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I have a detached barn with an existing sub panel fed by 2 120v lines. Been working fine for years until today many of my florescent lights weren’t working. I checked the voltage at the main panel and both legs show 120 there. I removed the cables from the lugs on the sub panel and one of the lines shows 80V. So it appears I’m dropping from 120 to 80 V along the line somehow.

If I need to run new lines I’m probably looking at several thousand dollars. So before I do that could this be something at the main or anything else - or do I just need to pull a new UL line?

Edit: hot to hot 185v….hot to neutral (not bonded) 80v…hot to ground 80v. It’s a newer sub panel with hot hot and neutral fed to it and ground to earth rod at the panel.

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  • Measure hot-to-hot, each hot to neutral, and each hot to ground. Edit to report results. Mention if you have an old sub-panel with hot, hot neutral only in the cable (might have a local grounding rod), or a 4-wire feed with hot, hot, neutral and ground in the cable (and neutral/ground isolated at the sub-panel location.) And welcome to why I prefer conduit and wires to direct burial cables. Pictures would also help answers.
    – Ecnerwal
    May 13, 2023 at 18:06

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Phantom voltage, probably

You're saying all your loads died completely (not dimmed, died) and that your inexpensive digital DVM is reading 80 volts open-circuit on the disconnected wires?

That's "phantom voltage" caused by the way inexpensive DVMs work. It should be treated as 0 volts. If you doubt that, go ahead and connect it to a load (you know, like that subpanel right there) and put any load on it - even an old incandescent "night light" will suffice. Does the voltage go away? Phantom voltage. It's caused by capacitive coupling between the wires and has no strength - not even enough strength to move the needle on an analog meter. It is caused when an entirely disconnected wire runs alongside a hot wire for some distance.

So, this suggests a connection problem at the origin (house end) of the feeder, or an intermediate splice. A common mistake is to fail to torque the lugs to the specification called out on the breaker or panel label. Another common mistake is to use a wrong-brand breaker under the misconception that all 1" breakers are compatible. They are not - bus stabs vary enough that the wrong one will arc and burn up the bus stab.

Failures of a wire enroute are exceedingly rare - rarer still if the cable was buried in conduit or direct-buried at proper depth and surrounded by fine material to avoid rock penetration.

3-wire feeders, though

Note that a newer installation requires 4-wire feeder from the main panel - hot, hot, neutral and ground as a wire. Many people get confused and think a ground rod "is the same thing" - no, it is not; it serves a completely different purpose and cannot serve the purpose of a ground wire.

If the installation is old enough that a 3-wire connection is legal, then ground needs to be bonded to neutral in the panel so that at least, neutral can attempt to serve the role of the ground wire. However, this is not particularly safe. While we were banning it, Britain permitted it, and they are now getting taken to school on why it's a bad idea. Note that the failure which causes all the problems is the exact failure you're having right now, except on a neutral instead of a hot. So not "rare" at all.

The other thing is that if any metallic utilities (metal water line; gas line; cable TV etc.) are running between the buildings, the 4th ground wire was mandatory even before 3-wire was outlawed here. That's because you're getting a 4th wire via that other utility, and that means lots and lots of galvanic corrosion on that other utility, or current for which it is not rated.

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Appreciate the answers and help. Local electrician stopped by and saw at main one line was shorted to ground. Then he looked at where the cable enters the barn and discovered it wasn’t direct burial cable at all. Guy who put it in 7 years ago screwed me. So his take is run new line in conduit with 4 wires - 2 hots neutral and ground.

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Not sure of how your panel is wired, but in Canada, it should have a neutral wire also, not bonded to ground (only at the main panel this is permissable to my knowledge) When you say measured 80 volts, was that between the leg and neutral? What do you read line to line? Also, what is the wiring method, conduit, direct burial cable? Bad neutral connections can give you wacky voltages, retightening the connections at the supply and load side can help. If not, using a megger to test insulation may be the next step before you start digging.

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  • I was reading 80 from hot to neutral and then 185 between 2 hot legs. The neutral is not bonded to ground at the sub. It’s UL buried wire. We’ve been doing a lot of electric at the main and it could just be coincidence this happened at same time.
    – DaveInPA
    May 13, 2023 at 19:17

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