If there are lights in your house that can be controlled from two different light switches (as opposed to just one), then the light is commonly referred to as a three-way light, and the switches are referred to as three-way light switches. Here’s how they work.
If you know anything about circuitry, then you at least probably know that an on/off switch is perhaps the simplest piece of circuitry there is. But once you add in a second switch to control the same object, things can get a bit complicated.
How a Light Switch Works
Before explaining how three-way light switches work, it’s important to know how a regular light switch works. These are called single-pole light switches, and they can turn on or off a light fixture from a single location. These are the most common types of light switches and are mostly found in bedrooms, bathrooms, and other simple room layouts where you would really only need one light switch.
Note: In the diagrams below, the neutral “return” wires and ground wires aren’t shown in order to make the diagrams as easy as possible to understand. If you’re worried about this, just know that the neutral “return” wire in the circuit doesn’t connect to any of the switches and simply continues on, whereas ground wires will connect to the green screw on every switch.
In a traditional wiring setup with a single light fixture and a single switch, you have a hot wire coming in from the electrical panel that supplies the power to the light fixture. However, a light switch is installed in line with that hot wire. So when the light switch is off, it breaks the connection of the hot wire so that power can’t get to the light fixture. When the switch is turned on, the hot wire is reconnected, thus supplying power to the light fixture.
Adding in a Second Light Switch
Things get a bit complicated when you introduce a second light switch into the mix, but it’s still pretty simple once you know how it all works.
A three-way light switch is different than a traditional single-pole light switch, as it contains an extra screw to connect an extra wire. This is known as the “common” screw and it’s usually black (instead of brass or silver). Another dead giveaway of a three-way switch is the absence of “On” and “Off” markings.
This is why whenever you need to replace a light switch in your house, it’s important that you get the right kind of light switch, since you can’t use a single-pole switch in a three-way circuit.
The diagram below provides a simple layout of a three-way light setup, and you can see the red wire is the extra wire that’s needed to make it all happen (conveniently enough, red wire is normally used in real life in three-way circuits as well).
How it works is that the hot wire coming in from the electrical panel is connected to the common screw of the first light switch in the circuit.
On the other side of this switch are two brass screws. The “traveler wires” (a.k.a. the wires that connect the two light switches to each other) attach to these two screws, and it doesn’t matter which of the two screws they each connect to.
On the other switch, the hot wire that continues on to the light fixture attaches to the common screw. And as with the previous switch, the two traveler wires will connect to the two brass screws (again, in no particular order).
This wiring setup uses the two traveler wires to allow for either light switch to control the light fixture. Not only that, but this also allows you to turn on the light from one switch and turn it off from the other switch. For example, the diagram below shows the inside of each switch and their “on/off” positions.
Currently, each switch is in a different state, but thanks to the top traveler wire, the circuit is still complete and the light is on. Flipping either of the switches will break the circuit and turn the light off. However, if you were to flip the other switch, then the circuit would use the red traveler wire this time and the light would turn back on.
It sounds complicated at first, but as you’ve now discovered, it’s actually pretty simple.
Beyond Three-Way Light Switches
Three-way lights are fairly common in a lot of larger houses, but you might also find four-way, or even five-way lights, in some houses.
Four-way circuits are just a tad different than three-way circuits, mostly since the light switch that you add in has to be a four-way switch rather than just another three-way switch. A four-way switch has an extra common screw to bring the total number of screws up to four: two common screws and two brass screws (not counting the ground screw).
This allows for the two traveler wires to travel through the four-way switch and onto the next switch in the circuit, which means four wires will connect to this switch rather than just three.
From there, you can add on as many four-way switches as you want, just as long as there are three-way switches on either end. You shouldn’t have to worry about anything more than a four-way circuit, though, as five-way circuits and higher are fairly uncommon in most residential households.