Why would one wire go live when another is disconnected?

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Old 04-17-12, 05:39 PM
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Why would one wire go live when another is disconnected?

Hey everyone - question about home wiring that was done when the house was built about 20 years ago (which putting it delicately...is very shoddy at best). I'd like to think of myself as fairly knowledgeable in basic home wiring...but this one has me baffled and I really hope someone can help.

I am redoing a screened-in porch at the back of our house. There is currently a working outlet, and single-pole switch that turns on an outside bug zapper, and a wire running coming into a second switch...then leading into nothing in the ceiling (presumably for a future ceiling fan / light install that was never completed). I gutted the entire porch and using one of those audible voltage testers, discovered the wire than ran into the second switch / ceiling box did NOT work.

Not thinking much about it, I tried figuring out how the first switch (to that outdoor bug zapper) was wired. Most of the wiring for that switch and the outlet I mentioned is behind the plywood sheathing of the house...so it's near impossible to figure out where it goes. Initially it LOOKED like the power from the circuit panel came into the outlet first...and then went out to the bug zapper switch. (Why did I think this? The outlet had a B/W wire coming into it...and another B/W going out back into the wall somewhere. Tracing that wire along the basement foundation best I could...it looked like it went right into the bug zapper switch). So wanting to completely eliminate the bug zapper switch, I disconnected all of the wires that were connected to the outlet to figure out which set was leading to it.

Here's where I got confused. When I did this and re-tested voltage at the outlet, one set of wires was live...and the other set was dead. And that makes perfect sense. Yet the bug zapper switch STILL worked. OK - so my conclusion: the power for this bug zapper switch isn't coming from the outlet. No problem. But just because I was near it, I decided to also check that 2nd switch / ceiling wire combo...and what do you know - it was LIVE!

I got super excited it was working but then super baffled because I didn't do anything to that ceiling wire at all. In fact - all I did was DISconnect 2 sets of wires at a nearby outlet.

Confused but happy, I was ready to pack it in for the evening and re-connected the incoming / outgoing wires back to the power outlet. Flipped the circuit breaker on. No issues. BUT - now all of a sudden the 2nd switch / ceiling wire that magically started working was NO longer working - just like it hadn't been since I started this whole process.

Logic is telling me it's obviously got something to do with the outlet wiring...since that's the only thing I messed with. But why would the ceiling wire all of a sudden become LIVE when the wires going into the outlet were all disconnected? It's certainly a first for me. Can anyone help me figure this mystery out? Thanks gang!
 
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Old 04-17-12, 08:41 PM
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Well, yes, that is confusing.

Apparently, the switch for the bug zapper and the switch for the ceiling fan/light are wired ahead of of independently of the receptacle. The receptacle might be on a circuit that feeds GFCIs, not that it matters.

Just a wild-hair guess, but might the ceiling fan/light switch be a 3-way?
 
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Old 04-18-12, 10:39 AM
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Thanks for your reply. I can't believe it - but I figured it out by complete accident. Today it was doing the same thing - ceiling switch would have power sometimes, but not others...and all I was doing was flipping the circuit breaker - I didn't even remove the wires from the outlet. So now I'm even MORE confused. But then is dawned on me - could it be connected to another switch I was using?

The circuit panel is located in a small basement room with a switch to two small lights for that room. THAT switch is what was controlling the outside porch ceiling switch. As I would go up and down to check things...sometimes I'd leave the basement light on, and sometimes I switched it off. Hence the sporadic voltage I was getting at the ceiling switch that I thought tied into the outlet I had been working on.

There you have it - another fantastic wiring job by the PO that accounts for my last 24 hours of hell. Now time to start cleaning it up. Thanks again!
 
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Old 04-18-12, 10:50 AM
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THAT switch is what was controlling the outside porch ceiling switch.
So the PO wired one SPST switch downstream from another one? Boy, and I thought the PO of my house was dim!
 
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