Standard light wiring

Question

I am wiring a standard light fixture with a black and white wire. The box that I am hooking the light to has 4 wires coming out. 2 black and 2 white. I have tried wiring it to each set of black and white wires and I have tried putting all three black and all three white together. I’m not sure how to proceed at this point.

Answer 1

You probably have one cable for power and one cable going to a switch.

You need to find which one is the power cable by separating all the wires except ground.

The power cable black will be the only one hot with the breaker on, so be careful. Test by using a multi meter and go from black to ground. It will read ~120 volts. It can be done also with a non contact voltage tester, but they read other wires being hot with them so close.

Old style switch loops connected the white from the switch to the black power, black from the switch to the black of the light, The white of the power to the white of the light.

Answer 2

You can use the light fixture if you don't have a multimeter.

As the other answer(s) say: Two cables indicate a hot cable and a "switch loop" cable. The switch loop is a hot going down to the switch, and a switched hot returning to the light. The Code normally requires that white wires only be used as a neutral, but makes an exception when a black/white cable is used as a switch loop.

The Code writers realized if they decided to use the black as the hot down to the switch then the white returning from the switch to fixture would be the switched hot. If somebody removed the light without marking the wires they would have the blacks from both cables connected together, and two whites to connect to the light, with no way of distinctly knowing which is neutral and which is the switched hot without additional testing or marking. They thought that was a bad idea.

They decided to use the white down to the switch as the hot, and the black returning from the switch to the fixture switched hot. You have a black and white to attach to the fixture, and they match the wires on the fixture. This works well during construction, before sheetrock the electrician can see the cable routing, and can easily see which cable goes to the switch, knows to connect the white from that cable to the black of the hot cable. This leaves a black and white to connect to matching colors on a light after the sheetrock is finished. They also decided the white used as hot should be permanently marked to indicate hot, but that requirement is often overlooked by inspectors.

If the original fixture had been wired correctly you should've been able to just disconnect the old fixture and match wire colors to the new.

Using the light as a tester. Connect black and white from light to one cable. Isolate and insulate the wires from the other cable. Turn on breaker. Then do one of the below:

If light comes on Turn off the breaker. You have identified the hot set, the other set is the switch loop down to the switch. The switch loop white should already be marked with tape or pen, if not then make it so. Disconnect the connected blacks, connect the black that was connected to the fixture to the marked white, that sends power to the switch instead of directly to the light. Connect the black from the switch loop cable to the light. Turn power on and test switch.

or

If light doesn't come on turn off breaker, connect light to other cable, turn on breaker and return to first previous step. If light still doesn't come on then you have a bad light, or you damaged the switch or breaker when you connected all blacks together and turned on switch and breaker.