Underground wiring

How long will standard NM 12-2 wire (like “Romex”) last if buried directly in the ground? Answer: about 11 years.

For past year or two, the GFCI outlet in our garage has tripped occasionally. We were never able to figure out exactly what was going on, despite lots of experimentation and replacing the GFCI outlet (in case it was faulty). The problem is, unfortunately, that our chest freezer is on the same circuit in the garage (there’s only one circuit in the garage…)

Well, on Friday, Dusty called me at work and told me that my homemade power outage alarm was going off. The GFCI had tripped again. We unplugged absolutely everything on that circuit, but the GFCI still tripped. This time I had an idea. It was pouring outside, and had been for some time. We have a post light in the front yard, installed by the original builders. They ran standard indoor wiring out through the garage wall, under a planting bed, under our front steps, and about 40 feet to the post light. I unwired the light and left the circuit open, but the GFCI still tripped. Finally, I disconnected the outside wire at the outlet, and lo and behold, no more ground faults… I deduce that the insulation on NM wire has decayed somewhere outside and has a ground fault when wet. I’ve never done any digging in that area, so I didn’t damage it.

Especially at the time the house was built, the proper UF wire (rated for underground) didn’t cost that much more than the NM wire. I used a whole bunch it when running power out to my shed, and it wasn’t too hard to work with. At this point, I need to run a new wire to my yard light, since that light is required by our local code. I’m going to either have to dig a 100ft trench to the breaker box or find a way to fish the new wire under the steps where the original wire is. If the installation had been done properly in the first place, I could have saved a lot of trouble…

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