An outlet wall plate with LED night lights is a good addition to any home, but many homeowners don't spend much time thinking about how their light switches work. Being able to turn off or on a light switch from up or down the stairs or at different entrances is often taken for granted. However, this isn't the behavior of a standard single-pole switch. It's made possible thanks to how a 3-way switch operates. A 3-way switched is wired quite uniquely.
The most basic thing to keep in mind is that 3-way switches are always used in pairs. If the local circuit features a 3-way switch, there's always another one to go along with it. When there are more than two, the other switch or switches will be 4-way switches. This allows you to turn on or off a light from 3 or more locations. Standard 3-way switches are paired, and thus they allow operation from only two locations, such as either side of a large room. A 3-way switch is never used in conjunction with a single-pole switch.
Identifying a 3-Way Switch
The easiest way to identify a 3-way switch is by looking at it. A standard single-pole switch has on and off markings on the switch. The top if usually on, and the bottom is off. These markings aren't present with a 3-way switch. Instead, whether you'll turn the light on or off is dependent upon the position of its paired switch's toggle.
Another way is by looking at the body of the switch and its screw terminals. With a 3-way switch, you'll notice three terminal screws and another ground screw. Two of these screws will be a light color, often copper or bronze. These are called travelers. The third screw terminal is usually a dark color and is known as the common terminal. You can identify the ground terminal because it's typically green. The arrangement of these terminals and screws will vary depending upon the manufacturer.
Wiring a 3-Way Switch
Wiring a 3-way switch depends upon where the switch falls within the circuit. The two switches could be before or after the light fixture in the circuit, or you could have one on each side and the fixture in the middle. The most important wire is the "hot" wire that is connected to the common screw terminal. It's usually colored black. This is the wire that delivers electricity to the switches and to the light fixture. Depending on where the switch falls within the circuit, it could be delivering power to the initial switch via the power source, or it may instead bring power from the second switch to the light fixture. If the fixture is in the middle, it'll instead go from the switch to the fixture and then to the second switch.
Traveler wires are connected to the traveler terminals. This is why a 3-way switch can turn on or off in two different ways. These wires provide two different potential pathways via which the circuit may be completed. It doesn't matter which traveler wire goes to which traveler terminal. They're effectively interchangeable. These terminal wires are "hot" because they can deliver power. A neutral wire is in the circuit as well. It bypasses the switches and connects to the fixture. The ground wire connects to the ground terminal in the switches and to the fixture.