To identify the common wire on a 3-way switch, look at the screws. The common terminal is the odd one out, a black or darker screw sometimes marked COM, while the two travelers sit on matching brass screws. Leviton's instruction sheet says the common terminal of a 3-way switch is usually labelled or black.
The wire on that screw is the common: in the usual layout it brings power in from the panel at one switch and carries power out to the light at the other. Wrap it with tape before you unscrew anything, because the wire itself carries no mark.
A 3-way switch setup uses two switches to control one light from two locations — top and bottom of stairs, both ends of a hallway, multiple entrances to a room. The wiring is more complex than a standard single-pole switch and many DIY installers wire it incorrectly. The fundamental concept: identify the 'common' wire on each switch and the two 'traveler' wires; wrong wiring on common produces a non-functional switch.
This guide covers a standard 3-way switch replacement (existing 3-way pair being replaced) or 3-way to 3-way upgrade.
- Skill level
- Advanced
- Time
- 30–60 min
- Steps
- 9
- Tools
- 4, listed below
- Materials
- 2, listed below
Before you start
Three things to settle:
1. 3-way switches have 3 terminals plus ground. Single-pole switch: 2 brass terminals + ground. 3-way switch: 3 terminals + ground. The 3 terminals are 1 'common' (different colour, often labelled COM) plus 2 'travelers' (matching colour, often brass).
2. Common wire identification is critical. The common terminal (different colour/labelled) connects to either the line wire (incoming hot) at one switch OR the load wire (going to the light) at the other switch. The two travelers connect between the two switches.
3. Wiring diagrams vary by configuration. Multiple legitimate wiring configurations exist for 3-way circuits. Always trace the existing wiring before disconnecting and replicate. Don't assume the new switch wires the same way as a different installation.
What you'll need
Tools
- Non-contact voltage tester
- Phillips and flathead screwdrivers
- Wire strippers
- Smartphone (for photos)
Materials
- Two 3-way switches (one for each location)
- Wire nuts (replacement)
Photograph existing wiring before disconnecting
3-way wiring is non-trivial to diagnose without a meter. The original wiring at each switch tells you which wire is common and which are travelers. Photograph carefully before disconnecting anything.
If photos lost or unclear, you'll need to identify the common with a multimeter — significantly more involved.
Finding the Common Wire When the Old Switch Is Already Off
If the switch came off before anyone marked the common, a non-contact voltage tester and a continuity tester or multimeter can still find it. The second step involves live wires; if that is outside your comfort zone, stop and call an electrician.
- Turn the breaker off. At both boxes, separate the three insulated wires so no bare end touches another wire or the box, and cap each one with a wire nut.
- Turn the breaker on. Hold the non-contact tester against each capped wire in turn. The box with one live wire is the switch fed from the panel, and that live wire is its common.
- Turn the breaker off again and confirm with the tester that nothing at either box is live.
- At the first box, twist the two remaining wires, the travelers, together under one wire nut.
- At the second box, test continuity between each pair of wires. The two that show continuity are the travelers. The third is the common that runs to the light.
- Untwist the travelers at the first box, then connect both switches as in the steps below.
If neither box shows a live wire, power enters at the light fixture, and the circuit has to be traced from the fixture box. That is electrician work.
ESA, Ontario's Electrical Safety Authority, allows a homeowner or occupant to do electrical work in their own home, as long as it meets the Ontario Electrical Safety Code and every required notification of work is filed with ESA.
Step-by-step instructions
Turn off breaker, verify with tester
Switch off the breaker for the circuit. Try both switches — light should not respond at either.
At each switch location, hold a non-contact voltage tester near the wires after removing covers. Verify no voltage.
Remove cover plates and unscrew switches
At each switch, remove cover plate and unscrew the switch from the box. Pull out gently — don't disconnect wires yet.
Photograph existing wiring at both switches
Take clear photos showing every wire and which terminal it connects to at each switch.
Note which terminal is COM (often a darker brass screw or labelled). The wire on COM at one switch is the line; at the other it's the load.
Note the wires at each switch
Switch 1 typically has: 1 wire on COM (the line, incoming hot), 2 wires on the traveler terminals (going to switch 2).
Switch 2 typically has: 1 wire on COM (the load, going to the light fixture), 2 wires on the traveler terminals (coming from switch 1).
Ground wire on each switch's green ground screw.
Disconnect wires from old switches
Loosen terminal screws and pull each wire free. Or, for back-stab connections, insert a small flathead into the release slot.
Inspect bare copper. Strip fresh 5/8 inch if oxidized or damaged.
