We take for granted being able to turn a light on from the bottom or top of the stairs. But a wall switch that allows a device or outlet to be turned on and off from two different locations is not a standard switch. It is called a 3-way switch. (Another variation, a 4-way switch is used when you need to control a fixture from three locations.)
Identifying a 3-Way Switch
The 3-way switch is always used in a pair.
The toggle lever on a 3-way switch has no ON or OFF markings, because either up or down lever positions may turn the light fixture on or off, depending on the sequence in which the switches are used.
Inspected side-by-side with a standard double-pole wall switch, you will notice that the body of a three-way switch has three terminal screws: two bronze or copper colored screws (called TRAVELERS) and a third screw that is darker in color. This screw is known as the COMMON, and it may be labeled as such. The arrangement of these screws varies depending on the switch manufacturer. On some three-way switches, the two traveler screws are on one side of the switch body, with the common screw is isolated on the other side. With other switches, though, the traveler screws are paired on opposite sides of the switch, with the common screw located on the other end.
The switch will also come with a ground terminal (green screw).
Wiring for a 3-Way Switch
Wiring a 3-way switch can be tricky, depending on where the switch falls in the circuit configuration. Even replacing one can be confusing if you do not pay attention to the wires and their locations on the switch when they are removed.
The circuit's hot wire, usually black, will be connected to the darker COMMON screw terminal on the switch.
Depending on where the switch falls in the circuit, the black wire will either be delivering power to the first switch from the power source, or it will be delivering power onward from the second swtich to the light fixture.
The other two terminals are used to wire up what are called "traveler" wires. It makes no real difference which traveler wire goes to which traveler terminal on the switch. The traveler wires run between the two three-way switches, offering two potential pathways to complete the circuit and send power onward to the light fixture. The light fixture will be ON whenever the switch levers are both in the UP position or when they are both in the DOWN position. Any time the switch levers are in opposite positions, the circuit is broken and the light fixture is OFF.
When Replacing a 3-Way Switch
When replacing a three-way switch, before disconnecting any wires, you'll need to identify the wire attached to the COM screw and label it, using a small tab of tape. Then disconnect all three wires from the switch, as well as the grounding wire. Because the other two wires, the travelers, are interchangeable, it doesn't matter which of the traveler screws you attach them to—there's no need to label them.
As you connect the new switch, make sure the ground wire is attached to the green grounding screw on the switch strap, then connect the marked hot wire to the common screw terminal. Then connect the remaining traveler wires to either of the traveler screws—it makes no difference which one.
- TIP: Please note, you cannot use any other type of switch in a three-way application.