Light switch wiring explained
By Azzeddine Kebbi, NAPIT-registered electrician and director of Kebbi Electrical Services · Updated 10 November 2026
Most homes use three kinds of light switch wiring: one-way (one switch), two-way (two switches for one light, such as the top and bottom of the stairs) and intermediate (three or more switches). Here is how each works and what the wires in the switch mean.
First, stay safe
- Always switch off the circuit at the consumer unit and prove it is dead before touching any wiring.
- If you are not sure, do not open the switch. Use a registered electrician.
The three types
One-way
One switch controls a light. It has two terminals, usually marked COM and L1.
Two-way
Two switches control one light. Each has COM, L1 and L2, linked by a three-core cable called the strappers.
Intermediate
Sits between two two-way switches so you can add a third switch or more, for example on a long landing.
What the wire colours mean
- Brown (or red in older homes): live.
- Blue (or black in older homes): in a switch this is often the switched live, so it should be sleeved or marked brown.
- Green and yellow: earth. It must connect to metal back boxes and metal switch plates.
- Grey, black and brown in a three-core cable: the strappers between two-way switches.
See UK wiring colours, old and new for more.
When to call an electrician
- A switch that buzzes, crackles, feels warm or smells of burning.
- Old wiring with no earth at the switch.
- Adding a new switch position or turning a one-way switch into two-way.
- Any work in a bathroom near the bath or shower.
Common questions
What is the difference between one-way and two-way switches?
A one-way switch controls a light from one place. Two-way switches let you control the same light from two places, such as the top and bottom of the stairs.
Can I use a two-way switch as a one-way switch?
Yes. A two-way switch works as one-way using the COM and one of the L terminals.
Why is there a blue wire on my light switch?
In switch wiring the blue or black wire is often used as the switched live. It should be sleeved or marked brown so everyone knows it can be live.
Do I need an electrician to change a light switch?
Like-for-like replacement is not notifiable, but it means working with mains wiring. If you are not confident and competent, use a registered electrician.
Need switches fitted or moved?
We fit and move switches, add two-way switching and fix faulty lighting circuits. See our electrical installations and repairs.