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About a year ago, I installed a secondary breaker panel in my detached barn. It is a very simple set up. I added a 50 amp single pole breaker on my main panel in the house, ran #4 wiring in conduit out to my barn. There I installed a sub panel with 3 breakers in it.

In the sub panel, the Black supply wire energizes the circuit rails, the white supply wire is connected to the Neutral bar and ground wire to grounding bar in panel. Breakers each have a single wire connected to each for 3 separate and very simple runs. One breaker controls a line of 3 outlets in the barn. The second breaker controls overhead lighting in the barn with one switch location. The third breaker is connected to a wire that is in conduit and goes underground to two smaller sheds to provide a light in each and an outlet in each.

These are all independent of each other, all done with new wire and have worked flawlessly for about a year now. The other day we had a very rainy day. I noticed that the barn lights were on, went in to check and confirmed that the light switch was off. This was obviously not right. I am not sure if the lights were fully illuminated or just partially, but they were on. I turned off the breaker controlling the lights and they were still on. Only when I shut the breaker going out to the sheds did the barn lights go off.

Somehow, the circuit controlling the shed lights and outlets is energizing the barn lights even with the breaker for the barn lights off and the switch off. I have rechecked my sub panel wiring and have confirmed that each circuit is wired properly and there is no visible way that those two separate lines should be energizing each other.

Somehow, power from one circuit is finding its way into a separate circuit and energizing the lights even with the breaker and switch turned off for that circuit. As I stated before, this system has worked flawlessly for about a year and this issue seems to come out of thin air. What did I do wrong?

Red wire is not connected to any source or to subpanel. The left most breaker is the one for barn lights. the right most breaker is the one that goes to shed and is causing my issues.

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    Pictures of the panels might help us see something. Most panels are made for split phase 240v and with you running only 120v something might have gotten cross. The heavy rain might also be showing a break in wire insulation somewhere. Assuming you are in North America.
    – crip659
    Jan 7, 2023 at 17:00
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    Pic added. Thanks. Yes, in North America. For clarification, red wire is not connected to anything. Not to source or to sub panel. Left most breaker is for barn lights, middle is barn outlets, right is for other sheds. (that is the one that seems to be causing my issue)
    – Steve
    Jan 7, 2023 at 17:23
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    Did you use wire/cable rated for wet/outside/underground conditions for the other shed and not NM-B/romex?
    – crip659
    Jan 7, 2023 at 17:51
  • It is regular romex in conduit. It is possible that there is some wetness in that underground run. We have been having crazy weather/ground frozen, heavy rain. I still don't understand how this could cause the issue I am having. The lights in barn are not connected to the circuit that may have gotten wet. And it seems to be bypassing the light switch in the barn.
    – Steve
    Jan 7, 2023 at 18:11
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    Bad insulation passing some power back though neutral wire might power the lights and be not enough of a short to trip the breaker. Undergrounds are expected to be wet, so romex not allowed.
    – crip659
    Jan 7, 2023 at 18:17

1 Answer 1

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Summarized from the comments and the OP's responses.

  • NM-B cable was pulled through underground conduit.
    • This is a code violation
    • The reason it's a code violation is now obvious - outdoor conduit is wet and water doesn't get along well with NM-B cable.

The solution is to pull the NM-B and replace it with properly rated wire.

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