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I'm swapping out my light switch. It has three black wires and I originally thought it was one hot, one to the light and one to the other switches (where-ever they may be). This is how my 1st floor switches works.

However on further testing and much frustration I found the following (at least 2 switches and on 2nd floor)...

  • one hot black wire into the top of the switch.
  • two black wires on the bottom of the switch BOTH feeding to the same light fixture.
  • I've replaced the light fixture before and don't recall two black wires.

Any thoughts as to why there are two black wires from the switch feeding the light fixture?

Also this light fixture is part of a bunch that are controlled by two 15 amp breakers (e.g. I have to shut down both breakers to cut the power to this circuit). Can anyone explain why it was done this way?

Thanks on-ca

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    I'm assuming this is not a 3 way switch? I'm not sure enough to make this an answer, but my first thought was that maybe a ceiling fan/light combo with separate fan and light switches was once installed there, and now one wire is extra. Also, it seems to me that if you have a fixture that requires turning off two breakers to cut power, you have a problem... – izzy Sep 8 '20 at 13:08
  • Hi izzy, yes this is 'not' a 3-way switch. My only guess is it is set up to support a 3-way and they just wired the two black wires to as the same out? This is all OEM from the builder back in 2003. Yes still not sure about the two breakers. It seems they fed two 15 amp breakers into the 2nd floor circuit to increase amps to 30? – on-ca Sep 8 '20 at 13:18
  • I'm not an electrician, but I believe it is likely a code violation if there are actually two breakers feeding one circuit. I think that usually when this happens it is a wiring mistake. I'd expect to find that one breaker is just for the questionable circuit, and the other breaker controls a different circuit as well as wrongly supplying power to the questionable circuit. – izzy Sep 8 '20 at 13:44
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    Are you sure about which is the hot and which is the load? The two breaker deal is probably a multi wired branch circuit where two hots on different legs share one neutral. both breakers should be tied together and both have to be off when working on the circuit. It is not a code violation. – JACK Sep 8 '20 at 14:37
  • as jack said 2 adjacent breakers sharing a neutral is a multiwire branch circuit, depending on the age of the home it could have been built prior the requirement for handel ties but that is a good safety to add if the breakers are both odd or even locations this would be a serious code violation wires below 1-0 are not allowed to be paralleled and residential paralleling breakers is a code violation. – Ed Beal Sep 8 '20 at 15:12

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