An intermediate switch (UK terminology – the US equivalent is a “4-way switch”) lets you control one light fitting from three or more switch positions. Common uses are long hallways, multi-floor staircases, and large open-plan rooms. The intermediate switch always sits in the middle of the chain, between two standard 2-way switches at the supply and load ends. You can daisy-chain multiple intermediate switches between the two 2-way endpoints for 4, 5 or 6-position control – each intermediate adds one more switch position. This guide shows the standard three-position arrangement (2-way + intermediate + 2-way) using 3-core and earth cable between the switches, in accordance with BS 7671:2018 (the IET Wiring Regulations, 18th Edition). If you have not wired a 2-way circuit before, start with our How to wire a 2-way light switch (UK) guide.

Important – the intermediate switch is always the middle switch in the chain. The two switches at the ends of the circuit must be standard 2-way switches – never intermediate. If you put an intermediate switch in an end position instead of the middle, the circuit will not work correctly.

Notifiable work: some electrical work in dwellings is notifiable under Part P of the UK Building Regulations. Replacing existing switches like-for-like on an existing circuit is generally not notifiable. New circuits, work in a special location (such as a bathroom), or consumer unit alterations usually are. If in doubt, use a Part P registered electrician.

Before you start: safety

1. Isolate the lighting circuit at the consumer unit (MCB or RCBO) and verify dead at every switch position using a GS38-compliant two-pole voltage tester. Prove-test-prove on a known live source at each location.
2. Lock or label the consumer unit so the circuit cannot be re-energised while you are working.
3. Non-brown conductors used as strappers (black and grey cores of 3-core and earth cable) must be identified with brown sleeving or tape at every termination (BS 7671 Table 51).
4. Metal-faceplate switches must be earthed via the earth terminal on the switch plate – not just the back box.

Full safe-isolation detail is in the 2-way light switch guide.

What you will need

• Two 2-way switches (for the supply end and the load end positions)
• One intermediate switch (for the middle position) – has 4 terminals, usually labelled L1, L2, L3, L4
• 3-core and earth cable between each pair of adjacent switches (Switch 1 to intermediate, intermediate to Switch 2)
• Twin and earth (6242Y) from the supply to Switch 1, and from Switch 2 to the light fitting
• Brown identification sleeving or PVC tape
• 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 at all three switch positions – the intermediate switch in particular needs the room for four terminations plus earth

How an intermediate switch works

A standard 2-way switch has three terminals: COM, L1, L2. An intermediate switch has four terminals: L1, L2 (incoming, from the previous switch) and L3, L4 (outgoing, to the next switch).

Internally, the intermediate switch has two positions:

• Straight-through: L1 → L3 and L2 → L4
• Crossed: L1 → L4 and L2 → L3

Toggling the intermediate switch flips between these two states, effectively swapping the strappers. Combined with the two 2-way switches at the ends, this means a toggle at any of the three switch positions can make or break the circuit. Adding more intermediate switches (4-way, 5-way control) just chains more cross-over points between the two 2-way endpoints – the operating principle is the same.

Wiring diagram (UK)

The diagram shows the standard UK three-position arrangement: Switch 1 (2-way) at the supply end, the intermediate switch in the middle, and Switch 2 (2-way) at the load end. Permanent live (brown) enters Switch 1 COM. Black and grey cores of 3-core and earth cable (both sleeved brown per BS 7671 Table 51) connect Switch 1 L1 and L2 to the intermediate L1 and L2. The intermediate outputs at L3 and L4, which connect through a second 3-core and earth cable to Switch 2 L1 and L2. Switched live exits Switch 2 COM and runs to the light. Neutral runs from the supply directly to the light fitting and does not pass through any of the switches – the standard UK loop-in topology. Earth is bonded to every back box and to the light fitting.

Step-by-step wiring

1. Isolate the circuit at the consumer unit and prove dead at every switch position.
2. Remove the existing wall switches if replacing. Photograph the existing wiring before disconnecting anything.
3. At Switch 1 (2-way, supply end), connect:
   • Permanent live (brown of supply twin-and-earth) → COM
   • Black core of 3-core and earth (sleeved brown at termination) → L1
   • Grey core of 3-core and earth (sleeved brown at termination) → L2
   • Earth conductor(s) → earth terminal on the back box, sleeved green/yellow where bare
4. At the intermediate switch (middle), connect:
   • Black core from Switch 1 (sleeved brown) → L1
   • Grey core from Switch 1 (sleeved brown) → L2
   • Black core to Switch 2 (sleeved brown) → L3
   • Grey core to Switch 2 (sleeved brown) → L4
   • Both earth conductors joined and terminated at the back box (or at the switch plate earth point if metal-faceplate)
5. At Switch 2 (2-way, load end), connect:
   • Black core from intermediate (sleeved brown) → L1
   • Grey core from intermediate (sleeved brown) → L2
   • Switched live (brown of cable to light) → COM
   • Earth conductor → earth terminal on the back box
6. Confirm brown sleeving on every black and grey strapper at every termination.
7. Gentle pull-test each termination. Loose conductors cause arcing and intermittent faults.
8. Reassemble: tuck conductors carefully into each back box. Make sure no insulation is pinched under the screw clamps.
9. Restore power and test that the light can be switched on and off from each of the three switch positions, in any combination of switch states.

Terminal label variations

Most UK intermediate switches use L1, L2, L3, L4 labelling, with L1/L2 grouped together on one side and L3/L4 on the other. Some makes (notably older MK and BG ranges) use numeric 1, 2, 3, 4 instead. Crabtree and a few others have a wiring diagram printed on the back of the switch showing the contact arrangement directly. The function is identical regardless of the labelling – always follow the diagram supplied with your specific switch.

Common faults

One switch position (often the intermediate) seems to operate “backwards” or does not toggle the light at all. Usually L1–L2 or L3–L4 are swapped at the intermediate terminals. Swap them and re-test.

The light works from two of the switches but not the third. A strapper is not landed correctly at the affected switch – typically at the malfunctioning switch or at the intermediate.

All switches operate the light but down = on / up = off. Cosmetic only – swap L1–L2 at one of the 2-way end switches to reverse the orientation.

Bulb flickers or sometimes will not switch off. Almost always a loose termination. Re-tighten every screw at all three switches.

Adding a dimmer to an intermediate circuit

Only one switch in the entire intermediate chain can be a dimmer (same rule as a 2-way circuit). The dimmer should replace one of the 2-way endpoints – not the intermediate. The intermediate switch and the remaining 2-way switch stay as standard non-dimming switches. Some dimmers require load-end installation; check the manufacturer’s instructions.

Smart control on an intermediate circuit

If you need smart control on an intermediate circuit, the SM323 smart dimmer range can replace one of the 2-way endpoints. The intermediate switch and the other 2-way switch remain standard. The smart dimmer at the endpoint controls the dimming level; toggling any of the other switches in the chain turns the light on or off at the last dimmed brightness.

• SM323 Zigbee Dimmer – Zigbee mesh; 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

All three are no-neutral dimmers – suitable for UK switch positions where the neutral wire is not available at the back box (the standard UK lighting topology).