Most of us use light switches every day to set or adjust the level of lighting we need. Our recessed fixtures, pendant lights, chandeliers and wall sconces are usually turned off and on by flipping a switch that's on a wall somewhere. Most of the time we don't give the switch itself much thought. But the kind of wall switch we use, and knowing the way each kind works and needs to be connected, can be essential if we want to make some changes or improvements to the way our lighting works.
There are only three kinds of wall switches that are used to control light fixtures: An On/Off switch, a 3-way switch, and a 4-way switch. Choosing which one you need depends on how you want to be able to control your lights. Light switches also come in several different styles, including toggle (Manufacturer’s Site), rocker (Manufacturer’s Site) and push button (Manufacturer’s Site). You can choose the style and color to match your decor. The important thing is to choose a switch that's made to do what you need it to do.
An On/Off switch is one that just turns the lights off or on from one location. For that reason, they're sometimes referred to as single location switches.
The technical, or proper, name of an On/Off light switch is a single-pole, single throw (SPST) switch. Single-pole means that only one "hot wire" can be connected to it. Single-throw means that when you switch it, it only connects to one other wire -- the wire going to your light.
Inside an On/Off switch, there's a spring-loaded gate. When you change the switch to On, that gate snaps closed. It closes the circuit and lets the power flow through the switch to the light. When you change it to Off, the switch snaps open. It opens the circuit and interrupts the flow of power to the light.
A 3-way switch is a single pole, double-throw (SPDT) switch. Single-pole, again, means that only one "hot wire" is connected to it. It also has two other wires connected to it, and double-throw means that when you switch it, instead of opening and closing the circuit, a 3-way switch changes the connection of the "hot wire" back and forth between two other wires.
Those other two wires are called the "travelers." They are connected, ultimately, to a second 3-way switch, and that second switch is connected to the wire that carries the power to your light.
Internally, a 3-way switch looks like a "V." The point of the V is the terminal where the hot wire coming from your breaker or fuse panel, or the load wire -- the one going to the light -- is connected. The two traveler wires are connected to the two open points of the V. The point of the V is called the common, or point terminal. It will look different, usually because it will have a dark, almost black, screw. The open points of the V are called the traveler terminals, and they will usually have bright brass screws.
Here's why, by installing a pair of 3-way switches, you can turn a light on or off from two different locations:
On one of the switches, the power coming from the panel is connected to the common terminal. Internally, that terminal is connected to one of the switch’s two traveler terminals, which takes it to one of the two traveler terminals on the second switch. If the second switch is not set to connect that traveler terminal to its common terminal, the light is off. If you flip one of the switches – either one --, then both of the switches will have their common terminal connected to the same traveler. The power will be connected to the light, and it will come on.
Similarly, if you have a pair of 3-way switches controlling a light and that light is on, then both of those switches are set so that both of their common terminals are connected to the same traveler wire. When that’s the case, flipping just one of the switches will connect that switch’s common terminal to its other traveler terminal.
The hot feed will no longer be connected all the way through, and the light will go off.
A 4-way switch is a double-pole, double throw switch. Double-pole means that two "hot," or potentially hot, wires are connected to it. Those are the two traveler wires from the 3-way switch that's connected to the power from the panel. It also has the two traveler wires from the 3-way switch that's connected to the wire that carries the power to the light. Double-throw means that when you switch a 4-way switch, it changes the connections between the two pairs of traveler wires.
Internally, a 4-way switch can be thought of as both an "X" and a pair of parallel lines -- either "||" or "=." In one position a 4-way switch connects the terminals that are diagonally opposite each other. That's the "X." When you change the switch, it disconnects the X and connects the terminals that are either next to each other -- "||" -- or the ones that are across from each other -- the =.
One of the two traveler wires coming from the 3-way switch that's connected to power will be hot. If the 4-way switch is set so that that wire is connected to the traveler that is disconnected at the second 3-way switch the light will be off. Changing any of the three switches will turn it on.
If you flip the 3-way switch that has power connected to it, you are changing which of its two travelers is carrying power. The power will now come into the 4-way switch on the terminal that it has connected to the traveler that the other 3-way switch has connected to the light to the wire and the light will come on.
If you flip that switch back and turn the light off, you can go to the 4-way switch and flip it. That will change its internal connections so that the traveler with power on it is connected to the traveler that the second 3-way has connected to the wire going to the light, and the light will come on.
Finally, if you flip the 4-way switch again, so that the light is off, and you flip the second 3-way switch, that switch will disconnect its common terminal from the traveler that doesn't have power on it and connect it to the one that does -- and the light will come on.
Two Things to Remember
If you want to replace one of your existing switches with a new switch, a timer or a dimmer, the new control needs to have the same functionality as the switch it's replacing. That is, you will need a single-location, or single-pole, single throw, switch, timer or dimmer to replace a single-location on/off switch, and you will need a 3-way switch, timer or dimmer to replace a 3-way switch. An on/off switch, timer or dimmer won't work where a 3-way control is needed. A 3-way switch, timer or dimmer can usually be made to work as an on/off control, but it can be tricky to get it connected, and a single-location device will usually cost less.
There aren't any hard-wired 4-way timers or dimmers, so you need to plan on replacing one of the two 3-way switches if you want to add timing or dimming to a switch circuit that has more than two switches.
The other thing to remember that the power is never off in a switch circuit. It means, of course, that you need to turn the power off at the breaker before you start to work on any switch. It also means that you might want to turn the power off at the breaker even if you are only going to replace a light bulb or two. That will eliminate the possibility that someone might turn the power to the fixture back on by flipping one of the two or three switches.