How to Wire a 3-Way Switch
A simple field guide for hallways, stairs, and two-location lighting
A 3-way switch lets you turn the same light on or off from two places. That is why you see them at both ends of a hallway and at the top and bottom of a stairway.
The device looks like a normal switch, but it is not. A regular single-pole switch has two circuit terminals. A 3-way has three circuit terminals plus a ground.
Know the screws first
Common — the dark or black screw. This is the important one.
Travelers — the two brass screws. These are the two paths between the switches.
Ground — the green screw.
One common gets the incoming hot feed. The other common sends the switched hot to the light. The two travelers run switch to switch.
How it actually works
Think of each 3-way as a selector. Flip the handle and the common moves from one traveler to the other. If both switches line up on the same path, the light is on. If they do not, the light is off. That is why either switch can change the light, no matter where the other handle is sitting.
The logic never changes. Only the cable routing changes.
Safety: Turn the breaker fully off and confirm the wires are dead with a tester before you open a box. If the wiring does not match a simple diagram, stop and call a licensed electrician.
Most common wiring: power at the first switch
Box AIncoming hot to the common (dark screw). Black and red travelers to the two brass screws. Whites spliced. Grounds to the green screw.
Between the switchesUse 14/3 or 12/3 cable. Black and red are travelers. White is the neutral. Bare is the equipment ground.
Box BCommon to the black wire that goes to the light. Travelers on the brass screws. Whites spliced to the fixture neutral. Grounds to the green screw.
The white neutral does not land on the switch. The ground is not a traveler.
The other layout: power at the light
Older homes often feed the fixture first. The idea is still the same: travelers between the two 3-ways, one common for the hot feed, one common for the switched hot to the lamp. Newer work more often feeds a switch box because a neutral is needed there for future controls.
If a white wire is used as a hot, it must be marked at every termination.
Mistakes that make it “only work from one switch”
Using 2-wire cable between the switches, so there is no second traveler
Landing the hot on a brass traveler screw instead of the common
Putting the neutral on a switch terminal
Mixing up which wire is the constant hot and which wire is the switch leg
How to test it
Turn the power back on and use both switches. The light should change from either location. If it only works when the other handle is in one position, the common is probably on the wrong screw. Kill the power before you open the boxes again.
The field rule
Two 3-way devices. One pair of travelers between them. Common in is power. Common out goes to the light. Neutral is not switched.
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