I recently bought a property with a house and shop. Previous owners ran 240v from a 100amp breaker in the main house panel out to a sub panel in the shop. So thats two 120v lines and a neutral for three wires going out there.

The problem I’m having is that in the shop, some circuits cannot support any load. They will test at 120v until you turn on a single light and then it goes down to 10-20v. I determined this to be from one of the 120v lines. Circuits on the other 120v line work fine. The bad line will test fine (120v) at the last connection point at the house before it goes underground, and test bad (10-20v) at the first connection point at the shop where it comes above ground. So my thinking is that the line has failed underground somewhere.

My questions are, is there any other possible explanation before I start digging? Open neutral? Something else I’m not thinking of?
If I were to find the damaged spot in the underground cable, can it be safely repaired or does the whole line need to be replaced? The lines were not run in conduit but they look like the kind of heavy duty plastic encased lines that you’re allowed to do that with.

Location: North Carolina, USA

 

It sounds like you could have a loose connection or broken wire. What's confusing is that you say you are getting 120 VAC until you put a load on the circuit. That leads me to think there is a loose connection just barely hanging on somewhere. You might even find that turning on a 240 VAC device like a water heater or electric heat will get power to the problem circuit while 240 VAC devices don't run or seem to be getting half power.

If you can't locate the problem at the box on either end of the shed supply line digging and replacing the wire may be the next step. Unless you have the test equipment to locate the break it's probably cheaper & easier just to trench the whole way and lay a new line. Don't forget the new should have four conductors; hot, hot, neutral and ground.

 

Definitely a bad leg. Today's code requires 4 conductors. You can repair what you have if you can find the bad spot or replace with new 4 conductor feeder.

 

Hi, are you testing the feeder at the sub panel when you turn a light on?
Geo 🇺🇸

 

Yes that’s what it seems like, that the line is just barely hanging on somewhere. Sometimes the circuit does work but a light plugged in flickers. Mostly it just doesn’t work. I don’t have a 240v device at the shop to try but I expect the behavior would be similar. Is there such a thing as a contactless voltage meter that can give multimeter-like readings without touching bare metal? I thought that could help find the break underground, if I dig in some spots and test the cable here and there to get to where the break is. Previous owner would have driven big trucks and equipment over where the lines are and I don’t think they’re buried very deep so I know a highly probable spot where the break is. How should it be repaired if I can find it? Other spots of the cable are under concrete and pavement so it’d be really ideal not to have to dig through that.

@Geochurchi, yes tested the feeders and the spot where the wires come into the shop from underground.

 

Contactless testers are marginal under the best of circumstances and your wires should be buried much deeper than a tester can sense. Years ago the underground service to my home broke a leg. The power company was able to use their special tester to know exactly where to dig. Without that specialized equipment I don't know of a way to locate an underground break especially if it is still partially connected.

 

@pilot dane, I was imagining digging to expose the cable and using some kind of tester to measure the voltage amount present in that spot. Not try to detect through the ground, but to dig to a spot in the line and measure voltage to tell if the break is up or downstream of that spot.

 

You didn't say how long the line was but finding an underground break wouldn't really be terribly helpful unless you're talking hundreds of feet. Underground breaks are pretty hard to fix as the lines get corroded quickly.

 

There is locating equipment out there that can find underground faults. The bad thing is they are quite expensive. If you call around you may find a contractor that has one and they can locate the fault. This is similar to what we have: https://accusrc.com/product-Dynatel-...RoC17oQAvD_BwE

We locate and repair faults all the time. Most cases we have to install a ground box to make a splice but if your lucky and can make a VERY good water proof splice you might be OK.

 

Thanks for all the input. My question now is, given that one of the legs has failed, is it hurting anything to leave it as is? Would it cause other issues in the house? Should I disconnect that leg from the main panel?

 

If that leg has failed it could be leaking current into the earth and costing you money. I would recommend disconnecting it from the breaker