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Old 04-17-2012, 08:00 PM   #1
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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 04-17-2012, 08:18 PM   #2
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Why would one wire go live when another is disconnected?


I don't ever completely trust a non-contact tester---get your self a good tester.

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Old 04-17-2012, 08:42 PM   #3
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Why would one wire go live when another is disconnected?


Never rely on a non-contact tester for such an important task.

You are reading phantom or induction voltage there, bud. Go get a real tester -- one with a solenoid load in it -- and check your wires again.
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Old 04-17-2012, 09:14 PM   #4
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Why would one wire go live when another is disconnected?


OK - well I'm pretty certain it's not a phantom voltage. But to be sure, I just took a volt meter and literally touched the ends to the contact wires at the ceiling switch. Just like before - no current running through at all...UNTIL I disconnect those wires at the outlet again. Then the current all of sudden shows up at the ceiling switch / wire.
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Old 04-17-2012, 09:22 PM   #5
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Why would one wire go live when another is disconnected?


You said that you disconnected all the wires to the outlet....

There wasn't another splice in the box was there? Like a black and red wirenutted together?
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Old 04-17-2012, 09:49 PM   #6
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Why would one wire go live when another is disconnected?


For those who would like a visual, I've attached a crude diagram for you.

KBuz - there was nothing else in the Outlet box aside from the wiring mentioned here.
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Old 04-17-2012, 10:20 PM   #7
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Why would one wire go live when another is disconnected?


Oh and i should also point out the wires going to the bug zapper itself were hard-wired. There is no outlet for the zapper to plug into.
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Old 04-18-2012, 04:24 AM   #8
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Why would one wire go live when another is disconnected?


If you're not going to listen to professional opinion here, there is not much more that we can do for you.

Someone else mentioned you may have a neutral back-feed, which is a real possibility. But measuring voltage without loads applied is an exercise in futility.

My tic-tester lights up and beeps when rubbing it on my shirt. Those things will pick up all sorts of stray voltage, static, etc.
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Old 04-18-2012, 07:01 AM   #9
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Why would one wire go live when another is disconnected?


The circuit that started working when you disconnected things -- Did you say it would power at least 100 watts worth of lights? (You can rig up an ordinary lamp socket with wires with alligator clips on the ends and put an incandescent bulb in it.)

Disconnect the receptacle unit so as to recreate the situation where the wire became live. But this time reconnect together all of the white wires you rendered loose. (Use a wire nut or lengths of wires ending in alligator clips.) Does the suspect wire still become live?
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Old 04-18-2012, 07:12 AM   #10
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Why would one wire go live when another is disconnected?


Quote:
Originally Posted by john123drury View Post
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).
Describe fully, all cables and wires that are in the box that houses the second switch or as you call it in the diagram the "switch to ceiling fan"
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Old 04-18-2012, 12:39 PM   #11
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Why would one wire go live when another is disconnected?


Thanks for your replies everyone. 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 04-18-2012, 12:51 PM   #12
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Why would one wire go live when another is disconnected?


SO a switch in the basement controlled a potential ceiling fixture in your screened in porch??

Your whole issue stemmed from guessing where cable/wires went when they left junction boxes.
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Old 04-18-2012, 01:16 PM   #13
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Why would one wire go live when another is disconnected?


Quote:
Originally Posted by kbsparky View Post
Never rely on a non-contact tester for such an important task.

You are reading phantom or induction voltage there, bud. Go get a real tester -- one with a solenoid load in it -- and check your wires again.
Unless you are chasing down circuits, is a non-contact tester sufficient for the basic DIY stuff?

I use my non-contact just to make sure what I'm about to work on is not live. I learned that lesson the hard way once, when wiring up a ceiling light that was switched off, but managed to shock me anyway.

I also have a neon tester and a cheapo multimeter, but rarely use them on house wiring.
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