A three-way wall switch is a variation of the standard single-pole switch that makes it possible to control a ceiling light or other electrical fixture from two different locations in a room. In a hallway or large room, for example, installing three-way switches at both ends lets you turn the light fixture on or off from both locations.
An additional switch adaptation, four-way switches, are used in conjunction with three-way switches to control lighting from more than two locations. This is not common in most homes, but you may see it in very large rooms, such as open-concept, kitchen/great room home layouts, where a central bank of lighting fixtures might be controlled from more than two entry points.
If you examine a three-way switch, you will notice several differences when compared to standard single-pole switches. First, the body of the switch will be thicker and bulkier than a single-pole switch. And the switch toggle lever will not have the ON-OFF markings found on a single-pole switch. The biggest difference, though, will be in the screw terminals on the switch.
While standard single-pole switches have two screw terminals on one side of the switch, plus a third green grounding screw terminal connected to the metal strap, three-way switches come equipped with another screw terminal.
If you look over the three-way switch, you will notice that this extra screw terminal is a darker color than the other two brass-colored terminals. This is known as the common connection of the switch. Depending on where the switch will be in the circuit layout, the purpose of this common connection is to either to deliver electrical current from the power source (the circuit-breaker box) to one of the switches or to deliver the current onward from the second switch to the light fixture.
The other two screw terminals on the switch body will be brass-colored. These are used to connect the circuit wires that run between the two three-way switches. These are known as the traveler terminals, and the wires running between the switches are known as traveler wires.
In a circuit situation, normally these traveler wires will have black and red insulation. When the switches are installed, these traveler wires allow electrical current to pass between the switches—or they interrupt the circuit flow to turn the light fixture OFF. At any given moment when the light fixture is ON, the power may be flowing through either the black or the red traveler wire. This will vary depending what position the switch toggle levers are in.
Three-way switches have different methods of connection, depending on the brand of the switch. The switch may also have several ways to make the wire connections. All switches have screws on the side, but some also come with push-fitting holes or slots to slide the wire into. Still others come with a quick-mount, spring-loaded slot alongside the screw terminals that are designed to hold the wires in place.
Although these push fittings or slot-fittings may be the quickest way to connect a switch, this method is not recommended, as it is generally less secure. Professional electricians who want to avoid callbacks always use the screw terminal connections which rarely come loose.
Three-way switches are tricky to install, especially for DIYers who are replacing a bad switch. One of the most common problems is improper wiring—connecting the circuit wires to the wrong screw terminals.
It's very easy to mix up three-way switch wiring when replacing a three-way switch, especially because in older wiring systems the standard color-coding of wires may look different than it does in newer installations. The best way to prevent this is to take the time to mark the wires before you remove any wires from the old switch, The wire connected to the common screw terminal is the most important to mark. It must always connect to the darkest-colored terminal screw. By placing a colored piece of tape or label on the wire, it will be easy to find when you connect the new switch.
It's also a good idea remove and reconnect one wire at a time when replacing switches. By doing this one wire at a time, you can ensure you are connecting the new switch correctly. This can sometimes be difficult, though, if the circuit wires in the wall box are too short—in this case, marking the wires is essential.
Problems with switches occur when wire connections come loose, or when the switch itself fails. Switches are mechanical devices that can wear out after hundreds or thousands of clicks.
If you suspect a loose wire connection, turn off the power to the circuit at the circuit breaker box before inspecting the wire connections on the switch. Make sure they are all tight. Also, check any wire nut connections on other circuit wires in the wall box. For example, there likely will be a neutral wire connection inside the box (usually these are white wires); make sure this wire nut connection is also secure. If you find that this neutral wire connection is made with electrical tape you should replace it with a wire nut.
If a switch makes a sizzling or popping sound when you turn the switch ON, it means that the switch contacts are becoming worn and aren't making good contact. This switch should be changed immediately to avoid bigger electrical problems.
By inspecting your switches periodically, you'll ensure safe and effective electrical connections.