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The Purpose of a Three-Way Switch
Wall switches used to control ceiling light fixtures come in three types. Most common is the single-pole switch, a type used to control a light fixture from a single location. The next most common is the three-way switch, which is commonly used to control a light fixture from two different locations. For example, a long hallway or stairway might use a pair of three-way switches at each end so that lights can be turned on when approaching one end of the hall or stairway, then shut off from the... other end. When used in combination with another type of switch—the four-way switch—you can arrange switches to control light fixtures from three locations
The name "three-way" may seem a little confusing. It refers to the fact that there are three different ways the switch toggles can be arranged: when both toggles are up or both are down, the circuit is complete and the light fixture will be illuminated. When the toggles are in opposite positions, the circuit is interrupted and the light fixture turns off. This allows either switch to control the light fixture at any given moment.
There are two clear giveaways that identify a switch as being a three-way type:
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- There are no ON/OFF marking on the switch toggle. Such markings are not needed with this type of switch.
- There are three screw terminals on the body of the switch (not counting the green grounding screw).
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Parts of a Three-Way Switch
A three-way switch has four different screw terminals on its body:
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- The green screw attached to the metal strap of the switch is always for the ground wire (this is the bare copper or green insulated wire). Grounding screws on switches were not always required, so if you are replacing an old three-way switch, you may find one without the grounding screw.
- 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. It is a darker color than the travelers, usually dark brass, copper, or black. This screw serves one of two purposes depending on where it is positioned in the circuit run: either it accepts the incoming black (hot) wire from the power source, or it connects to the black (hot) wire that leads onward to the light fixture.
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The Grounding Screw Terminal
For safety, always install a three-way switch that has a grounding screw. It may be located on the bottom of the switch, as shown here, or it may be on the side or another location, but will always be directly connected to the metal mounting strap on the switch. An older switch may not have a grounding screw but should be replaced with one that does.Continue to 4 of 4 below.
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Three-Way Switch Wiring
Three-way switches can be wired in a number of different ways, depending on where they are located relative to the light fixture in the circuit cable runs. For example, they can be arranged so that the feed cable runs to the first three-way switch, then to the light fixture box, then to the second three-way switch.
Or, as shown here, they can be wired so that the cables run through both three-way switches, then to the light fixture. This is a relatively common configuration, in which the wiring... connections are done in this manner:
- At the first switch location, the feeder wire from the power source is a 2-wire cable with ground. This means that there is a black hot wire, a white neutral wire, and a bare copper grounding wire. At this first switch, the black feed wire is connected to the common screw on the switch. The grounding wire is connected both to the switch using a pigtail, and to the second cable run passing onward to the next switch. If the switch box is metal, it also must be pigtailed to the grounding wires.
- The cable run linking the two switches is made with 3-wire cable. The black and red wires are "travelers" and are connected to the traveler screw terminals on the two switches. This provides two alternate pathways for hot current to flow between the switches—this is what allows the switches to turn the lights on and off in a flexible manner.
- Because switches do not have white neutral wire connections, the neutral wires at the switch boxes are simply joined together so they pass through, onward to the light fixture box.
- At the second switch box location, the wiring is similar to the first switch, with the traveler terminals connected to the traveler wires coming from the first switch. However, at this second switch, the common screw terminal is connected to a black hot wire that leads onward to the light fixture. Once again, the white neutral wires are simply joined together, and the grounding wires are joined together with pigtails connecting the switch and to the box, if it is metal.
- The cable run from the light fixture requires a 2-wire cable with ground. At the light fixture, completing the wiring is simply a matter of connecting the black and white circuit wires to the matching wire leads on the light fixture. The ground wire is connected to the light fixture lead and is pigtailed to the box, if it is metal.
Note that wiring configurations can vary greatly depending on how the circuit is arranged. But if you keep in mind the path of electrical flow, and remember that traveler wires must connect the two switches, it should be easy enough to correctly wire a light fixture controlled by two three-way switches.