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.
Where the mystery red wire goes in the light fixture
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. Safety first.
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 the light switch controls while the black wires are always live. So the red wire is probably the one you want.
In my friend’s case, knowing he had a red wire in his electrical box, the solution was simple if not intuitive. It was three steps.
- Connect the white wire from his light fixture to the bundle of white wires in the box.
- Connect the black wire on his light fixture to the red wire in the box. Not the bundle of black wires.
- Leave the bundle of black wires in the box tied together but not connected to the fixture.
After he did all that, his light switch operated his new light the way one would expect it to.
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 black 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. And if you have the opposite problem, where the old fixture worked but the new one doesn’t, and you verified you wired up the red wire correctly, the problem may be the wire nut. It’s happened to me.
Red wires and ceiling fans
What if you’re installing a ceiling fan? Then the red wire in your box goes to the whichever wire on your ceiling fan isn’t white. Frequently it will be blue. The white wires on the fan go to the bundle of white wires. The black wire goes to the bundle of black wires. If you only have black wires in your box, connect both the other-than-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. You can figure out where the red wire goes in a light fixture.