Swapping a plain light switch for a dimmer is one of the most satisfying hour-long jobs in the house. But it trips people up more than it should, usually at the wiring stage.
Isolate the circuit at the consumer unit. A standard one-way dimmer has two terminals — the switched-live cable cores land on them, and it isn't fussy about which core goes where. Fit the dimmer where the old switch was, restore power, done. A two-way setup (one light, two switches) uses three terminals — COM, L1 and L2 — and only one of the two switches needs to be the dimmer.
That's the shape of the job. Now the detail that separates a clean install from a flickering, buzzing mess — because with modern LED lighting, the wiring is the easy part. Choosing the right dimmer is where it goes wrong.
1. Safety first — this is non-negotiable
Turn the circuit off at the consumer unit, not just at the wall switch. Pull the lighting MCB or the fuse for that circuit. Then test that it's actually dead with a voltage tester on the terminals before you touch a core. Lighting circuits can share neutrals and surprise you.
Trade Tip: Replacing a switch like-for-like is generally allowed as a competent DIY job in England and Wales, but any new circuit or work in a special location (a bathroom, for instance) is notifiable under Part P and belongs with a registered electrician. If in doubt, it's a twenty-minute job for a pro and worth it. We'll say when to call one — we won't pretend everything's a DIY.
2. One-way dimmer wiring
The most common swap. Behind your existing one-way switch you'll typically find a switched-live pair: a permanent live feeding in and a switched live going up to the light. On a one-way dimmer there are two terminals — often marked with arrows or simply “1” and “2.” The two cores land one on each. There's no polarity to worry about on a basic one-way dimmer, so you can't get the two the wrong way round.
Sleeve any blue core being used as a switched live with brown (or a red/brown marker) so the next person knows it's live, not neutral. Fold the cores gently back into the box, screw the dimmer to the back box, and you're done.
One thing that catches people out: a dimmer body is deeper than a plain switch. If your existing back box is a shallow 16mm one, the module can foul the cores and refuse to sit flush. Swap it for a 25mm box before you start rather than fighting it at the end — a five-minute job done early beats a cracked faceplate done late.
For a single light, the Knightsbridge SN2161 1-Gang 2-Way Dimmer Switch is the workhorse. It's a leading-edge dimmer rated 40–400W for traditional lamps and 3–100W for LED, and being a 2-way unit it'll happily run as a simple one-way dimmer too — you just don't use the third terminal. White finish, standard single plate, drops straight onto a UK single back box.
3. Two-way dimmer wiring (two switches, one light)
Landing, stairs, a big lounge with a switch at each door — that's a two-way circuit. Here each switch has three terminals: COM, L1 and L2. The two “strapper” cores run between L1 and L2 of the two switches, and the COM terminals carry the feed and the switched live to the light.
The golden rule: only one of the two switches needs to be the dimmer. You dim from one position and use a standard 2-way switch at the other. Trying to put dimmers at both ends of the same circuit is the classic mistake — they fight each other and you get flicker or dead spots. One dimmer, one plain 2-way switch. That's it.
Running two circuits from one plate — say two lighting zones in an open-plan room — the Knightsbridge CU2162 2-Gang 2-Way Dimmer Switch gives you two independent leading-edge dimmers on a single white plate, each rated 40–400W and 3–100W for LED. Two knobs, two circuits, one back box.
4. Leading edge vs trailing edge — the LED trap
This is the bit nobody tells you, and it's where the money gets wasted. Older dimmers are leading edge, designed for the big resistive load of filament bulbs. Modern LEDs are a tiny, fussy electronic load. Pair a cheap leading-edge dimmer with low-wattage LEDs and you can get flicker, buzz, a narrow dimming range or lamps that won't switch off fully.
Trade Tip: Two things save you. First, use a dimmer with a proper LED rating and stay inside it — our featured units are rated 3–100W for LED, so four 8W downlights (32W) sit comfortably in range. Second, and most important: use dimmable LED lamps. A non-dimmable LED on any dimmer will flicker or fail no matter how good the dimmer is. Trailing-edge dimmers give the smoothest result on LEDs — worth asking for if we've got them in — but a correctly-loaded leading-edge dimmer with genuinely dimmable lamps will do the job cleanly.
Building a custom grid or matching an existing plate range, the Knightsbridge SN400M Dimmer Module is the flexible option — a 40–400W leading-edge, 2-way dimmer module you drop into a grid plate of your choice. Same wiring logic as the switches above, just modular.
See the full range of dimmer switches and, when you're buying lamps to go with them, our dimmable LED bulbs.
5. Summary table
| Setup | Terminals | Key rule |
|---|---|---|
| One-way dimmer | 2 (no polarity) | Two switched-live cores, one per terminal |
| Two-way dimmer | 3 (COM, L1, L2) | Only ONE end is a dimmer; other is a plain 2-way switch |
| LED load | — | Stay inside the LED wattage rating (e.g. 3–100W) |
| Lamps | — | Must be dimmable LEDs — non-dimmable will flicker |
| Edge type | — | Trailing edge = smoothest on LED; loaded leading edge works |
6. Expert Help in the West Midlands
We've been selling switchgear from Villa Road, Birmingham since 1975, and dimmer-plus-LED mismatch is the single most common “why's it flickering?” that walks through the door. Bring the wattage and the number of your LED lamps to the trade counter and we'll match you a dimmer that behaves — and tell you straight if your lamps aren't dimmable in the first place. No guesswork, no returns trip.
Frequently asked questions
Does it matter which wire goes where on a one-way dimmer?
No. A basic one-way dimmer has two terminals with no polarity, so the two switched-live cores can go on either terminal. On a two-way dimmer (COM, L1, L2) the terminals do matter and follow standard 2-way switch wiring.
Why does my LED flicker on a dimmer?
Usually one of two things: the lamps aren't dimmable, or the total LED wattage is outside the dimmer's rated range. Fit genuinely dimmable LED lamps and keep the load inside the dimmer's LED rating.
Can I put a dimmer at both ends of a two-way circuit?
No — use a dimmer at one position only, with a standard 2-way switch at the other. Two dimmers on the same circuit interfere with each other and cause flicker or dead spots.
Replacing a switch like-for-like is generally a permitted DIY task in England and Wales, but new circuits and work in bathrooms or other special locations are notifiable under Part P — use a registered electrician. Always isolate and test dead before working.