This guide shows how to replace one of the two switches in a 2-way lighting circuit with a dimmer, in accordance with BS 7671:2018 (the IET Wiring Regulations, 18th Edition). It assumes you already have a working 2-way switch arrangement – if not, start with our how to wire a 2-way light switch (UK) guide.

Important – only one of the two switches can be a dimmer. Standard 2-way dimmers will fight each other if installed at both positions, causing flicker, hum, or premature dimmer failure. The other switch must remain a standard 2-way switch (or a matched dummy dimmer for aesthetics). The exception is dedicated master / slave dimmer systems with proprietary protocols (such as Varilight V-Pro Multi-Point, or smart dimmers paired with a secondary controller).

Notifiable work: some electrical work in dwellings is notifiable under Part P of the UK Building Regulations. Replacing an existing switch with a dimmer 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 circuit at the consumer unit (MCB or RCBO) and verify dead using a GS38-compliant two-pole voltage tester. Prove the tester on a known live source, test the target circuit at the working position, then re-prove the tester to confirm it’s still functioning.
2. Lock or label the consumer unit so the circuit cannot be accidentally re-energised.
3. Black and grey conductors used as strappers must be identified with brown sleeving or tape at every termination (BS 7671 Table 51).

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

What you’ll need

• One 2-way dimmer rated correctly for your load (see Choosing the right dimmer below)
• One standard 2-way switch for the other position (or leave the existing one if it’s in good order)
• 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 – most UK dimmers need this for comfortable installation; shallower boxes can be tight and risk damage to the dimmer’s heat sink

How a dimmer fits into a 2-way circuit

A 2-way dimmer has three terminals just like a standard 2-way switch – one common terminal (labelled COM, C, L or Common depending on the manufacturer) and two strapper terminals (L1 and L2). The wires from the existing 2-way switch transfer across to the dimmer’s terminals on a like-for-like basis: whichever wire was in the COM terminal of the old switch goes to the dimmer’s common terminal; the other two go to L1 and L2 in either order (L1 and L2 are interchangeable for standard 2-way operation).

Some 2-way dimmers must be installed at the load end of the circuit – the switch position from which the cable runs to the light fitting, rather than the position where the permanent live enters. This is manufacturer-specific; check the wiring instructions supplied with your dimmer. If your dimmer specifies load-end installation but the existing arrangement has the permanent live at that position, you may need to swap the supply and load ends of the circuit.

Wiring diagram (UK)

The diagram shows a 2-way circuit with the dimmer at the load end and a standard 2-way switch at the supply end. Permanent live (brown) enters the standard switch at COM. Black and grey strappers from 3-core and earth cable (both sleeved brown per BS 7671 Table 51) connect L1 to L1 and L2 to L2 between the two units. The dimmer’s common terminal outputs the switched and dimmed live to the light fitting. Neutral runs from the supply directly to the light fitting and does not pass through either switch – the standard UK loop-in topology, and the reason most UK switch positions don’t have a neutral wire available. 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.
2. Remove the existing wall switch at the position where the dimmer will go. Photograph the existing wiring before disconnecting anything – this is your reference if you need to revert.
3. Identify the common conductor – the wire that was in the COM terminal of the old switch. This is the wire that goes to the dimmer’s common terminal.
4. At the dimmer, connect:
   • The wire that was in the old switch’s COM → dimmer’s common terminal (labelled C, COM, Common or L depending on the manufacturer)
   • The other two wires (from L1 and L2 of the old switch) → dimmer’s L1 and L2 terminals in either order
   • Earth conductor → the dimmer’s earth terminal (mandatory for metal-faceplate dimmers) or the back box earth terminal
5. Confirm brown sleeving on the black and grey conductors at every termination.
6. Gentle pull-test every termination.
7. Reassemble: tuck conductors carefully without pinching them under the dimmer’s heat sink or the screw clamps. The dimmer should sit flush against the back box.
8. Restore power and test from the other 2-way switch position – the light should turn on and off from the standard switch, and the dimmer should control brightness when the light is on.

Dimmer terminal label variations

UK manufacturers don’t all use the same labels. The function is identical – one terminal for the common conductor and two for the strappers – but the labels vary:

• Most standard 2-way switches: COM, L1, L2
• Varilight 2-way dimmers: C, L1, L2 (with C positioned next to L1 and L2, not at the opposite end as on a standard switch)
• Some other dimmers: L, L1, L2 (L being equivalent to common)
• Some dimmers: L, Load, L1 (where L is the permanent live in, Load is the switched live out, L1 is a single strapper)

Always follow the wiring diagram supplied with your specific dimmer.

Common faults

The light dims but doesn’t switch off fully, or flickers wildly. Two dimmers may be installed at both positions, which standard 2-way dimmers can’t handle. One must be replaced with a regular 2-way switch (or a matched dummy dimmer).

The light flickers when dimmed. Usually a dimmer / lamp mismatch – LEDs generally need a trailing-edge dimmer; older filament lamps tolerated leading-edge designs. See our upcoming leading edge vs trailing edge guide.

The dimmer hums or buzzes audibly. A faint hum is normal at full brightness on many dimmers; loud buzzing usually indicates either a dimmer rating too low for the load, or a non-dimmable lamp on the circuit.

The dimmer is hot to touch. Some warmth is normal; very hot dimmers usually mean the load is close to or exceeds the dimmer’s maximum rating. Remember LED de-rating (see below).

The dimming works in reverse, or only across part of the rotation. The wire from the old switch’s COM may have gone to L1 or L2 of the dimmer instead of the common terminal. Re-check by photo or by looking at the other 2-way switch’s wiring.

Choosing the right dimmer for your LEDs

For LED lamps, you’ll usually want a trailing-edge dimmer rated for the total LED wattage on the circuit. LED loads need significant de-rating from the dimmer’s incandescent rating – as a rough guide a dimmer rated 400 W for incandescent might only support around 120 W for LEDs, with a per-lamp count limit (often around 10 lamps).

Older leading-edge dimmers can work with some LEDs but tend to cause flicker, audible buzz, or “snap” behaviour mid-range. Modern LED installations should specify trailing-edge.

Smart control on a 2-way dimmer circuit

If you also need smart control on the dimming – Alexa, Google Home, Apple Home, Home Assistant, app dimming, or schedules – the SM323 smart dimmer range sits at the primary dimmer position with a standard 2-way switch (or a matched dummy dimmer for consistent aesthetics) at the secondary position:

• 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

All three are no-neutral dimmers designed for UK 2-way circuits where the switch position doesn’t have a neutral wire available.