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Introduction to Three-way Switches
Three-way switches are special light switches that are always installed in pairs and are used to control one or more light fixtures from two locations, such as at the top and bottom of a stairway. One clue that gives away a three-way switch is the absence of ON/OFF markings on the switch toggle. This is because sometimes a three-way switch is off when the toggle is up, and vice versa. It depends on the position of the other three-way switch. Sometimes two three-way switches are installed with a... four-way switch between them, giving you control from three locations.Continue to 2 of 4 below.
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Parts of a Three-Way Switch
A three-way switch has four different screw terminals on its body:
Continue to 3 of 4 below.
- The green screw is always for the ground wire (this is the bare copper or green insulated wire).
- The two brass-colored screws across from each other are called the traveler screws. These carry the power from one switch to the other.
- The last screw is the common terminal and is a darker color, usually dark brass, copper or black. This connects to the incoming source (hot) wire on one switch and to the black (hot) wire leading to... light fixture on the other switch.
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The Ground Terminal
For safety, always install a three-way switch that has a ground screw. It may be located on the bottom of the switch, as shown here, or it may be on the side or another location.Continue to 4 of 4 below.
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Three-Way Switch Wiring Diagram
Here is a mock-up of a household circuit with three-way switches and a light fixture. The switches would normally be on opposite sides of a room, staircase or hallway.
The power source, or "feeder wire" is a 2-wire cable with a ground, meaning it has one black (hot) wire, one white (neutral) wire and a bare copper ground wire.
The black hot wire from the feeder cable is connected to the common terminal on the first switch. The white neutral wire is spliced at each device location and... carries on to the light fixture. Neutral wires do not connect to switches. The ground wire connects to the first switch and carries on to the next switch and the fixture.
The connections between the switches are made with a 3-wire cable, which has four wires: red, black, white and ground. The red and black wires are the traveler wires, and each connects to a traveler terminal on each switch.
Between the second switch and the fixture, a black wire from a 2-wire cable connects to the switch's common terminal and to the hot lead on the fixture. The white wire connects to the fixture's neutral lead. The ground wire connects to the fixture's ground lead or grounding screw terminal.
If any of the electrical boxes (not shown) in this circuit were metal, there would also have to be a grounding pigtail (short length of ground wire) connecting the circuit ground to each box. Plastic boxes don't need to be grounded.