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Bought an order home with a 200 amp service. In the main panel is a 100 amp breaker that goes to a sub panel in a workshop out back. In the workshop there's two 20 amp breakers on one side and a two pole 30 amp on the other with no main cutoff/disconnect for the sub panel in the shop. None of the dozen plus 8 foot ancient florescent lights worked when we moved in but everything else ran fine. Recently, all power has stopped without any breakers tripping in the shop or on the main in the house. If I dial my meter down to it's lowest setting, I can read maybe half a volt, give or take in the shops sub panel. The 100 amp breaker still shows 120 volts. I have tried resetting the breakers, reseated the ones in the shop, trimmed off and redid the connections for the feeder wires - which I discovered were aluminum. I have traced the wires through the attic where they come out and down a wall into the ground seeing no signs of damaged lines or junctions. Short of digging up about a 100ft of yard or ripping open the shop wall, I can't find anything to explain it.

After some testing, I have found some current on the feed lines in the main for the 100 amp breaker to the workshop. It's small, and I register some current on the lines as they enter the ground. We're talking less than an amp. A non contact tester alerts that voltage is present on the lines entering the ground. No current on the lines in the workshop. However, the non contact tester is alerting along the ground, cinder block foundation and lower walls of the workshop.

Thoughts?

Here's some pictures of the sub panel and the main.

Any advice would be greatly appreciated. What am I missing?1

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    Electricity usually a simple path, if power at the beginning and no power at the end, something is broken. Do not have enough knowledge to tell by the picture, but that 100 amp breaker might not be the correct one for that panel.
    – crip659
    Commented Nov 15, 2022 at 14:33
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    In sub panels the grounds and neutrals need to be separated/unbonded. Should not be the cause of your problem now, but should fixed soon. Usually there are ground buses for the panel you buy.
    – crip659
    Commented Nov 15, 2022 at 15:05

2 Answers 2

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Some ideas:

  • Confirm 240V between the two terminals of the 100A breaker when on, 120V between either one and neutral, 0V between the subpanel feed terminals and 0V between either of them and neutral. Check all and confirm. If that's not what you see, comment here and modify the question.
  • Use a non-contact voltage tester to check the wires at the house end, once in the attic and once outside just before they enter the ground. This might help pinpoint the location of a break if there is one, or perhaps the location of a junction box you haven't found.
  • Switch off the three breakers in the shop and switch on the 100A feed. Use a clamp meter to look for current in the feed wires in the main panel. There should be none. I'd be surprised if something underground severed both wires without leaving some current path to ground. Maybe not enough to trip the breaker. But zero would be surprising. If zero current I'd look harder for a loose connection above ground.
  • If everything points to an underground problem I'd disconnect all the wire ends and run a wire overground to do a continuity test on them before digging.
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  • Great answer! I really like your last bullet point on doing a continuity check before digging. I suppose we should also have asked if there had been any recent dirt work that could have severed the cable. Commented Nov 15, 2022 at 23:39
  • Here's what I know so far. 100 amp breaker in the main reads 240 ac across and 120 on each one with neutral. Reseated it, no obvious signs of damage. On the sub, I get 1/2 a volt across both incoming lines and ground, but only if the bottom right breaker is on(20 amp). Only getting 1/2 a volt on the bottom connection on each side, and the incoming. Nothing on the top two (white/left and black/right). I'll have to try and get my hands on a non contact and clamp meter to try the other checks. Thanks again for the help. Any thoughts? Color me puzzled. Commented Nov 16, 2022 at 16:07
  • Something on the bottom right circuit is producing half a volt. I would probably ignore that, but it should be easy to track down in a shed. Unplug everything, disconnect circuit from breaker and bus, etc. Probably not interesting.
    – jay613
    Commented Nov 16, 2022 at 16:42
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    A cheap Combo clamp/NCV meter is adequate for this job. Just buy one. That's how DIY works. You spend money on toys and you learn.
    – jay613
    Commented Nov 16, 2022 at 16:45
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    That actually isn't funny! :(. You should add this info to the question. Probably time to start digging. You may be able to locate the break with the meter probe but it may be dangerous to do that. At some random point you may create a nice current path through you.
    – jay613
    Commented Nov 19, 2022 at 11:42
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The spiders and their webs shorted everything out and ate thru the wires! OK, more seriously: A few observations.

  1. You mentioned 15 amp breakers in the sub. They are 20 amps. No big deal, just a minor correction suggested.

  2. The 100 amp breaker looks like it's loose. It might just be the fact that it's a larger breaker and it's fine.

  3. Obviously the 100 amp breaker is turned off. When turned on, do you get normal voltage at the breaker? Then on the sub, no voltage, or minimal voltage? If so, it's a simple problem? You've got break(s) in the wires leading to your shop. Simple problem, difficult to fix.

  4. Lastly, and this doesn't directly answer your question, but the ancient florescent lights (your 8' t12s) have a very difficult time starting if the shop is very cold. But it's unlikely all would not be working. There may have been a failure in one the the feed wires to to the sub to start with. If you used the 240v/30 amp breaker for anything and it worked, that means both hots were OK.

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  • 1.) Thanks for pointing out that oversight. I have been staring at breakers for so long trying to sort multiple things out that I must have gotten some mixed up. Appreciate it. 2.) I'll have to try and reseat that one later when I can kill the main. Daughter is on a feeding pump. 3.) Normal at the breaker, I believe and 1/2 to 2-3 volts on the sub. 4.) I assumed that the 30 amp fed the lights. As they never came on. The 30 amp looks like it feeds the circuits in the ceiling but I haven't been able to get access to verify/map out those lines yet. Commented Nov 15, 2022 at 21:15
  • It's pretty safe to re-seat a breaker with the main power on, but if you're not comfortable with that, it's best to turn main power off. I do it all time time, but I'm comfortable with live panels. So turn off the main power when convenient. Oops, hit enter by accident...continuing: Pull the breaker and check for any signs of over-heating, arcing, etc. Reseat the breaker, turn on main power, then turn on the breaker and as I suggested before, check voltage at the breaker and at the sub, if voltage at the breaker, but non at the sub, there's your clue! Commented Nov 15, 2022 at 23:27

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