I am replacing an old light fixture with a new one, which normally I do pretty easily. However, when I took down the old fixture, there are 3 black wires all tied off with a wire nut, 2 white wires together and 1 single white wire, plus 2 ground wires. I know the fixture was older (1983) and had black and brown wires, but none were attached to the black wires. I'm confused on how to proceed as the new fixture only has 1 white, 1 black, and the ground. Does anyone know how to do this?
The likely scenario is that the single white wire is the switched hot and the bundled white wires are the neutral. You need to confirm this.
With all wire bundles clear of each other and any metal, turn on the switch and then restore power at the breaker. With a non-contact tester, check the black wire bundle, the white wire bundle and the single white wire.
The black bundle should register hot. The white bundle should register not hot. The White wire should register hot.
Now turn off the switch (not the breaker) and check again. All should be the same except the white wire, which should now be not hot.
If that is the case, we have confirmed the setup.
Turn off the power at the breaker. Attach the white wire from the fixture to the white bundle. Attach the black fixture wire to the single wire. Attach the ground to the bare or green ground wire (if present) or to the metal box if no ground wire wire, or leave alone if no wire and a plastic box.
All should be well. If not, write back.
P.S. If this does work, the single white wire should have a black marking on it to show it is sometimes hot. Tape or a marker will do.
You have a switch loop
In things that weren't built in the last decade or so, one way of wiring a light switch was to bring power to the light then run a switch loop from the light location to the switch location. Instead of having hot and neutral like a normal power cable, a switch loop for a regular light switch has a hot and a switched hot, but no neutral, so they reused the white wire as either the hot or the switched and were supposed to designate it as a hot with a paint mark or tape flag. However, harried electricians and ignorant DIYers often forget said marking, leaving you in the situation you are in now.
- Turn the power off to the fixture at the breaker (if that's not already done)
- Use black electrical tape to flag the single white wire as the switched hot
- Wirenut the single white wire to the black wire from the fixture
- Wirenut the junction of the two white wires to the white wire from the fixture
- Connect the ground appropriately i.e. to the bare or green wires in the box if they are present, or to a metal box if no bare or green wires are present in the box. Simply wirenut it off if the box is plastic and there are no bare wires in it.
- Button things back up
- Turn the power to the fixture back on at the breaker
P.S. in stuff built according to today's Code, the problem you run into no longer exists because switch locations are required to have a neutral in most cases. This provision was added due to the proliferation of "smart switches" and other sophisticated switching devices, such as light dimmers, that require a neutral wire to power internal electronics. This also requires newly installed switch loops to use more...normal wire colors.