3-Way Switch Wiring Diagram: How It Works and Which Wire Goes Where
A 3-way switch pair controls one light from two places. Each switch has one common screw and two traveler screws. Here is the wiring diagram, the terminal map, the step-by-step swap, and the mistakes that make a 3-way work from only one switch.
Still stuck after this guide? You can ask an electrician online — see pricing, expert credentials and common questions first.
The Short Answer
A 3-way switch setup lets you turn one light on and off from two places. Each 3-way switch has three terminals: one dark "common" screw and two brass "traveler" screws, plus a green ground. The hot wire from the panel goes to the common of the first switch, two traveler wires link the brass screws of both switches, and the common of the second switch feeds the light. The neutral never connects to either switch.
If you are replacing an existing 3-way switch, the one thing you must get right is which wire is on the common screw. Mark it with tape before you take the old switch off. The two travelers are interchangeable.
Work only with the breaker off and the wires tested dead. If you find aluminum wire, scorch marks, no ground, or more wires than this guide describes, stop and ask an electrician before going further.
How Does a 3-Way Switch Work?
A normal (single-pole) switch simply opens or closes one wire. A 3-way switch is different: it has no "off" position. It always connects its common terminal to one of its two traveler terminals, and flipping the toggle moves that connection from one traveler to the other. That is why 3-way toggles have no ON or OFF printed on them.
The light is on when both switches are connected to the same traveler, because then there is a complete path from the hot wire, through both switches, to the light. Flip either switch and the path breaks.
| Switch A connects common to | Switch B connects common to | Light |
|---|---|---|
| Traveler 1 | Traveler 1 | On |
| Traveler 1 | Traveler 2 | Off |
| Traveler 2 | Traveler 1 | Off |
| Traveler 2 | Traveler 2 | On |
This is also why the toggle positions do not line up the way you expect. "Up" can mean on or off depending on where the other switch sits, and that is normal.
Which Terminal Is Which on a 3-Way Switch?
| Terminal | How to spot it | What connects to it |
|---|---|---|
| Common | Darker screw, usually black or dark bronze. Some switches print COM next to it. | On the first switch: the incoming hot (line). On the second switch: the wire going to the light (load). |
| Traveler 1 and Traveler 2 | Two lighter brass screws, often on opposite sides or at the same end of the switch | The two traveler wires that run between the switches, usually the red and black wires of a 3-conductor cable. Either traveler can go on either brass screw. |
| Ground | Green screw, usually on the strap at one end | The bare copper or green ground wire, spliced to the other grounds in the box |
Do not go by screw position alone. Brands place the common in different spots, and some switches use push-in holes labeled on the back. Read the markings on the switch you have.
3-Way Switch Wiring Diagram (Power Comes In at the First Switch)
This is the most common layout in newer homes and the easiest one to follow: power from the panel enters the first switch box, a 3-conductor cable runs to the second switch box, and a 2-conductor cable runs from the second switch to the light.
| Wire | Where it goes |
|---|---|
| Black (hot) from the panel cable | Common screw of Switch 1 |
| Red wire in the 3-conductor cable | Brass traveler screw on Switch 1 and a brass traveler screw on Switch 2 |
| Black wire in the 3-conductor cable | The other brass traveler screw on each switch |
| White (neutral) from the panel | Spliced with a wire connector to the white in the 3-conductor cable in the Switch 1 box, then spliced to the white going to the light in the Switch 2 box |
| Black from the cable to the light | Common screw of Switch 2 at one end, the light fixture's hot lead at the other |
| Bare or green grounds | Spliced together in each box, with a pigtail to each switch's green screw and to a metal box if there is one |
Cable is sized to the circuit: 14-gauge (14/2 and 14/3) on a 15-amp breaker, 12-gauge (12/2 and 12/3) on a 20-amp breaker. Never mix in a smaller wire.
What If the Power Comes In at the Light Instead?
Older homes are often wired with power at the ceiling box, or with the light between the two switches. The colors in each box will not match the table above, but the rule never changes:
- The hot from the panel lands on the common of one switch.
- The common of the other switch goes to the light.
- The two travelers connect brass screw to brass screw.
- Neutrals and grounds are spliced, never put on a switch screw.
In these layouts a white wire is often used as a hot or a traveler. The electrical code requires a white wire used that way to be re-marked, usually with black or red tape around it, so the next person does not mistake it for a neutral. If you find a white wire on a switch screw with no tape, wrap it now.
If you are replacing a switch, you do not need to understand the whole layout. You only need to know which wire was on the old common screw. If you are wiring from scratch or the old wiring was disturbed and you cannot tell what is what, this is the point to bring in an electrician. Guessing at 3-way wiring is how lights end up working from only one switch, or breakers start tripping.
How to Replace a 3-Way Switch Step by Step
The general switch-swap routine is in our guide to replacing a light switch. These are the steps that are specific to 3-way switches.
| Step | What to do | Why it matters |
|---|---|---|
| 1. Kill the power | Turn off the breaker for the light. Flip both switches to confirm the light stays off. | A 3-way circuit can still be live at one box even when the light is off. |
| 2. Test it dead | Check every wire in the box with a non-contact voltage tester. Test the tester on a known live outlet first. | Boxes sometimes contain wires from a second circuit. |
| 3. Pull the switch out | Remove the cover plate and the two mounting screws, then pull the switch straight out without disconnecting anything. | You need to see the wires in place. |
| 4. Mark the common | Find the dark common screw (or the COM label) and wrap the wire on it with a piece of tape. Take a photo. | The common wire is the only one whose position matters. |
| 5. Move wires one at a time | Move the marked wire to the common on the new switch, then the two travelers to the brass screws, then the ground to the green screw. | Moving one wire at a time prevents mix-ups. |
| 6. Make good connections | Wrap wires clockwise around the screws and tighten firmly. Avoid the push-in (backstab) holes. | Loose connections cause flickering, heat and failed switches. |
| 7. Fold and mount | Fold the wires back neatly, mount the switch and plate. | Pinched wires can short against the box. |
| 8. Test all four positions | Restore power and try every combination of the two switches. | The light should go on and off from either switch in any position. |
Tools and parts: non-contact voltage tester, a multimeter if you need to find the common on an old switch, screwdrivers, wire strippers, needle-nose pliers, electrical tape, wire connectors, and a replacement 3-way switch rated for the circuit (15 A or 20 A). A single-pole switch will not work in a 3-way location.