Connect wires to new switch 1
On the new 3-way switch, identify the COM terminal (darker colour or labelled).
Connect the wire that was on COM of the old switch to COM of the new switch.
Connect the two traveler wires to the two traveler terminals (either traveler wire to either traveler terminal — they're interchangeable).
Ground to green screw.
Connect wires to new switch 2
Same process: COM wire to COM terminal, two travelers to traveler terminals, ground to ground.
Push switch into box, mount with screws, install cover plate.
Restore power and test from both locations
Switch breaker back on. Test:
- Toggle switch 1 — light turns on. Toggle again — light turns off.
- Toggle switch 2 — light turns on. Toggle again — light turns off.
- From any state, toggling either switch should change the light's state. If only one switch works (or only switches 'on' but not 'off'), the wiring is wrong.
Troubleshoot common issues
If only one switch controls the light: COM is on the wrong terminal at one of the switches. Re-verify which wire was on COM at the original install.
If light doesn't respond at all: connections are loose or wrong. Re-check.
If light flickers: connections aren't tight. Re-tighten.
Common mistakes that cost you
- Common wire on traveler terminal. Switches don't function correctly. Verify common identification before connecting.
- Working live. Always verify power off with tester.
- Not photographing existing wiring. Without reference, common wire identification requires a meter. Always photograph first.
- Mixing 3-way and 4-way without understanding. Don't try to add or modify 3-way circuits without understanding the wiring. Hire an electrician.
- Permit not pulled for new wiring. Adding new 3-way circuit (not just replacing) requires ESA permit in Ontario.
Where Ontario Requires a 3-Way Switch
The Building Code asks for 3-way switches on stairs. A stairway with four or more risers in a house needs switches at the head and the foot, controlling at least one light (Ontario Building Code, Division B, Article 9.34.2.3). The exception is a stair to an unfinished basement that has no outside entrance or garage door and serves a single unit, which can run on one switch at the top.
Ontario's guide to adding a second unit repeats the rule for basement apartments: a switch at both the top and the bottom of the stairs.
Finishing the basement ends that exception, and the City of Ottawa requires a building permit to finish a basement. So a basement renovation in Ottawa turns a single switch at the top of the stairs into a 3-way pair, with a new cable run between the two boxes. That is new wiring, so the ESA notification applies.
Frequently asked questions
Can I install a smart 3-way switch?
Yes — but most smart switches need a neutral wire. Older 3-way installations sometimes don't have neutral at the switch.
Lutron Caseta and some other smart switches work without neutral — buy these for older installations.
What if I have a 4-way switch in the circuit?
4-way switches are used in 3-or-more switch chains (lights controlled from 3+ locations). 4-way switches go between 3-way switches in the chain.
Replacement is similar — identify which wires are travelers and which are pass-through. More complex; see specialty guides for 4-way wiring.
How do I find the common terminal on a 3-way switch?
The common terminal on a 3-way switch is the one in a different colour, often a darker brass screw or one labelled COM. The other two terminals are travelers and usually match each other. At one switch the wire on common is the line, the incoming hot; at the other switch it is the load going to the light.
Photograph the old wiring before disconnecting, because finding the common without photos means working with a multimeter.
Why does only one of my 3-way switches control the light?
When only one 3-way switch controls the light, the common wire is on the wrong terminal at one of the switches. Check the photos of the original wiring to see which wire sat on COM at each location and move it back.
If the light does not respond at all, the connections are loose or wrong. If it flickers, the connections are not tight enough. Wired correctly, toggling either switch changes the light from any state.
Do I need a permit to add a 3-way switch circuit in Ontario?
Yes. Adding a new 3-way circuit, rather than replacing an existing pair of switches, requires an ESA permit in Ontario. A like-for-like replacement is a smaller job: turn off the breaker, confirm no voltage at both boxes with a non-contact tester, and move each wire to the matching terminal on the new switch.
Adding circuits, mixing in 4-way switches or troubleshooting a 3-way installation that never worked is work for an electrician.
When to call us instead
Adding new 3-way circuits or troubleshooting non-working 3-way installations requires an electrician. The wiring complexity exceeds most DIY skill levels.
Book a free 60-minute site visit →Dream Touch Renovations · 33 Roydon Pl, Suite 210, Ottawa, ON K2E 1A3 · (613) 862-4555
Renovating homes across Ottawa, Kanata, Nepean, Barrhaven, Orléans and Stittsville.
Related Ottawa services
If you're updating switching throughout a home, our team coordinates licensed electrical work as part of renovation scopes. Book a free 60-minute consultation.
Official references
- Do I need a building permit?, City of Ottawa