I'm replacing a light fixture in my house and am a bit confused about how the old fixture was wired.

There are two pairs of wires, 2 black, 2 white and 2 ground. The two black wires were capped off together and not hooked up to the old fixture at all.

The white and beige wire were connected two the old light I removed. I guessing one set of wires is power coming from the box and the other set is from the switch. The switch is a dimmer.

The light I bought has black and white wires. Any ideas on how to tell which ones to hook them up too?

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Warning: You'll be working with live electrical wires during this procedure. If you don't feel comfortable doing so, please contact a local licensed Electrician.

  • Set up your multimeter (or voltmeter) to measure volts AC.
  • Very carefully remove the caps from the white wires, making sure the wires do not come into contact with anything.
  • Turn the switch to the ON position.
  • Touch one probe from the meter to the bare grounding conductors.
  • Touch the other probe to the exposed end of one of the "white" wires.
  • Touch the probe to the other "white" wire.

You should have measured 0 volts to one "white" wire, and line voltage (~120 volts) to the other. Using a bit of black tape, or a black marker. Mark the "white" wire that measured line voltage to ground.

  • Turn the breaker off, and verify power to the circuit is off.
  • Connect the white fixture wire to the unmarked "white" wire from the ceiling.
  • Connect the black fixture wire to the marked "white" wire from the ceiling.

Warning: You'll be working with live electrical wires during this procedure. If you don't feel comfortable doing so, please contact a local licensed Electrician.

  • I like your warning. – WarLoki Aug 24 '15 at 10:43
  • Do you mind if i use it in the future. – WarLoki Aug 24 '15 at 10:45
  • @WarLoki Go for it. – Tester101 Aug 24 '15 at 10:49
  • What would be the worst that would happen if I just hooked it up? If I have the wires reversed will it just not work? Or will something crazy happen with sparks flying everywhere? If the worst that could happen is the breaker would flip I'm tempted to just hook it up, and if it doesn't work just hook up the other way. – Walter Kuhn Aug 24 '15 at 15:35
  • The light bulb will absorb light, instead of emitting it. – Tester101 Aug 24 '15 at 15:38

I partially agree with the answer from WarLoki. I'm concerned though. You mentioned that when you initially removed the old fixture that the black wires were wire- nutted together and the two white wires were connected to the fixture wires. This could also mean that the fixture is controlled through the Neutral line. As mentioned , get yourself a Voltage Tester or DVM (digital voltage meter) or you can wire a pigtail to a light bulb socket, so you can find which of the 4 wires is the common or hot. Check for voltage by opening and than closing the known toggle switch. If you find that a wall switch is controlling one of the white wires It is unsafe because the line will always be energized. I believe that may be the situation you have from the photo you posted and from your description. If this is the only fixture and there is only one single pole switch controlling that fixture and the power line starts at the light fixture this would indicate the wires are reversed. the white wires should be twisted together and the two blacks wires should be connected to each of the light fixture wires. I think you should double check this circuit and be positive that a single switch is controlling just this lone light.

  • The old light was hooked like that for several decades without any problems. When you say unsafe do you mean risk of starting a fire? Or of someone getting electrocuted? – Walter Kuhn Aug 24 '15 at 17:16
  • I apologize. I didn't mean for my answer to worry you. I did want to point out (as others have) that one of the white neutral wires in the photo you posted, which is connected at the switch is probably a hot and should be labeled as such. That is the only concern that I wanted to point out. – ojait Aug 24 '15 at 19:55

The fastest way to check is to use a volt meter and check between the white wire on the left and the (used to be) white wire on the right you should get 120v +- between the two. Now test between the white wire and the ground and see which one gives you 120v, that should be your switch leg, power to the fixture. With that out of the way, turn off the switch or breaker before wiring the fixture. I think the white wire that is to the right of the picture is the switch leg that goes the switch. That would need to be marked with black tape(that is the hot/power) and makes up to the back wire on the fixture. I think that the white wire on the left is a neutral wire and makes up to the white wire on the fixture. The green or bare wire in the fixture makes up to the bare wire in the back off the box. I see that you have aluminum wiring so make sure if the fixture has copper wires to use the correct wire nuts.

  • How can you tell just by looking which is the switched hot, and which is the neutral? – Tester101 Aug 24 '15 at 9:53
  • Without been there it is a guess I will modify my answer, but lets see. – WarLoki Aug 24 '15 at 10:27
  • I think the white wire on the left is the one going to the dimmer switch just because the direction of that set of wires goes toward the dimmer switch, and the other comes from the direction of the box. Not sure if that's sound reason though as I'm not really familiar with how electricians would run wire when building a house. – Walter Kuhn Aug 24 '15 at 17:13

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