Where the red wire goes in a light fixture

Last Updated on May 12, 2023 by Dave Farquhar

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.

It turns out the red wire was the key to solving his light fixture problem.

Light stays on all the time after changing light fixture

If your light stays on all the time after changing your light fixture no matter whether the light switch is on or off, and there was a red wire in the box, that’s the key to the problem. Once you determine the wire coming from the light switch, you’ll be able to fix it.

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.

where the red wire goes in a light fixture
If you change a light fixture and find a red wire in the electrical box, chances are the black wire from the fixture needs to go to the red wire in the box.

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 for your lights.

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.

And while you’re changing light fixtures, you may have to patch some holes in walls. Here’s my advice on fixing small holes and large holes cheaply and relatively quickly.

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4 thoughts on “Where the red wire goes in a light fixture

  • November 25, 2015 at 12:31 am

    I wish I could figure out what they were thinking when they wired up the light switches in our master hallway.

    There are three separate switches — let’s call them 1, 2, and 3. When #1 is “on,” switches #2 and #3 operate as you would expect them to and turn the hallway lights off and on. When #1 is “off,” #2 and #3 don’t do anything at all, and the lights remain off no mater what position or combination #2 and #3 are in.

    Ultimately we just avoid using #1, which is inconvenient as it’s the switch closest to our bedroom door. I’ve considered removing the light switch altogether and replacing it with a blank plate just to keep me from accidentally flipping it off and then wondering why none of the other switches work.

    • November 25, 2015 at 8:43 pm

      Sounds like they used the wrong type of switch by your bedroom door, Rob. By using three-way switches at the ends of the circuit and a four-way switch in between, your hall light would work properly. It sounds like they may have left out the four-way switch or positioned it wrong.

  • November 25, 2015 at 12:54 pm

    One other common use of the red wire: for a three-way circuit. If you have a fixture(s) controlled by two or more switches (ex: a set of hallway lamps with switches on each end of the corridor) the red wire is often a feed between the switches.

    Semi-related: none of the upper (or lower) outlets work in a single room (typically a master suite): try that switch by the door that you could never figure out. Older homes wired an switch on the entry panel to half-plugs (what that break apart on the outlet is for) to control the owner’s night stand lamps on either side of the bed.

    Continuing the core dump: “Caution!” I have seen the red wire used to supply the ‘other side’ of the 220 (110-N-110) to rooms. This is a bit old, but likely something you could find in a renovation. The line is supposed to be clearly tagged, but a coat of paint covers all. Your “Light Up Stick” tester will show you ‘hot’, but it does not tell you that between red and black is 220v.

    • November 25, 2015 at 8:46 pm

      Thanks, Dan. I forgot about the 220v wrinkle, though in houses of a certain age, 220v outlets certainly are pretty common, for powering window air conditioners.

      I’ve also encountered the wall switches powering half outlets, on more than one occasion. One time I spent hours trying to figure out a dead outlet, only to finally realize it was powered by a switch.

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