Understanding domestic electric lighting circuits
(See this page for the 2004 changes in cable colours)
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Always isolate any electrical circuit before working on the circuit.
In modern domestic properties in the UK, the main electric lighting circuits are separate from the power ring main circuit. Each house should ideally have at least two lighting circuits; each protected by a 5 amp fuse or 6 amp trip in the consumer unit. A single 5/6 amp circuit can cope with up to twelve 100 watt lamps, it is usual in a multi-storey house, to have at least one lighting circuit for each floor even if the number of lamps are less than 12 on each level.
Shaver units may also be connected to the lighting circuit (treat it as equivalent to one 100 watt lamp) - where installed in a bathroom or a room containing a shower, the shaver unit must incorporate an isolating transformer.
Unlike the ring power circuit, the lighting circuit does not form a loop returning to the consumer unit. The consumer unit is normally connected to the first lamp, which in turn is connected to the second lamp and so on.
The cable used is a 1sq mm PVC twin core and earth rated for up to 12 amps. It consists of a red insulated core for live, black insulated core for neutral with a bare earth conductor between them. The three conductors are laid side by side within a PVC sheath. When connecting the cable, the exposed earth connector must be covered with a sleeve coloured yellow and green (to denote that it's an earth).
The lighting cable is routed from the consumer unit to a series of lighting points for ceiling roses or wall light fittings. The power to each lamp is connected via a wall or ceiling mounted switch. Some light units incorporate their own switch, for these fittings, the power circuit is then connected directly to the fitting.
Most room lights are controlled by wall mounted toggle switches (although alternatively touch sensitive or rotary light dimmers can be fitted), The cable normally runs down the wall within conduit within the plaster. A flush fitting wall box is sunk into the wall to take the switch, or alternatively a surface mounted box is fitted. Multi-switch units enable more than one light to be controlled from one position.
In bathrooms and shower rooms, the switch must be a 'pull string' type.
These switches can also be used when new lights are being installed - they can easily be screwed under a ceiling joist with minimal disturbance to the decorations. There is a tendency to feel that pull switches are only suitable for bathrooms etc., however this limits the opportunities and should be avoided. When a new light is to be positioned over a work surface or even an external light fitted, there is no reason why a pull switch should not be mounted in any convenient position.
Two wiring methods
There are two basic methods of wiring lights - by ceiling rose and by junction box.
Systems using the ceiling roses make all the connections at the ceiling rose. While this removes the need for one junction box per lamp, it is often more awkward for the average diy'er.
With the junction box system (an old standard but still found in older installations), a cable is taken to a series of junction boxes, one for each light fitting/switch. The junction boxes are generally located between the ceiling joists or under floorboards close to the switch. Junction box type connections are required for fluorescent lights and other fittings that do not use a ceiling rose.