I have a tool shed and an outdoor outlet 100 feet away that is supplied with electricity from a GFCI mounted on the outer back wall of my cabin. The outdoor line is buried in PVC conduit (see attached photo). About one year after installation, the GFCI started to trip. I dug up the conduit where the splice was, and sure enough the conduit was filled with water. I had spliced the line in the conduit box with wire nuts, with no attempts to waterproof it beyond sealing the box with silicone – a naive move, indeed. After I replaced the (now corroded) wire nuts, I sealed the connections with a rubber insulating compound, and filled the conduit box with non-corrosive silicone sealer.
Two months later, after a few days of rain, the GFCI is starting to trip again. Any suggestions?
Replies
The GFI may be shot. Try replacing it first. The darn things are very touchy-they don't like moisture.....go figure. A permanent fix would be to splice into the line before it hits the current GFI box and install another box inside with the GFI in it and replace the current one with a regular recp. Be sure to run the line going out to the load side.LOL :) Lite
Thanks for the reply. I tried replacing the GFCI outlet when it went out the first time, hoping I wouldn't have to reach for the shovel. No such luck. If I had it to do over again, I wouldn't have an underground splice period. Live and learn.
M
If it were me, I'd pull out the whole thing and replace the line with continuous, non-spliced wire. A ground fault circuit interrupter is designed to be extremely sensitive to any current leakage so that it can trip fast enough to save a life. It wouldn't take much moisture to create a fault large enough to trip it.
If you do, of course, attach a pull string to the old cable. Pull the pull string through by pulling the old cable out, then use the pull string to pull the new wiring through.
Silicone is not good to use around electrical components, because the acetic acid it gives off when it cures is corrosive.
Underground conduits are always considered to be a "wet" location. I have yet to find an underground conduit of any length and installed for any substantial period of time that didn't have some water in it. PVC conduit rarely seals water tight for long. Even above ground it is more raintight than actually sealed. A smart electrician will try to use bells, as opposed to couplings, on vertical runs and point all bells down to drain. I once had to work on a panel under a six story vertical run that constantly dripped water on the busses. Random pops and sizzles. No fun with 480v. A poor installation.
Those conduits that don't allow in water from outside allow air to move, I know that there are seals but here again rarely are they perfect for long, and moisture to condense. Within a few weeks the conduit is filled. Even explosionproof seals commonly leak a bit just not enough for any trapped gasses to pass a flame if the explosion occurs inside the conduit.
I would replace the GFI first. Make sure the GFI stays dry. An intermittently damp GFI can confound troubleshooting. It gets damp and trips. Dries and resets easily. Typically the electrician only sees it dry. In this case it is easy to replace the line, box and the GFI several times and still not solve the problem. I once had one tripping only when the HOs used the sprinkler system. In a wet season the sprinkler might only be used once a month or less.
Several different electricians had looked at it and replaced parts. It was only when I rolled up in the dry wintertime and the sprinkler operating that the problem became clear. A retractable sprinkler head was twisted around and spraying directly on the GFI defeating the "weatherproof" cover.
The "in use" covers, ones with a dome and opening for cords so they remains weather resistant even when something is plugged in also tend to seal and drain better than the cheaper flip covers. The good ones are well worth the $20.
Also check to see that the box on the house isn't allowing conditioned air in. This can allow any moisture coming from the underground conduit to condense and partially fill the cooler box. Same thing can happen if heated, moist, air is allowed to flow into a box exposed to cooler outside temperatures. Seal, as well as possible, both routes or conduits coming into the box. Duct seal, an inert and removable putty that comes in blocks, can be stuffed firmly into the conduit openings. Don't use regular caulk or foam. Replacing conductors can become much more difficult if you do.
When none of the above works replace the conductors with continuous runs of wire to eliminate the splice. THHN/THWN would be the most popular choice for insulation. With a 100' run, the electrons see 200' (there and back), to the tool shed I would tend to oversize the conductors a bit to keep the voltage drop in line. I don't have the figures here to calculate the voltage drop so I'm thumbnailing this. Assuming a 20A circuit I would go with 3-#10s and transition back to #12 at each end.
#10s commonly come in longer lengths on reels, don't buy it by the foot unless your made of money or get a really good deal, so I would go with a common color to keep from being stuck with lots to spare, white would be my first choice, and phase the hot with black tape, red would be more definitive but black tape you probably have, and strip back all exposed grounds.
Another thought is that if the conduit is buried deep enough, typically 18" below grade, you could move the sensitive GFI to the shed end of the run. A GFI protected run can be run without conduit as shallow as 12" but at this depth without conduit expect trouble from aerators, shovels and rototillers.
Some runs leak enough current along their length to trip a receptacle type GFI intermittently, typically the more expensive breaker type GFIs are less sensitive and more reliable so this can be a not so cheap option. A megohm or highpot test can spot this situation easily. Especially if the ground in the conduit is bare. Sometimes a single conductor is defective with insulation with pinholes or a bit thin from the factory. Sometimes the insulation has been damaged when it was pulled in.
"Even explosionproof seals commonly leak a bit just not enough for any trapped gasses to pass a flame if the explosion occurs inside the conduit."
BTW, explosionproof equipment is not designed to be gas tight, but rather to have a long enough path that an internal explosion is cooled off enough by the time that it gets outside that it does not ignite gasses on the outside.
Thanks for the detailed reply. I am sorry to report that the GFCI is dry, still tripped when replaced, and that there is no conditioned air entering the conduit.
This circuit has one long run (>100 feet) to an outdoor light and outlet, and a short run (~20 feet) to a tool shed. The long run was a lot of work to install, and I am loathe to replace it. I am considering moving the underground splice above ground by installing a weatherproof outlet on a post at the start of the long run (with the splice inside this box), and replacing the short run to be able to reach the new splice. Of course, it will require some careful conduit cutting and gluing to do this. Does this approach sound reasonable?
M
Sounds like it is worth a try. I would pull the splice up and make a quick wire nut connection, above ground as a test, to see if the splice is really the problem. No need in installing an above ground junction box if it won't solve the problem.
Unfortunately an underground run can sometimes have pinholes, or even weak insulation, that can leak enough current to trip a GFI. Doubly frustrating this seem to happen more in longer runs where replacement is a pain. This is another advantage to installing conduit. It makes replacement a breeze.