

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A code compliant work around might be to GFCI protect the deck receptacle and then bypass the deck GFCI with the shed feeds and just replace the receptacle in each shed with a GFCI.
I would tend to agree with this fix if the cable was in conduit but without any protection someone might cut into the cable and get shocked if they were digging or putting in plants. In this case the OP has no idea what depth the cable is at so removing the GFCI protection on the feeds to the sheds might not be the best solution.
 

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How far away are the sheds from the GFCI receptacle? How long is each run?
 

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I think you will find the trouble with a resistance ohm check without needing a megger. We do it all the time with just a vom. many times an audible tone tester will tell if there is a problem. clear all of the ends and check for resistance between all of the leads at each end. If there is anything less than a complete open there is trouble.
 

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Many times when I’ve been asked to check why a customers electricity bill is too high it is because of this exact thing. Damaged underground feeds to a shed or garage constantly leaking current to ground.

Do the customer a favour and take an amp reading on the feeds. Guarantee there is a decent amount of draw on the wires with no loads
 

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Many times when I’ve been asked to check why a customers electricity bill is too high it is because of this exact thing. Damaged underground feeds to a shed or garage constantly leaking current to ground.
Okay new to the contracting business people, this is great sales info! :)
 
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