A friend was replacing a light fixture in his bathroom and ran into something confusing–a red wire in the electrical box along with the usual black and white wires. And when he hooked up his new light fixture, he did what I would expect the majority of homeowners to do–he connected the white wire on the fixture to the white wires in the box, and the black wire on the fixture to the black wires in the box. And then his light switch wouldn’t work–the light stayed on all the time.
He was on the right track when he asked what the red wire is for. That was the key to solving his problem.
Let’s get this out of the way first. When working with electricity, turn off the power to the circuit at the breaker box. If you can’t determine the correct circuit breaker, throw the main.
Normally you’ll see a red wire in places you would expect a ceiling fan. But in places like a bathroom or a wall light, if you see a red wire, it’s a good bet that the red wire is the wire that’s connected to the light switch. So the red wire is probably the one you want.
In my friend’s case, the solution was simple if not intuitive. I had him to connect the white wire from his light fixture to the white wires in the box. Then I had him connect the black wire on his light fixture to the red wire in the box. I had him leave the black wires in the box tied together but not connected to the fixture. When he did all that, his light switch operated the light normally.
On a related note, if you see an extra black wire in an electrical box separate from another set of black wires tied together in the back of the box, chances are that extra wire is the one that operates the light.
I also have a post about a red wire in a light switch, if you need that information.
What if you’re installing a ceiling fan? Then the red wire in your box goes to the non-white wire on your ceiling fan (which is often blue). The white wires on the fan go to the bundle of white wires, and the black wire goes to the bundle of black wires. If you only have black wires in your box, connect both the non-white wires from the fan to the black wires in the box.
Figuring out the intent of whoever originally wired the room can be a little bit difficult, but it’s doable. The thing to remember is someone who knows less than you has figured this out before, so you can too.