To wire a 2-way light switch (known as a 3-way switch in the US/Canada), you must connect the permanent hot (line) wire to the Common (black) screw on Switch 1, run two traveler wires (red and black in 14/3 NM-B) between the brass screws of both switches, and connect the switched hot (load) to the Common (black) screw on Switch 2. In the UK/AU (where it is strictly called a 2-way switch), connect the brown line to COM on Switch 1, run blue and yellow strappers between L1 and L2 on both switches, and connect the load to COM on Switch 2.
The Terminology Trap: US 3-Way vs. UK/AU 2-Way
The single biggest point of failure in DIY lighting projects is regional terminology. If you search for a '2-way switch' in the US, you will be handed a standard single-pole on/off switch. If you use that to wire a two-location circuit, you will fail. Use the decision table below to identify your region's standard and buy the exact parts required.
| Region | Correct Switch Name | Cable Required (Between Switches) | Concrete Part Pick (15A / 6A Lighting) |
|---|---|---|---|
| US / Canada | 3-Way Switch | 14/3 NM-B (or 12/3 for 20A) | Leviton 5603-2W (15A) or Eaton 2131W |
| UK / Ireland | 2-Way Switch | 1.0mm² or 1.5mm² 3-Core + Earth | MK Logic Plus K2142WHI (10A) |
| Australia / NZ | 2-Way Switch | 1.0mm² or 1.5mm² TPS (3C+E) | Clipsal 2000/2 (10A) |
Tools, Materials, and Device Ratings
Before opening a single junction box, stage your workbench. Assuming a standard residential 15A (US) or 6A (UK) lighting circuit on copper conductors, here is your exact bill of materials.
- Wire Strippers: Klein Tools 11054E (US 14-10 AWG) or a precision VDE stripper for 1.0/1.5mm² (UK/AU).
- Voltage Tester: Fluke 1AC-II Non-Contact Voltage (NCV) tester for initial sweep, plus a CAT III digital multimeter (e.g., Fluke 117) for hard verification.
- Wire Connectors: Ideal Wire-Nuts (Yellow for 14 AWG pairs, Red for 12 AWG) or Wago 221-413 lever nuts. In the UK/AU, use insulated crimp terminals or push-in switch terminals.
- US Cable: 14/2 NM-B (feed to first switch, and switch to light) and 14/3 NM-B (between the two switches). Ampacity: 15A at 60°C column per NEC 310.16.
- UK/AU Cable: 1.5mm² 3-Core + Earth PVC. Ampacity: ~16A clipped direct, but protected by a 6A MCB or Type B RCBO at the consumer unit.
- Pigtail Wire: 14 AWG solid THHN (green for ground, black for hot pigtails).
Critical Safety: De-Energize and Verify
Working on lighting circuits involves exposed 120V (US) or 230V (UK/AU) conductors. A shock from a 230V circuit can induce ventricular fibrillation in milliseconds.
1. Turn OFF the lighting breaker at the main panel or consumer unit.
2. Apply a lockout/tagout (LOTO) device or tape the breaker in the OFF position.
3. Test your NCV tester on a known live circuit (like a plugged-in lamp) to prove it works.
4. Test the NCV against the switch faceplate and exposed wires.
5. Verify dead with a multimeter: measure Line to Ground and Line to Neutral. Both must read 0.00V.
Note: NEC-style guidance and IET Wiring Regulations (BS 7671) require verification of dead before touching conductors. Your local AHJ has final authority on permitting requirements.
Wiring the US 3-Way Circuit (14/3 NM-B)
This procedure assumes Switch 1 receives the power feed (Line) from the panel, and Switch 2 sends the switched power (Load) to the light fixture. This is known as a 'Line-to-Switch, Load-to-Switch' topology.
- Switch 1 (Line Box): Strip the 14/2 NM-B feed. Connect the bare copper ground to the green ground screw on the switch and pigtail to the metal box. Wire-nut the white neutral wires together and push them to the back of the box (they do not connect to a standard 3-way switch). Connect the black hot (line) wire to the black (Common) screw on the switch.
