A 2-way light switch lets you control one light fitting from two different positions – for example, the top and bottom of a staircase, or both ends of a long hallway. This guide shows the standard UK wiring for a 2-way arrangement using 3-core and earth cable between the two switch positions, in accordance with BS 7671:2018 (the IET Wiring Regulations, 18th Edition).

Notifiable work: some electrical work in dwellings is notifiable under Part P of the UK Building Regulations. New circuits, work in a special location (such as a bathroom), or alterations to the consumer unit are usually notifiable and should be done by a Part P registered electrician. Replacing a like-for-like switch on an existing circuit is generally not notifiable, but if you are not confident with the work, use a qualified electrician.

Before you start: safety

1. Isolate the circuit at the consumer unit. Turn off the relevant lighting circuit MCB or RCBO. If you are unsure which circuit feeds the switch, turn off the main switch.
2. Lock or label the consumer unit so the circuit cannot be accidentally re-energised while you are working.
3. Prove dead with a GS38-compliant two-pole voltage tester using the standard safe isolation procedure: prove the tester on a known live source, test the target circuit at the working position, then re-prove the tester on the known live source. Non-contact “stick” testers alone are not sufficient for safe isolation.
4. Brown-sleeve all switched-live conductors. BS 7671 Table 51 requires non-brown conductors carrying line voltage – including the black and grey cores in 3-core and earth cable when used as strappers – to be identified with brown sleeving or tape at every termination.

What you will need

• Two 2-way light switches (each with COM, L1 and L2 terminals)
• 3-core and earth cable, 1.0 mm² or 1.5 mm² – the run between the two switches
• Twin and earth cable (6242Y), 1.0 mm² or 1.5 mm² – supply and run to light, if not already in place
• Brown identification sleeving or PVC tape (for the black and grey strappers)
• Green and yellow earth sleeving
• GS38-compliant two-pole voltage tester
• Insulated terminal screwdriver, side cutters, wire strippers
• Back boxes at least 35 mm deep for comfortable wiring – shallower boxes work but make routing the conductors harder

How 2-way switching works

Each 2-way switch has three terminals: COM (common), L1 and L2. COM is the pivot – in one switch position it is electrically connected to L1, and in the other position it is connected to L2.

To control a single light from two positions, the two switches are connected by two strappers (also called travellers) running between their L1 and L2 terminals. The permanent live enters one switch’s COM, and the switched live leaves the other switch’s COM and runs to the light fitting. Toggling either switch alters which strapper completes the circuit, switching the light on or off independently of the other switch’s position.

In UK domestic installations the strappers run inside 3-core and earth cable (brown, black and grey cores, plus a circuit protective conductor) between the two switch boxes. The cable from the supply to the first switch, and from the second switch to the light, is normally twin and earth (6242Y).

UK 2-way switch wiring diagram

The diagram shows the standard UK 2-way arrangement. Permanent live (brown) enters Switch 1 COM. The black and grey cores of the 3-core and earth cable connect L1 and L2 of Switch 1 to L1 and L2 of Switch 2 as strappers – both sleeved brown at every termination per BS 7671 Table 51. The switched live exits Switch 2 COM and runs to the light fitting. Neutral runs from the supply directly to the light fitting and does not pass through either switch – this is the standard UK loop-in topology and the reason most UK switch positions do not have a neutral wire available. Earth (the circuit protective conductor) is terminated at every back box and at the light fitting.

Step-by-step wiring

1. Isolate the circuit at the consumer unit and prove dead with your voltage tester (see safety section).
2. Remove the existing switch faceplates if replacing existing switches. Photograph the existing wiring before disconnecting anything – useful reference if you need to revert.
3. Run the cables if installing new wiring: 3-core and earth between the two switch positions, and twin and earth from the supply to Switch 1 and from Switch 2 to the light.
4. At Switch 1, connect:
   • Permanent live (brown of supply cable) → COM
   • Black core of 3-core (sleeved brown at termination) → L1
   • Grey core of 3-core (sleeved brown at termination) → L2
   • Earth conductor → earth terminal on the back box, sleeved green/yellow where bare
5. At Switch 2, connect:
   • Black core of 3-core (sleeved brown) → L1 (matching Switch 1)
   • Grey core of 3-core (sleeved brown) → L2 (matching Switch 1)
   • Switched live (brown of cable to light) → COM
   • Earth conductor → earth terminal on the back box
6. Gentle pull-test every conductor after termination. Loose terminations are a common cause of arcing and intermittent faults.
7. Reassemble: tuck conductors carefully into each back box. Make sure no insulation is pinched under the screw clamps and that the faceplate sits flush.
8. Restore power at the consumer unit and test that the light can be switched on and off from either switch in any combination of switch states.

Common faults

The light only works from one of the two switches. The most common cause is the switched live (or permanent live) connected to L1 or L2 instead of COM. Re-check both switches: the brown wire from the supply must enter the COM of Switch 1, and the cable to the light must exit the COM of Switch 2.

The light is permanently on or permanently off regardless of switch position. One of the strappers may be connected to the wrong terminal on one side – swapped between L1 and L2. Swap the strappers at one of the two switches.

The light flickers or buzzes (when on a dimmer). This is usually a dimmer / lamp mismatch rather than wiring – LED lamps generally need a trailing-edge dimmer; older filament lamps tolerated leading-edge designs. Our upcoming leading edge vs trailing edge guide will cover this in detail. Samotech’s rotary dimmer switches use trailing-edge dimming designed for LED loads.

Adding a dimmer to a 2-way circuit (no neutral required)

If you’re replacing one of the 2-way switches with a dimmer, and your switch position does not have a neutral wire – the case for most UK installations, because the neutral runs directly to the light fitting and never passes through the switch – you need a no-neutral dimmer that wires to live and switched live only.

If you also need smart control – Alexa, Google Home, Apple Home, Home Assistant, app dimming, or schedules – the SM323 smart dimmer range uses the same no-neutral wiring with your choice of protocol:

• SM323 Zigbee Dimmer – Zigbee mesh; works with Home Assistant, Philips Hue Bridge, SmartThings, Hubitat
• SM323-WF WiFi Dimmer – direct WiFi; Tuya / Smart Life, Alexa, Google Home
• SM323-MT Matter Dimmer – Matter-over-Thread or WiFi; Apple Home, Alexa, Google Home