3 Way Light Switch Wiring: Traveller Wires, Common Terminals and Light Control

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Three-way switch wiring allows a single light fixture to be controlled from two separate switch locations using a pair of traveller wires that carry the switched hot between the two switch boxes before reaching the light.

A three-way switch is a single-pole double-throw (SPDT) device — it has one common terminal and two traveller terminals. The common terminal is identified by a darker screw (black or copper-coloured) separate from the two brass-coloured traveller screws. The switch toggles the common terminal connection between traveller 1 and traveller 2, but it is not an on/off device by itself — it completes the circuit only in combination with a second three-way switch. In a standard three-way circuit, power arrives at the first switch box via the hot conductor. That hot connects to the common terminal of switch 1. The two traveller wires — typically the red and black conductors of a 14/3 or 12/3 cable — run from the traveller terminals of switch 1 through the wall to the traveller terminals of switch 2. The common terminal of switch 2 connects to the hot terminal of the light fixture. Neutral bypasses both switches completely, running from the supply to the neutral terminal of the fixture. Ground connects to the metal box and switch ground terminals in every box. The circuit completes when both switches have their common terminals connected via the same traveller wire — both travellers on the same side both up or both down. When the switches are on opposite travellers, the circuit is open and the light is off. Toggling either switch changes which traveller is connected to the common on that switch, either completing or opening the path. The NEC now requires a neutral conductor in every switch box, even when the switch does not use it, to allow future installation of smart switches that require neutral to power their electronics. Where power enters the light box rather than the switch box, a different wire run arrangement called power-through-light or California three-way uses a different physical cable layout while achieving the same electrical result.

How to wire 3 way light switch wiring

Identify the circuit and de-energise Locate the circuit breaker for the three-way circuit. Turn it off and verify zero volts on all wires at both switch boxes with a non-contact tester. Tag the breaker.
Plan cable runs From supply to switch box 1: 14/2 or 12/2 cable. Between switch boxes: 14/3 or 12/3 cable. From switch box 2 to fixture (if applicable): 14/2 or 12/2 cable. Alternatively, power can enter the fixture box depending on the home layout.
Wire switch 1 Connect the supply hot (black) to the common (dark) terminal. Connect the red and black conductors of the 14/3 traveller cable to the two brass traveller terminals. Connect ground to the switch ground screw and bond to the metal box.
Wire switch 2 Connect the red and black traveller conductors to the two brass traveller terminals. Connect the outgoing hot to the light fixture (via the 14/2 cable from this box or directly if fixture is at this location) to the common (dark) terminal.
Wire fixture and test At the fixture, connect the incoming hot from switch 2 common to the fixture hot terminal and the bypassed neutral to the fixture neutral. Restore power and confirm the light toggles correctly from both switch locations.

Specifications

Switch type	Single-pole double-throw (SPDT) — three-way switch
Cable between switches	14/3 NM-B (15 A) or 12/3 NM-B (20 A circuit)
Common terminal identification	Dark (black or copper) screw, labelled COM
Traveller terminal identification	Two matching brass-coloured screws

Safety warnings

Verify which screw is the common terminal before disconnecting an existing switch — connecting the hot to a traveller terminal instead of common prevents circuit function and creates a safety hazard.
The white conductor in a three-wire cable used as a traveller must be re-identified with black tape or marker at both ends — using white as a switch leg without re-identification violates NEC 200.7.
De-energise the circuit at the panel and verify zero volts on all wires before touching any three-way switch terminal — multiple wires in the box may be on different circuits.

Tools needed

Non-contact voltage tester for confirming de-energisation of all wires in switch boxes
Continuity tester or multimeter for identifying common and traveller terminals on unmarked switches
Wire strippers and needle-nose pliers for forming hook connections on traveller terminals
Screwdriver with non-conductive shaft rated for 1000 V for terminal work

Common mistakes

Connecting the hot conductor to a traveller terminal instead of the common (dark) terminal on switch 1, causing the light to operate from switch 2 only.
Running only a two-wire cable between switch boxes, which provides no path for the second traveller wire needed for three-way operation.
Connecting neutral to the switch instead of bypassing it directly to the fixture, which would work incorrectly and violates NEC wiring method requirements.

Troubleshooting

Light always on regardless of switch position: Cause: Both traveller wires shorted together or to the hot conductor Fix: De-energise and use an ohmmeter to check traveller-to-traveller and traveller-to-ground resistance. A short between travellers bypasses both switches. Inspect wiring in both boxes and in conduit for pinched conductors.
Light always off regardless of switch position: Cause: Open traveller wire or hot connected to wrong terminal Fix: Verify hot is on the common (dark) terminal of switch 1. Use a continuity tester to confirm both traveller conductors are unbroken between boxes. Check that switch 2 common connects to the fixture hot.
Switch works but light flickers: Cause: Loose connection at common terminal or traveller terminal of one switch Fix: De-energise and re-torque all switch terminal screws. Check for wire strand damage at screw connections. Verify the fixture neutral connection is secure.

Frequently asked questions

How do I identify the common terminal on a three-way switch?

The common terminal has a darker screw — black or copper-coloured — while the two traveller terminals have matching brass-coloured screws. The common terminal is also usually labelled "COM" or "COMMON" on the switch body. When replacing an existing switch, remove one wire at a time and note which screw it came from before disconnecting, or use a marker to identify it before removal.

Why does my light only work from one switch?

The most common cause is that the hot conductor is connected to a traveller terminal instead of the common terminal on switch 1. This means switch 1 does not toggle the hot onto either traveller — only when switch 2 happens to connect to the same (fixed hot) traveller does the circuit complete, making it appear to work only from switch 2. Check that the hot connects to the dark common screw.

What is a traveller wire?

Traveller wires are the two conductors that run between the two three-way switches. They carry the switched hot from the common terminal of switch 1 to the common terminal of switch 2, creating the two alternative paths that allow either switch to complete or break the circuit. Travellers are typically the red and black conductors in a 14/3 NM-B cable.

How do I add a third switch location?

A third switch location requires a four-way switch inserted between the two three-way switches. A four-way switch has four traveller terminals and internally cross-connects or straight-connects them depending on switch position, allowing any of three or more switches to control the light. Each additional switch position after the first two requires one additional four-way switch.

Can I use a smart switch with three-way wiring?

Many smart switches require a neutral wire that standard three-way switch boxes may not have unless the panel-to-switch cable includes a neutral. Smart three-way systems typically use a smart master switch at one location and a smart remote or accessory switch at the other location, communicating wirelessly or over existing wiring. Verify the smart switch's neutral requirement and wiring instructions before purchasing.

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