- Run the Travelers: Strip the 14/3 NM-B cable running between boxes. At Switch 1, connect the red and black wires of the 14/3 to the two brass (Traveler) screws. Order does not matter. Wire-nut the 14/3 white neutral to the feed white neutral. Connect the 14/3 bare ground to the ground pigtail.
- Switch 2 (Load Box): The 14/3 arrives here. Connect the bare ground to the green screw. Wire-nut the white neutral to the 14/2 neutral heading up to the light fixture. Connect the 14/3 red and black travelers to the two brass screws.
- Switch 2 (Load Termination): Take the 14/2 NM-B heading to the light. Connect its black wire to the black (Common) screw on Switch 2. Connect its bare ground to the switch ground.
- At the Light Fixture: Connect the fixture's black wire to the 14/2 black (switched hot), white to white (neutral), and fixture ground to the box ground.
Wiring the UK/AU 2-Way Circuit (3-Core + Earth)
In the UK and Australia, the standard method is the 'COM-to-COM' conversion or standard strapper method. We will use the most common configuration: Line into Switch 1 COM, Load out of Switch 2 COM.
- Switch 1 (Line Box): Strip the 1.5mm² 3-core + earth feed. Connect the bare earth (sleeved in green/yellow) to the earth terminal. Connect the brown (Line) wire to the COM terminal.
- Run the Strappers: Connect the 3-core + earth cable running to Switch 2. Connect the blue wire to L1 and the brown (or yellow, depending on AU/UK specific cable colors) wire to L2. Connect the earth to the earth terminal. Note: The neutral (black with blue sleeve in UK old colors, or blue in new) is connected via a terminal block to pass through to the light; it does not terminate on the switch.
- Switch 2 (Load Box): The 3-core + earth arrives. Connect earth to earth. Connect the blue strapper to L1 and the brown/yellow strapper to L2. (Maintain the same L1/L1 and L2/L2 pairing as Switch 1).
- Switch 2 (Load Termination): Take the cable heading to the light. Connect its brown (Switched Line) wire to the COM terminal on Switch 2.
- At the Light Fixture: Connect the fixture's brown to the switched line, blue to the passed-through neutral, and earth to the fixture earth.
Verify and Test: Expected Multimeter Readings
Do not just throw the breaker and hope. Use your multimeter to verify the logic before energizing the lighting MCB/breaker.
| Test Point (Power OFF) | Multimeter Setting | Expected Reading | What it Proves |
|---|---|---|---|
| Line to Ground (at Switch 1 feed) | Continuity / Ohms | OL (Open Loop) or >1 MΩ | No dead short to ground before power-on. |
| Traveler 1 to Traveler 2 (Switch disconnected) | Continuity / Ohms | OL (Open Loop) | Travelers are not shorted together in the wall. |
Energize the circuit and perform these live voltage checks (CAUTION: LIVE VOLTAGE):
- US (120V System): Measure between the Line (black feed) and Ground at Switch 1. Expected: 114V - 126V AC. Measure between Traveler 1 and Ground. Flip the switch. The traveler reading should swap between ~120V and 0V.
- UK/AU (230V System): Measure between Line (brown) and Earth at Switch 1. Expected: 220V - 240V AC. Measure between L1 and Earth. Flipping the switch should toggle L1 and L2 between ~230V and 0V relative to earth.
The Most Common Botch (And How to Fix It)
If you finish the job, turn on the breaker, and find that the light only turns on and off when the other switch is held in a specific position (e.g., Switch A works fine, but Switch B only works if Switch A is flipped UP), you have committed the most common 2-way/3-way wiring botch.
The Physics: A 3-way/2-way switch is a single-pole, double-throw (SPDT) device. The Common terminal is the wiper that routes power to either Traveler 1 or Traveler 2. If you feed power into a Traveler, the switch can only pass power when the internal wiper happens to be touching that specific Traveler. The other switch is left completely dead in one state.
The Fix: De-energize the circuit. Identify the wire that is the permanent feed from the panel (or the wire heading directly to the light fixture). Move that specific wire to the black screw (US) or COM terminal (UK/AU). The two wires running between the two switch boxes must be the only wires on the brass / L1 / L2 screws. Retest.
