Last Updated on September 7, 2023 by Dave Farquhar
After reading where the red wire goes in a light fixture, someone asked me where to connect the red wire to a light switch.
That’s a good question. The first time I saw a red wire on a light switch, it certainly confused me. I have one in my kitchen and it scared me for years.
When you see a red wire in a light switch box, it can mean two things. It could be there is (or once was) a three-way switch in play. Or it could be the red wire is the wire going to the fixture, and the black wires are the live wires.
We’ll cover the easier scenario first. But first, let’s get my disclaimer out of the way: Never work on a light switch with the power on. Turn off the power at the breaker or fuse box. And if you don’t know which breaker controls the light you have, throw the main. Don’t risk injury.
Nothing fancy, one switch, one light
So on an occupancy switch like the Enerlite HOSS that cares about which wire is live, the red wire in the switch box connects to the red wire on the switch.
But what about a regular old light switch? If the switch that was there is a standard light switch with two brass screws on it and one green screw, you can connect the red wire to either of the brass screws. Connect the black wire to the other brass screw. If you have a ground wire, connect the ground wire to the green screw. If you don’t have a ground wire, leave the green screw unconnected.
Wiring a three-way switch
A three-way switch is different. This type of switch lets you turn the light off or on from more than one place in the room. This type of switch has two brass screws, usually on the bottom, and a copper or black screw on the top left.
Ideally, the old switch is still there, so you can wire the new one up the same way as the old one. Put a piece of tape on each wire with a number on it. Put a piece of tape with the same number on each terminal. Label the terminals on your new switch the same way, then disconnect the old switch and connect the new switch exactly the same way.
If you’ve already taken the old one out, find the other three-way switch. Check that one, and wire your new switch like that one. The red wire will go on one of the brass screws, but whether it goes on the left or the right depends on how the room is wired. If you don’t know how it was, connect the red wire to one of them, turn the power back on, and try the lights. If the switches malfunction, switch the lower wires on the new switch.
I hope this helps you figure out the right way to connect the red wire to a light switch.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
What if I wanted to add a second switch to that light switch with a red wire? We are taking out a light in our bathroom, and adding a light with a fan. How can I add the other switch for the fan? I got it all to work on one switch, then I added the second set of wires, and tried to run a pigtail from the hot wires in the box (black and red pigtailed together) to the bottom screw, and the white wire on the top screw. In the attic had all whites and all blacks together. It flips the switch now, and the one switch that works turns off the lights over my vanity, not just the light over my toilet. I’m guessing the problem is in the attic again?
Adding switches isn’t my strong point but it sounds like you shouldn’t have the black and red tied together now since they’ll be running to separate switches.
why would 1 switch only function properly if the other switch is on? they should both turn on independently correct?
Yes they should. That’s an indication that one or both switches aren’t wired properly.