Three-way Switches

Three-way switches are a deceptively simple modification to standard residential light switches. They are required in certain locations such as stairways as a safety measure that lets you illuminate the area without having to ascend or descend in darkness. Three-way switches are also useful in hallways, rooms with multiple entrances such as a kitchen, or any other area that multiple light switches would be helpful. Unlike standard light switches, there is no designated “on” or “off” position. Instead, there are three positions (hence the name): both up, both down, or mixed up and down. There are many ways to wire these devices, but I will explore the simplest method for ease of illustrating the concept.

In a standard light switch, there is an up position that is “on”, which completes the circuit from power supply to light. There is also a down position that is “off”, which breaks the circuit and prevents power from traveling the full path.

A single three-way switch does not have an “off” position. For this illustration, don’t think of circuits. Instead, picture a railroad switch on a train track. As the train travels down the single pathway, it reaches the railroad switch and, depending on the position of the lever, can take the left track or the right track. Back to our single three-way light switch, it functions similarly: there is no stopping the electricity, just as there is no stopping the train. You either direct the power (or the train) to the left or to the right. That single switch could be used in uncommon applications, such as directing power to an overhead light fixture OR to a night light – one or the other will always have power. But we are not stopping with specialty applications, let’s add in the second switch to understand a true three-way light switch circuit.

Adding a second three-way switch utilizes the exact same switch. There is not a switch model for upstream power and downstream power – they should be wired the same. One switch will receive power from the breaker or fuse panel, and the second switch will send power to a light fixture. Between the two switches, instead of a single hot wire, there are two switched wires. These two wires are called travelers. When both switches are in the up position – power enters the first switch, flows through the top traveler wire, and enters the top of the second switch before it finally powers your light. Similarly, when both switches are in the down position – power enters the first switch, flows through the bottom traveler wire, and enters the bottom of the second switch before it finally powers your light. In both cases, the lights are on, and the switches are pointed the same direction.

To turn off a light with a three-way switch, you simply mismatch the switch positions. If one switch is up and one switch is down, the light will turn off. Power enters the first switch, exits through the top traveler wire, but the second switch is positioned down and expects power from the bottom traveler wire. In this case, no complete circuit is made to the light bulb. Just as a train on a single track may choose the left track to travel through Spokane, or the right track to bypass the city – the railroad switch will need to be operated to the same pathway both BEFORE and AFTER passing through the city, so the train can get back on the single track. The concept may be a tad confusing, so I have drawn a simplified version of what should happen.

Remember, this is the simplest method. There are many other ways to wire three-way circuits, whether right or wrong. A DIY homeowner may have miswired travelers or not aligned the direction of the switches – so there are many deviations that may occur.

Too Long, Didn't Read Version: Three-way switches give you access to light from two locations using two traveler wires to bridge the gap in a circuit. If both switches expect power through traveler “A” or if both switches expect power through traveler “B”, the light turns on. If they are mis-matched, the light is turned off.

-Marshall Tramp, CPI, RMP


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Marshall Tramp, CPI, RMP
Author: Marshall Tramp, CPI, RMP
Top Notch Inspection Services llc
Serving Eastern Washington & Northern Idaho