How to Install a Three-Way Switch: Wiring Two Switches for One Light (2026)
Three-way switches control a single light from two locations. This guide covers understanding three-way wiring (travelers and common terminal), replacing a defective three-way switch, and adding a second switch location.
A three-way switch has three terminals: a dark-colored COMMON terminal and two lighter-colored TRAVELER terminals. The common terminal connects to either the power supply wire or the wire going to the light fixture (depending on which switch in the pair you're working on). The two traveler wires connect to the traveler terminals (either way — they work the same in both positions). Getting the common terminal wrong is the only way to incorrectly wire a three-way switch.
Frequently Asked Questions
How does a three-way switch work?
A three-way switch has two positions, and either position can complete the circuit depending on what position the other switch is in. The two switches are connected by two wires called travelers. The common terminal on each switch connects to either the power supply or the fixture — one switch gets power, the other controls the load. When both switches are in the same position (both up or both down): the circuit through the travelers is broken, and the light is off. When they are in different positions: current flows through the travelers and the light is on. This is why flipping either switch toggles the light.
How do I identify which wire is the common when replacing a three-way switch?
Before removing the old switch: look at the terminals. The common terminal is usually the dark brass or black terminal — it's set apart from the two lighter traveler terminals. Take a photo of the wiring before removing anything. The wire on the common terminal must go on the common terminal of the new switch. The two traveler wires can connect to either traveler terminal — their order doesn't matter (either arrangement produces the same result). If you're unsure which wire was on the common: use a non-contact tester with power on to find the always-hot wire — that's the common wire in the power-supply switch.
What is a four-way switch and when do I need one?
A four-way switch is used when you need to control a light from three or more locations. The wiring sequence is: three-way switch — four-way switch(es) — three-way switch. You can have as many four-way switches between the two three-ways as needed. A four-way switch has four terminals and has no common — all four are traveler connections. Four-way switches cannot be used alone; they always go between two three-way switches.
Can I add a second switch location to an existing single-pole circuit?
Yes, but it requires running new wiring between the existing switch and the new switch location. The existing single-pole switch must be replaced with a three-way switch, and the new location gets a three-way switch, with three-wire cable (14/3 or 12/3 — the extra wire carries the traveler) run between them. If running new wire through finished walls is not feasible: a wireless smart switch system (Lutron Caseta, Sonos) can add a second control point without rewiring — the wireless switch sends a radio signal to the smart switch at the original location.
After replacing the three-way switch, the light only works from one switch. What went wrong?
If the light only turns on when one specific switch is in one specific position, and the other switch doesn't toggle it: the common wire is on the wrong terminal at the malfunctioning switch. The most common error. Remove the switch again, confirm which wire was on the old common terminal (check your photos), and move it to the correct (dark) common terminal on the new switch. The travelers can be on either of the remaining terminals.
A three-way switch has three terminals: a dark-colored COMMON terminal and two lighter-colored TRAVELER terminals. The common terminal connects to either the power supply wire or the wire going to the light fixture (depending on which switch in the pair you’re working on).
Three-way switch wiring is only confusing the first time — understanding the common terminal makes it logical.
What you need
- Non-contact voltage tester
- Flathead and Phillips screwdrivers
- 3-way switch (replacement or new — must be a 3-way, not a regular single-pole)
- Electrical tape
- Wire stripper (if cutting wires is needed)
Step 1: Turn off the circuit
Turn off the breaker controlling the light. Confirm power is off with a non-contact voltage tester at the switch.
Step 2: Remove the old switch and photograph wiring
Remove the cover plate. Unscrew the switch from the box. Pull it out. Before removing any wires: take a photo clearly showing which wire is on the dark (common) terminal.
Step 3: Identify the common wire
The wire on the dark/black terminal is the common. In 3-wire cable (black-white-red), the common is typically the black wire. Mark this wire with a piece of tape labeled “COMMON” before removing it.
Step 4: Disconnect and reconnect
Disconnect all wires from the old switch. On the new switch:
- Common wire → dark (COM) terminal
- The two traveler wires → either of the two lighter traveler terminals (order doesn’t matter)
Ground wire (bare copper or green) → green ground screw.
Step 5: Test before closing
Fold wires back into the box, being careful not to disconnect anything. Restore power at the breaker. Test: flipping either switch should toggle the light. If only one switch works: the common wire is on the wrong terminal — redo.
Turn off the circuit, screw the switch into the box, install cover plate.
Related guides
- How to Replace a Light Switch — single-pole switch replacement (simpler version)
- How to Install a Dimmer Switch — add dimming to the three-way circuit (requires 3-way compatible dimmer)
- How to Add an Electrical Outlet — related electrical work
- Turn off the circuit and verify power is off
Turn off the breaker controlling the light. Use a non-contact voltage tester at the switch to confirm no voltage. Do not rely on the light being off — the hot wire to the switch may still be energized even with the switch in the off position.
- Remove the old switch and photograph the wiring
Remove the cover plate and unscrew the switch from the box. Pull the switch out to expose the wires. Before removing any wires: take a clear photo showing which wire is on the dark-colored COMMON terminal. This is the single most important piece of information — getting the common wire on the wrong terminal is the only way to incorrectly wire a three-way switch.
- Identify and mark the common wire
The common terminal is the dark brass or black terminal on the switch — it's set apart from the two lighter-colored traveler terminals. Mark the wire that was on the common terminal with tape labeled 'COMMON' before disconnecting anything. The two remaining wires are travelers — they connect to either traveler terminal on the new switch (the order doesn't matter).
- Connect wires to the new switch
Disconnect all wires from the old switch. On the new switch: connect the COMMON wire to the dark (COM) terminal. Connect the two traveler wires to either of the two lighter traveler terminals in any order. Connect the bare copper or green ground wire to the green ground screw. Use a screwdriver to tighten each terminal screw firmly — loose connections cause intermittent failures.
- Test before closing the box
Fold wires loosely into the box without fully pushing them in. Restore power at the breaker. Test: flipping either switch from either location should toggle the light. If the light only turns on from one specific switch in one specific position: the common wire is on the wrong terminal — turn off the breaker and move it to the dark terminal. Once confirmed working, turn off the breaker, push wires into the box, screw the switch in, and install the cover plate.
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