How to Wire a Light Switch (One-Way Switching, UK)
Updated: Oct 9
A one-way light switch has two conductors to connect. The permanent live goes into the terminal marked COM, the switched live that runs back up to the light goes into L1, and the earth goes to the earth terminal on the back box or plate. Before you touch any of it, the lighting circuit has to be switched off at the consumer unit and proved dead, because a switch still has a live conductor in it when the light is off.
That covers one light controlled from one place. A light with a switch at the top and bottom of the stairs uses two-way switching and a three-core cable, which we cover in our guide on how to wire a two-way light switch. Three or more switches on one light need an intermediate switch in the middle, explained in our post on intermediate switch wiring.
Can you replace a light switch yourself?
Yes. In England, replacing a light switch like for like is not notifiable under Part P of the Building Regulations. Approved Document P limits notifiable work to a new circuit, a replacement consumer unit, or additions and alterations in the space around a bath or shower, and says replacements, repairs and maintenance anywhere else are not notifiable.
Not notifiable does not mean anything goes. The same guidance says non-notifiable work should still be done to BS 7671, and a local authority can take enforcement action if it finds the work unsafe. Swapping a worn or cracked switch for a new one is a reasonable job for a careful householder. Moving a switch, adding a new switch position or running new cable into a wall is where our lighting installation and repairs team usually takes over. A cable buried less than 50 mm deep in a wall must run in a safe zone (straight up, down or across from an accessory, or within 150 mm of the top of the wall or a corner) and must also have 30 mA RCD protection unless it is mechanically protected, for example in earthed metal conduit (BS 7671 regulations 522.6.202 and 522.6.203).
Safe isolation before you start
Never assume the circuit is off. Work through these steps every time.
Switch off the lighting circuit at the consumer unit, either its circuit breaker (or fuse) or the main switch.
Stop anyone switching it back on: tape over the breaker and leave a note, or use a lock-off if your board takes one.
Check your voltage tester works on a proving unit or a known live source.
Remove the switch plate and test between every conductor, and between each conductor and earth.
Check the tester again on the proving unit, so you know it did not fail halfway through.
Use a two-pole voltage indicator that makes contact with the conductors. HSE guidance GS38 says non-contact "voltage sticks" are for identifying live equipment, not for proving something dead, and that any device used to prove dead should be checked before and after use.
What you will find behind a one-way switch
In most homes a one-way switch is fed by a single twin-and-earth cable that drops down from the ceiling rose. This is called the switch drop. In wiring installed to the current colours it contains:
a brown core, the permanent live coming down from the light
a blue core, the switched live going back up to the light
a bare earth, which should be sleeved green and yellow
In older installations wired before the 2004 colour change, you will see red in place of brown and black in place of blue.
The blue (or black) core in a switch drop is not a neutral. The switch only ever breaks the line, so both cores are carrying live when the switch is on. Because the blue is working as a line conductor, it should be marked brown with sleeving or tape at each end. A lot of older work skipped this, and correcting it while the plate is off costs nothing.
If you find two or more cables, a neutral bunch, or three-core cable, stop and take a photo. That points to two-way switching, a loop at the switch, or a supply for a smart switch, and the one-way method below will not be right for it.
Wiring the new switch, step by step
Take a clear photo of the old switch before disconnecting anything.
Loosen the terminals and free the conductors one at a time.
Check the conductor ends. If the copper is blackened, nicked or brittle, cut it back to clean copper and strip just enough insulation for the conductor to sit fully in the terminal.
Put the brown (or red) permanent live into COM.
Put the blue (or black) switched live into L1, with brown sleeving over it.
Leave L2 empty. Most switches sold are "two-way" switches with COM, L1 and L2, and they work as one-way switches using COM and L1 only.
Connect the earth. A metal back box has an earth terminal, and a metal switch plate has its own earth terminal that must be connected to it with a short earth lead. On a plastic plate, the earth goes to the terminal in the box. Never cut an earth off because there seems nowhere to put it.
Tighten every terminal firmly and give each conductor a gentle tug.
Fold the cables back so none can be pinched by the plate screws, then fit the plate without overtightening.
Restore power and try the switch.
If the light comes on with the rocker up, switch off at the consumer unit, prove dead again, then move the conductor from L1 to L2. In the UK a switch is normally on when pressed down.
What usually goes wrong
Loose terminals: the classic switch fault. A loose conductor heats up, and the first signs are a warm plate, a crackle or buzz when switching, or a light that flickers.
Earth left off a metal plate: if the plate ever became live, nothing would clear the fault.
A light that will not switch off: both conductors are usually in the same terminal, or the switch is not the only thing controlling the light.
When it is a job for an electrician
Stop and get help if the cable has rubber or fabric insulation that crumbles when you move it, if there is no earth conductor at all and you want a metal plate, or if you find scorch marks or melted plastic inside the box. Crumbling insulation often means the wiring is due for a proper inspection rather than a new switch.
The same goes for a bathroom switch that sits within 0.6 metres of the bath or shower (a wall switch belongs outside the zones), a new switch position, a dimmer that needs to match LED lamps, or a smart switch that needs a neutral. A standard switch drop does not carry one.
Light switch questions, answered
Do I need to turn off the main switch or just the lighting breaker?
Switching off the lighting breaker is enough if you have proved the switch dead afterwards. If your board has old rewireable fuses and the labels are unclear, turning off the main switch is simpler.
Can I fit a two-way switch where a one-way was?
Yes. Use COM and L1 and it behaves exactly like a one-way switch. That is why most shops sell two-way switches as standard.
Need a hand with switches or lighting?
As an electrician in Shifnal, SM Electrical works across Wolverhampton, Telford, Shropshire and Staffordshire, handling everything from a single faulty switch to full lighting redesigns. If you have found something in the back box you are not sure about, get in touch and we will take a look.
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