Three-way switches for lighting in a home provide a way to control one light with two separate switches in two different locations. Three way switches are primarily found in hallways and stairwells, but can also be found in rooms with multiple entry and exit points, or in larger sized rooms. The point of a three-way switch is to be able to turn a light on or off when entering, or exiting a room, hallway or staircase. Lack of a three-way switch in these areas is actually a safety concern, since moving through these spaces when it is dark can lead to injury. Home inspectors will usually notify their clients if there is a location in the home that does not have a three-way switch lighting circuit, but should, since this condition is considered to be a health and safety issue.
It is common during a home inspection to find improperly wired three-way switches. In these cases, you may be able to turn lights on from each location, but only if the other switch is in a certain position, and same goes for turning the light off. There are a few different reasons why this issue can occur. One reason is that the installer did not install a light switch that is meant for a three-way circuit. Another reason is that the wiring configuration is incorrect. Lastly, there may be not enough wires installed for that particular circuit.
A three-way light switch is different than a normal light switch. A normal light switch is called a single pole switch, whereas a three-way light switch is called a double pole switch. A single pole light switch only allows electricity to flow one way, and only allows the lighting circuit to turn on or off. A double pole switch allows electricity to flow in two different ways, and this function is used when another double pole switch is also installed in the circuit. When two double pole switches are installed in a lighting circuit, the light can be turned on or off from two different locations, since there is more than one path the electricity can travel.
Modern three-way light circuits are different from normal light circuits because they require an additional wire for the circuit to work properly. The three wires for a three-way switch circuit are referred to as the traveler wires, and common wire. There are two traveler wires and one common wire at each switch. The traveler wires are both connected from one switch to the other, and the common wire is connected to the power source and light fixture. When one or the other switch is activated, the power will either be routed to complete the circuit and turn on the light, or will disconnect power from the light fixture, making the light turn off.
When replacing a three-way light switch, it is important to make sure that the traveler wires and common wire are all connected to the double pole switches properly, in order to ensure proper function of the three-way light circuit.
Check out the video below for more information on how to replace a three-way light switch.