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Wiring a light switch or dimmer

One of the simplest electrical wiring jobs you can carry out is to replace an existing light switch, either with a new one in a different style (perhaps to match the colour scheme in a newly-decorated room) or with a dimmer switch that will allow you to vary the intensity of the room's lighting from full brightness down to mere glimmer.

Most switches contain just one cable, linking the switch to the light it controls. One-way switches made of plastic have just two terminals on the back of the faceplate, to accept the live and switches live cable cores. The earth core goes to the terminal in the base of the mounting box. Two-way switches have three terminals on the back; if they are wired for one-way switching , the top terminal and either of the bottom terminals are used to connect the switch cable. All three terminals are used for connecting the switches in two-way switching arrangements.

Wiring light switch Wiring light dimmer switch

What to do?

Turn off the power at the mains. Unscrew the face-plate of the switch you want to replace, and disconnect the switch cable cores from the terminals on the face-plate. Leave the earth connection attached to the mounting box. If more than one cable goes to any terminal, stick a tape tag on each core before you disconnect it and label it to show which terminal it was connected to. Alternatively, make a sketch.

Now reconnect the cores to the new switch, ensuring that you copy the original wiring arrangement precisely. The black(or blue in a new regulations) core is live when the switch is on, so identify it with a flag of red (brown) PVC tape.

If you find a bare earth core within the box, disconnect it and cover it with some green and yellow PVC sleeving. Fold the cable back neatly and secure the face-plate to the box. To fit a dimmer switch, follow the wiring instructions included with it.

Two way switch wiring diagram

Point ot check:

Two-way light switch diagram

Each light around the home is usually controlled from just one switch, which makes or brakes the circuit to the light as it is operated. However, there are a number of situations where it would be more convenient to be able to switch a particular light on or off from more than one position.

This arrangement is known as two way switching, and involves using two switches connected with special cable containing three colour-coded cores plus an earth. Each switch must be suitable for two-way switching, which means there must be three terminals on the back of each plate.

A typical wiring diagram for two-way switch is shown below. Two-way switching gives you the convenience of being able to control bedside lights from the bed as well as by the door.

Two-way light switch wiring diagram

In stairwells you can provide partial two-way switching as shown at this diagram. The one-way landing light switch is replaced by a one-gang two-way switch, and the existing one-gang one-way hall light switch is replaced by a two-gang two-way switch. The two are linked with three-core cable. The hall light is still controlled just from the hall, but the landing light can be switched on or off from either switch position.

Two-way landing switch wiring diagram

If you want full two-way switching, with both lights controllable from upstairs and downstairs, replace both swithes with two-gang two-way switches, and link them with two separate 3-core cables. Connect the cores as shown at the diagram below.

Two-way staircase switch wiring diagram

If you want more than two switches to control a particular light, you can extend the two-way swithing arrangements to a multi-way set-up by using extra switches called intermediate switches. These are fitted, as the name suggest, between the first and last switches, which are both the two-way type. Three core cable is again used to link the switches.

Intermediate switches have two pairs of terminals which are used to connect in the blue and yellow cable cores. The red cores are linked via a strip connector.

Multi-way switch wiring diagram

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