Why Does My 3-Way Switch Only Work From One Switch?
| Symptom | Most likely cause | Fix |
|---|---|---|
| Light works only when the other switch is in one position | The common wire and a traveler are swapped on one switch | Power off, find the wire that belongs on the common at that switch and swap it with the wire currently there |
| Light never comes on at all | Commons swapped on both switches, a dead bulb, or a loose traveler | Check the bulb, then check each connection. Use a multimeter to confirm the common on each switch. |
| One switch does nothing, the other works | A broken or loose traveler, or a failed switch | Tighten connections; replace the switch if it tests bad for continuity |
| Breaker trips when you flip a switch | A hot wire is connected to a traveler or neutral by mistake, creating a short | Stop and call an electrician if you cannot see an obvious error |
| Switch is warm, buzzes, or the light flickers | Loose or backstabbed connection | Move wires to the side screws and tighten. See why lights flicker. |
Finding the common on an unmarked old switch: with the switch removed and no power anywhere near it, set a multimeter to continuity. One terminal will show continuity to one of the other two in one toggle position and to the other in the opposite position. That shared terminal is the common. For other switch faults, see light switch not working.
What About Dimmers, Smart Switches and 4-Way Switches?
- Dimmers: use a dimmer labeled 3-way. In most simple setups only one of the two switches can be the dimmer; the other stays a standard 3-way switch. Some dimmer systems allow dimming from both locations with a matching companion switch.
- Smart switches: most smart 3-way kits replace one switch with the smart unit and the other with a companion or auxiliary switch, and many need a neutral wire in the box. Follow the maker's diagram exactly; it often does not match standard 3-way wiring.
- Neutral at the switch box: recent editions of the National Electrical Code (section 404.2(C)) generally require a neutral at switch locations in new work, with some exceptions, because electronic controls need one. Older homes often do not have it.
- 4-way switches: to control one light from three or more places, a 4-way switch goes between the two 3-way switches. It has four terminals for two pairs of travelers and simply crosses or straightens them when flipped. The two end switches are still 3-way switches.
When Should You Call an Electrician?
Call a licensed electrician if:
- The wire is dull silver (aluminum). See aluminum wiring.
- You see scorching, melted insulation or brittle cloth-covered wire. See black marks around an outlet or switch.
- There is no ground wire, or the box has more cables than the layouts above.
- The original wiring was disconnected and nobody marked the common.
- You are adding a new 3-way location, which means fishing new cable through walls.
- The breaker trips or anything gets warm after your work.
Rules on homeowner electrical work vary by state and city. Many places let you replace a switch like for like in your own home, while new circuits and new switch locations often need a permit. Not sure which wire is the common? An electrician can look at a photo of your box and tell you.
Notes for Canada, the UK and Australia
- Canada: the switches and wire colors match US practice. Permit and inspection rules for homeowners differ by province, so check with your provincial electrical safety authority before doing wiring work.
- UK: the same idea is called two-way switching, and the terminals are marked COM, L1 and L2. Circuits run at 230 V. Newer cable uses brown (live), blue (neutral) and grey or black for the strappers, while older homes use red and black. Under Part P in England, replacing a switch like for like is not notifiable, but new circuits and some work in bathrooms are.
- Australia: also called two-way switching. Electrical wiring work, including replacing switches, must be done by a licensed electrician.
Pro Tips
Frequently Asked Questions
What is the common wire on a 3-way switch?
The common is the wire on the darker (black or bronze) screw, sometimes labeled COM. On one switch it is the hot from the panel, and on the other it is the wire to the light. Mark it with tape before you remove an old switch, because it is the only wire whose position matters.
Does it matter which traveler goes on which brass screw?
No. The two travelers are interchangeable, so either one can go on either brass screw. What matters is that the common wire goes on the common terminal of each switch.
Why does my 3-way switch only work when the other switch is up?
Almost always because the common wire and a traveler are swapped on one of the switches. Turn the breaker off, identify the correct common wire at that switch, and move it to the dark common screw.
What wire do I need between two 3-way switches?
A 3-conductor cable with ground: 14/3 on a 15-amp circuit or 12/3 on a 20-amp circuit. The red and black wires are the travelers and the white is usually the neutral passing through.
Can I put a dimmer on a 3-way switch?
Yes, but it must be a dimmer rated for 3-way use. In most simple setups only one location can be the dimmer and the other stays a regular 3-way switch, unless you buy a system with a matching companion switch.
Do 3-way switches need a neutral wire?
Standard mechanical 3-way switches do not connect to the neutral. Most smart switches do need a neutral in the box, and recent editions of the electrical code generally require a neutral at switch locations in new work.
Can I replace a 3-way switch with a regular single-pole switch?
Not as a direct swap. A single-pole switch has only two terminals and the light will not work properly from the other location. If you no longer want two-location control, have an electrician rewire it so the unused travelers are safely capped.
