Just changing the main lighting in a room by installing a dimmer switch can make a huge difference. You can decide between full-on illumination and a warm, ambient glow – or somewhere in between – with just one setup. It’s pretty easy to achieve, too. In this article, we’ll have a look at how to install a dimmer switch and what needs to be taken into account beforehand.

Before you start

First of all, there are a few things you need to consider before installing a dimmer switch. In the past, when incandescent (tungsten and halogen) bulbs were the norm, it was a pretty straightforward job. All you had to do was remove the old on/off switch and replace it with the dimmer. That’s because those types of switch just regulated the voltage between low and full, and that directly affected the amount of light the bulb produced. 

LED bulbs are quite different, however. First of all, they require much less energy, as they don’t work by heating up a filament. That means they have a smaller wattage – typically about an eighth of that of a tungsten bulb of similar brightness. Second, while any incandescent bulb can be dimmed, LED bulbs have to be specially designed to be dimmable. A standard LED bulb for a normal domestic bayonet or screw fitting is designed to be in the on or off state – not anywhere in between. So make sure you buy LED bulbs that are dimmable.

Also, because of the lower power requirements and the way they dim, you’ll need to get a dimmer switch that’s designed for use with LEDs. There are two types of dimmer switch, called trailing-edge and leading-edge. It’s quite a technical difference and describes the part of the electrical wave at which the power cuts out and restarts many times a second. The best dimmer switches for LEDs are trailing-edge switches. It’s worth knowing this, although the product description should also make it clear that they are optimised for LEDs. They will also work with legacy incandescent bulbs.

Installing the switch

We’ve written about installing a regular light switch elsewhere in our help guides. This process is fundamentally the same, with the above considerations noted. Switching from a standard on/off switch to a dimmer switch would fall under a “like for like” replacement under Electrical safety: Approved Document P of the Building Regulations. You will not need to change the wiring or undertake any significant electrical or building work. However, the same warning applies – only attempt this job if you are competent and experienced, or have help from someone who is. Read up on electrical safety and take precautions such as wearing insulating gloves and safety glasses. If you’re in any doubt, please contact a qualified electrician.

1. Isolate the power

Go to the consumer unit (you might know it as a fuse box) and locate the switch for the circuit you’re working on. They should all be labelled, e.g. “downstairs lights” or “upstairs sockets”. If it isn’t clear which switch isolates the circuit you’re working on, flip the main switch, which will turn everything off. You might need to reset a few things, such as clocks and timer switches when you turn it back on later.

2. Remove the old faceplate

Take off the faceplate with a screwdriver. If it’s a screwless faceplate, it should clip off, and there will be a screwed plate underneath it. All the while, do not touch any wiring – treat everything as if it is live. Pull it as far as it will go without forcing it too much. There will be some resistance and the wires are quite stiff, but it should move.

3. Test the current

Using a voltage tester, check the wires for any current to confirm that you have turned off the correct circuit. Follow the tester’s instructions, but normally you are simply touching an exposed part of the circuit such as a wire or a terminal with the prong of the tester.

4. Note which wires connect to which terminals

There should be three wires connected to the old switch:

  • Brown or Red → Common terminal
  • Blue or Black → L1 terminal
  • Green and yellow→ Earth terminal (⏚)

Take a note of each, and take a photo if it helps you to remember.

5. Remove the wires

Using a VDE screwdriver, undo the screws connecting the switch to the wires. Now you can put the old switch to one side. Don’t throw it out – you might need to reconnect it if the dimmer doesn’t work.

6. Inspect the old box and compare the switches

Dimmer switches often need a little more depth than regular switches because they have different components. Make sure that the new dimmer switch will fit in the space left by the old switch. If it’s too shallow, you’ll need to have a new back box installed. This is no longer a task for an amateur, so call a professional electrician. If you are installing a surface pattress box (i.e. a box that stands proud of the wall rather than being sunk into it), you should be able to swap it for a deeper one without an electrician, however.

7. Connect the wires

If you’re satisfied that there’s enough room for the dimmer switch, reconnect the wires as per the colour guide above. Make sure they are screwed firmly to the terminals so they won’t come out if you tug them hard, but don’t overtighten them as that can damage the wire or the terminal. Ensure that the terminal screw is gripping the exposed copper wire, not the plastic insulation.

8. Attach the dimmer switch faceplate

Now, you can attach the dimmer switch to the back box using the screws provided. Make sure you don’t trap any of the wires or bend them excessively, as that can cause them damage or make them come loose. The new faceplate should fit snugly against the box, with no visible gaps. If the screws are getting difficult to turn but there’s still a gap, you might have a wire trapped somewhere. Release it fully and try again. If it still won’t close, you might need a new back box, so call an electrician or get a deeper pattress box if you have that type of switch.

9. Turn the power back on

Now, you can turn the power back on at the consumer unit. Test your dimmer switch. If it’s working, that’s great. If not, make sure you have the correct type or bulbs (i.e. dimmable) and switch (i.e. trailing edge). If there’s no power at all, go back to step 1 and try again, or call an electrician, who will be able to test your circuitry, switch and bulbs.

Whether you’re doing it yourself or hiring a pro, check out our huge collection of dimmer switches – there’s bound to be one to suit your home.