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In my bathroom, I had an old wall lamp hanging above the mirror, that was connected to a junction box. I dismounted the old lamp and didn't pay attention to which wires it was connected. Now I need to connect a new lamp and I don't know which wires from the junction box should be used.

The photo of the junction box is below. There is one strand of wires (black, white, ground) coming from the top, and two strands of wires coming from the bottom. All black wires are connected together, and all ground wires are connected together. Now, white wires -- it looks like only two of them were connected, though I'm not sure about that -- maybe one disconnected from other two when I dismounted the old lamp. So the question is: which wires should I use to connect my lamp?

Additional detail: there is a two-way switch on the wall that controls the lamp. The new lamp has standard white, black and ground wires.

enter image description here

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2 Answers 2

up vote 4 down vote accepted

First recommendation; put labels on stuff. Sorry, nobody needs a Monday Morning Quarterback.

Do you have a meter? I would try metering out which wire gets voltage with the lightswitch going on and off.

My guess, based on the two way switch, is that the three black wires are; Line ( from the breaker ), feed to the outlet, and feed to the switch. The two whites in the wirenut would be the Neutral to the breaker box, and the outlet. I am guessing that the single white wire is a carrier from the switch, or in your case, switches. With this premise in mind I would check the voltage with a meter on the single white against ground while blinking the switch. If the meter blinks with the switch, then put the fixture black onto this wire, and white with the wirenuted whites.

If this guess is wrong, I have more questions. Did / does the outlet loose power when the lightswitch is off? If yes, then it's probably wired with all blacks together and all whites together. But again, I'd check with a meter before closing in all the boxes.

good luck, let us know the outcome

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1  
Why are we working with the power on? –  Tester101 Dec 24 '11 at 14:15
    
@Tester101 - that's the only way I know of for finding out where the power goes when it's on... –  RQDQ Dec 24 '11 at 22:33
    
@Trout, your guess was right, thanks! –  Leo Jan 30 '12 at 0:46

It's hard to tell from your picture (I can only see one disconnected wire), but you should have two wires, likely both white. In a perfect world one of the wires should be coded (marked) as the switched hot, however, in the real world that's hardly the case. So the challenge here is determining which wire is switched hot, and which is neutral.

You could test this with the power on, using two hands to hold the multimeter, and your third hand to flip the switch that may or may not be across the room. Or you could do it the safe way, with the power off. The first thing you'll need is a multimeter that can check continuity. Next with the power off, touch one lead to ground (The metal box should be grounded, but if you can use an actual ground wire that would be better). Touch the other lead to each disconnected wire in turn. One of the wires will give you a low resistance reading, while the other will give you an infinite reading. The wire that read resistance is your neutral wire, the other one is the switched hot. This works because somewhere in your house (main panel, service drop, etc.), the neutral and ground are bonded together.

As always, if you don't feel comfortable, don't have the tools, and/or knowledge to do the work. Contact a local licensed electrician.

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