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I've moved into an older home and am trying to figure out the wiring. There's a small addition at the back of the house where I'm trying to figure out the wiring behind a blank wall panel.

Before digging in I'd like to turn the power off to the area - I've flipped every single breaker (in a poorly labeled box) but one and the power never went out to the room. The only one I didn't flip I couldn't flip - it seems stuck which I imagine could be a problem. Should I have it replaced or should I just try harder? Is it something I could DIY - is turning off the main breaker in the breaker box enough to replace a sub-breaker, or do I need an external disconnect?

There isn't much rhyme or reason to this box, poorly labelled with breakers turned on that aren't wired to anything as far as I can tell and have now been off for a week with no noticeable effect. The only breaker that was off I tried turning back on and it triggered the main breaker for the whole box - I assume that's horrible and dangerous and that there's got to be a short in the box itself?

In the attached image, the breaker that's on is the one that seems stuck, and the one that's off is what triggers the big breaker at the top of the panel (not pictured.) For reference, I believe we have 100A service.

Breaker panel with two breakers highlighted

EDIT:

This breaker is probably fine right - just throw it back in the panel /s

Tandem circuit breaker with visible damage

The burn may be from the breaker in the slot beneath this one, the breaker that tripped the main breaker. The one pictured here was the one I thought was stuck, turns out it wasn't, but I put a new one in anyway.

Referring to the panel with breakers 4L and 6R in red: Turns out the wires from the same cable were wired to two breakers - a black wire to 4R and a red wire to 6R (6L has no wire.) It seems to me that kind of setup requires a two pole breaker with the black and red each being on one pole?

Right now the red wire is still connected to 6R that remains off (and labeled "DANGER".) 4R is actually powering a small circuit with two lights and one outlet.

I was up to my elbows in the breaker panel today - there was a whole wad of disconnected wires, some with a pair (white) still connected to the panel neutral.

  • Did the breaker that you tried turning back on also trip when the main breaker tripped? – ThreePhaseEel Mar 16 at 20:18
  • Good question. I can't remember if I flipped it off before turning the main breaker back on. Could you illustrate what each scenario might tell you? – Matt Mar 16 at 20:22
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    If the branch breaker didn't trip when the main breaker tripped, then the branch breaker's defective (welded shut somehow -- not normal for a QO!). If the branch breaker did trip when the main breaker tripped, then the breakers are fine. Either way, there's a fault on that circuit that needs to be rectified. – ThreePhaseEel Mar 16 at 20:25
  • “QO” - is that a brand or type of breaker? – Matt Mar 17 at 14:14
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    Yes -- your breakers are Square-D QOs (very nice breakers, on the whole) – ThreePhaseEel Mar 17 at 14:16
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QO's are quality breakers, so your "off breaker" is probably a tripped breaker because the circuit it's feeding has a big short. When you reset it, the short was strong enough to also trip the main breaker. (i.e. trip the main first). That sometimes happens. The repair here is to hunt down and fix the short. It's either amateurish or failed wiring at a junction box, or a nail driven through the Romex.

Your breaker that won't turn off is a very serious problem and that breaker should be pulled immediately. It is an old-school "double-stuff" breaker. The new style QO "double-stuffs" look different, the switches are abreast of each other. That is fine.

  • This is good information - I didn’t realize there could be shorts that trip the main breaker, I thought it would always be the branch breaker (is that the correct term?) Thank you! – Matt Mar 17 at 14:09
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    @Matt yeah, breaker trip curves are actually published, and there's a big swath of "gray area" running right through the chart. It's absolutely possible to have an overload that is in the gray area of both the branch and main breaker, and then it's anyone's guess which will trip first. Or, if the overload is in the "instant trip" zone of both breakers, then it's a plain old race condition, again, anyone's guess. – Harper Mar 17 at 14:20
  • Are there any concerns about using a tandem breaker? It seems "code" in some places is that you can use whichever breaker configurations the manufacturer specifies on the panel, but the labels in my panel have no breaker illustrations on them (including any mention of tandems.) – Matt Mar 18 at 2:16
  • @matt you already have a tandem aka double-stuff breaker. That's what you'd be replacing. You'll need to use a tandem or you won't have enough circuits. – Harper Mar 18 at 2:29
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    Sure, that would be fine if you'd rather do that. – Harper Mar 18 at 4:59

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