So I decided to replace the ailing motion sensor light on the garage eave of my nearly 50 year old split-level.
I have replaced motion sensor lights several times in my small barn, but all the wiring there is exposed, the wire colors are all to code (I did the original wiring) and apart from the usual hassles of filth, old stiff insulation and hard rafters, life is good.)
My 1st and fundamental mistake --- I didn't take a picture of the old wiring when I removed the eave light. I'll never do that again.
After taking off the old light, I discovered two 3-lead cables coming into the box.
Cable #1 had a hot (black) lead with 120v tested against its own ground (bare) and neutral (white). (Using a voltmeter)
Cable #2 was also black/white/bare ground. THAT black lead was carrying no voltage when black was tested against its own white and ground. HOWEVER, if I tested the two blacks, I got 120v.
I was confused.
The line cable was pretty obvious and I figured (oh, silly me) that black/black and white/white made sense. I know - in retrospect, really stupid. I connected black/black and white/white and threw the breaker on. The breaker did NOT activate and the light seemed to work.
However, my wife noticed that the porch light (to the left about 15 feet) went on although the single pole switch controlling it was off. Whaaa? My wife tried turning the switch to the on position and the breaker blew. I reset the breaker and then the light stayed on regardless of the switch position without throwing the circuit breaker.
At this point, I disconnected everything at the eave. Then the porch light and switch worked just fine.
I decided the better part of valor was to cap the 2nd black line and simply connect all the grounds, use the hot black lead from cable #1 to power the eave light and connect all the whites.
Now everything works normally. I have absolutely no idea where cable #2 goes, but it obviously interacts with the porch light circuit.
Nothing in the house seems to be affected by leaving black on cable #2 capped and unused.
Several questions: I tentatively am scheduled to have an electrician come out and look at this mess in 2 days. Good idea, yes? OR given the lack of voltage between the whites, I could simply call it good. OR I could disconnect the white lines, cap white #2 and just consider the entire cable to be an orphan. Finally, any idea as to how this happens?
Sorry this is wordy, but I figure with spaghetti wiring the more precise the description the better.