Common Wire on a 3-Way Switch
This is a free printable common wire on 3 way switch: download the diagram as SVG or open it and print to paper or PDF.
The common wire on a 3-way switch connects to the darker-coloured screw (COM terminal) and carries either the incoming line or outgoing load — identifying it correctly is the key to making 3-way switching work.
A 3-way switch is a single-pole double-throw (SPDT) switch used in pairs to control one light or load from two different locations — at the top and bottom of a staircase, or at each end of a long corridor, for example.
Each 3-way switch has three terminals: - **COM (Common):** the single darker screw, usually black or dark bronze in colour. This is the pivot point of the switching action. - **Traveller terminals (two):** the pair of lighter-coloured screws, usually brass or silver. These connect to the traveller wires that run between the two switches.
The common terminal on the supply-end switch receives the incoming line (hot) conductor from the source. The common terminal on the load-end switch sends the switched hot out to the light fixture. The two traveller terminals on each switch connect to one another through a pair of wires routed between the switches — these are the traveller wires.
Inside the switch, a rocking mechanism connects the COM terminal to whichever of the two traveller terminals is selected. When both switches select the same traveller, the circuit is complete and the light is on. When one switch is toggled, it selects the opposite traveller, breaking the circuit and turning the light off — regardless of which position the other switch is in.
The most common wiring error — and the reason most 3-way switch installations fail — is connecting the line or load wire to a traveller terminal instead of the COM terminal. If the line is on a traveller terminal rather than COM, the light may work in some switch combinations but not others, or may be permanently on.
In practice, the common wire is identified by: 1. The darker screw colour on the physical switch. 2. The label 'COM' or 'C' stamped on the switch body, often on the side opposite the screw. 3. With a multimeter: the COM terminal is the one that maintains continuity to the line wire when the switch is in either position, because it is the terminal the internal blade always contacts.
North American wiring practice (NEC NFPA 70) uses a specific colour code for 3-way wiring, though cable colour conventions vary by cable type (NM cable vs individual conductors in conduit). Always verify with a multimeter rather than relying on colour alone. Consult IEC 60364, BS 7671, AS/NZS 3000, or your local code for requirements outside North America.
This is a generic educational reference. All switch wiring must be performed by a licensed electrician in accordance with applicable electrical installation codes.
How to wire common wire on 3 way switch
- Isolate the circuit and verify dead Turn off the circuit breaker controlling the 3-way circuit. Use a non-contact voltage tester at both switch boxes to verify no voltage is present on any conductor before touching any wiring.
- Identify the COM terminal on each switch Before removing existing wires, confirm which screw is COM on both switches — look for the darker screw or the 'COM' marking. If replacing old switches, photograph the existing wiring first.
- Identify the common wire at each switch In a typical NM cable installation: at the supply switch box, the cable from the panel arrives. The black (line) wire connects to COM. At the load switch box, the cable to the fixture arrives. The black (switched hot to the load) connects to COM. Between the two switches, a 3-wire cable carries the two traveller wires.
- Connect the COM wire at the supply switch Connect the incoming line wire to the COM (dark) screw terminal of the first switch. Tighten the screw firmly — a loose connection will cause arcing and intermittent operation. Do not overtighten on wrap-screw terminals; strip the wire to the gauge recommended on the switch body.
- Connect the traveller wires at both switches Connect one traveller wire to each brass or silver screw on the first switch. Repeat at the second switch. It does not matter which traveller wire goes to which traveller terminal — swapping them only inverts which switch position turns the light on, not whether the circuit functions.
- Connect the COM wire at the load switch Connect the switched hot wire going to the light fixture to the COM (dark) screw of the second switch. If the fixture cable has a white conductor used as a switched hot, re-identify it with black tape at both ends.
- Restore power and test Restore the circuit breaker. Test the light from both switch positions. The light should turn on and off from each switch independently, in any combination of positions. If the light does not respond to one switch, the common wire at that switch is likely on a traveller terminal.
Specifications
| Switch type | Single-pole double-throw (SPDT) — marketed as '3-way' in North America, 'two-way' in some other regions |
|---|---|
| Number of terminals | 3 (1× COM, 2× traveller) |
| COM terminal colour (typical) | Black or dark bronze screw; travellers are brass or silver |
| Typical current rating | 15 A or 20 A at 120/277 V AC (North America); 10 A or 16 A at 250 V AC (IEC) |
| Conductor size (15 A circuit) | 14 AWG copper (NEC); 1.5 mm² (IEC/BS 7671) |
| Conductor size (20 A circuit) | 12 AWG copper (NEC); 2.5 mm² (IEC/BS 7671) |
| Minimum box volume (single 3-way switch with 14/3 cable) | 18 cubic inches (NEC Table 314.16(B)) |
Safety warnings
- 3-way switch wiring involves mains voltage. Always isolate the circuit breaker and verify with a non-contact voltage tester that all conductors are dead before handling any wiring. In many jurisdictions, replacement of switches and light fixtures must be performed by a licensed electrician. Consult NEC NFPA 70 (USA), BS 7671 (UK), AS/NZS 3000 (Australia/NZ), IEC 60364, or your local code.
- The white conductor in the traveller cable between two 3-way switches may be re-identified and used as a live conductor (switched hot or common). It is not a neutral. Re-identify it with black or coloured tape at both ends to prevent future shock hazard.
- Never connect the earthing conductor to the COM screw or traveller terminals of the switch. Earth must connect only to the green or bare copper earth screw if the switch has one, or be joined to other earths in the box with a connector.
- Do not exceed the current rating of the switch. Using a 15 A switch on a 20 A circuit with a load approaching that rating risks overheating the switch terminals.
Tools needed
- Non-contact voltage tester
- Digital multimeter (continuity and voltage)
- Wire strippers
- Insulated flat-blade screwdriver and cross-head screwdriver
- Needle-nose pliers (for forming wire hooks for wrap-screw terminals)
- Electrical tape (black, for conductor re-identification)
- Torch/flashlight for working inside wall boxes
Common mistakes
- Connecting the line wire to a traveller (brass) screw instead of the COM (dark) screw at the supply switch — the most common cause of 3-way circuit failure.
- Connecting the switched hot to a traveller screw at the load-end switch instead of COM, causing the light to work correctly only in one switch combination.
- Using two 2-wire cables between the switches (only two conductors) instead of a 3-wire cable, leaving no second traveller and making proper 3-way switching impossible.
- Treating the re-identified white conductor in the traveller cable as a neutral and joining it to the neutral bundle, creating a potentially dangerous situation where the light does not turn off.
- Swapping neutral and traveller wires, which can result in the switch housing becoming live relative to earth — a shock hazard.
- Failing to verify dead with a tester before starting work, assuming the correct circuit breaker was switched off.
Troubleshooting
- Light is always on regardless of either switch position
- Cause: Both traveller terminals on one switch are connected to live conductors, or the COM is shorted to a traveller terminal internally Fix: Isolate the circuit. Verify which terminal is COM on each switch using a multimeter. Ensure only the line wire connects to COM at the supply switch. If a traveller wire is accidentally on COM, the circuit cannot switch off — rewire correctly.
- Light works from one switch location but not the other
- Cause: The common wire at the non-working switch is connected to a traveller terminal rather than COM Fix: Identify which switch is the load-end switch (the one with the wire going to the fixture). Verify its COM terminal has the fixture wire connected. If it is on a traveller terminal, move it to the darker COM screw.
- Light flickers or switches fail intermittently
- Cause: Loose connection on the COM terminal, or a traveller wire connection that is intermittently making and breaking under vibration Fix: Isolate, remove each switch from its box, and inspect all screw terminals. Re-strip and re-terminate any wires that are not fully under the screw head. Ensure back-stab connections (if present) are replaced with screw-terminal connections, which are more reliable.
Frequently asked questions
How do I identify the common terminal on a 3-way switch without the packaging?
Look for the single screw that is a darker colour than the other two — typically black or dark bronze rather than brass or silver. The word COM or the letter C may also be stamped on the switch body. Confirm with a multimeter: the COM terminal has continuity to one traveller or the other depending on switch position, but always to exactly one.
What happens if I connect the line wire to a traveller terminal instead of COM?
The light will work in some switch-position combinations but not all, or will be permanently on. Specifically: with the line on a traveller, one position of that switch energises the traveller, and if the other switch also connects that traveller to COM, the light comes on. The circuit logic becomes unreliable and the wiring is incorrect.
What are traveller wires on a 3-way switch circuit?
Traveller wires are the pair of conductors that run between the two 3-way switches. They carry the alternating path that the switch blade selects. In NM (Romex) cable, a 3-wire cable (black, red, white) is commonly used between switches, with black and red as the travellers and white re-identified as the common conductor.
Can I use a standard single-pole switch as a 3-way switch?
No. A standard single-pole switch has only two terminals (line and load) and simply opens or closes one circuit. A 3-way switch has three terminals and routes current to one of two paths depending on position. A single-pole switch cannot replicate this SPDT function.
Why does my 3-way switch have four terminals? Is it a 4-way switch?
If a switch has four terminals, it is likely a 4-way switch, which is used in the middle of a 3-switch (or more) arrangement where three locations must control one light. A 4-way switch has two pairs of traveller terminals and no COM. The 4-way sits between two 3-way switches in the daisy-chain circuit.
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