How to understand lighting circuits

Shining a light on lighting circuits

Lighting circuits are wired in two different ways, using either junction-boxes or loop-in ceiling roses. These days, the loop-in system predominates - though individual circuits often combine the two for the most economical use of cable.

Unlike power circuits, lighting circuits are always of the radial type. The cable leaves the consumer unit and runs to each outlet position before terminating at the last fitting on the line. Most houses have at least two lighting circuits - usually one upstairs and one down.

For your safety, electrical products must be installed in accordance with local Building Regulations. If in any doubt, or where required by the law, consult a competent person who is registered with an electrical self-certification scheme. Further information is available online or from your Local Authority.

Never take risks with electrical safety. Before you start any type of electrical work, you must follow these following safety precautions:

  • Switch off the main power at the consumer unit/fuse box. Isolate the circuit you plan to work on by removing the circuit fuse. Put this in your pocket to avoid accidental replacement
  • Or switch off the breaker and lock it if you can
  • Attach a note to the unit to advise you are working on the circuit
  • Check the circuit is dead with a socket tester or voltage tester/meter for lighting circuits

The reason why the wiring of a lighting circuit is more complex than a power circuit is that each outlet or light fitting needs to be controlled by a separate switch. One way of providing for this is to install a four-terminal junction box, in which the cable from a light and the cable from its switch can be connected to the circuit cable.

In practice, instead of a junction box for each lighting outlet, you might find that the cables run to a single, bigger junction-box in the attic or ceiling void, where all the connections for the circuit are made. Or there may be a combination of junction-box and loop-in wiring on one circuit.

Two of the cables entering the junction box are the main circuit cables. Their brown (live), blue (neutral) and earth (green/yellow-sleeved) cores are each connected to a different terminal. (In older installations these cores will be red (live), black (neutral) and green/yellow (earth)).

One of the other cables entering the box runs to the light, the last cable to the switch. The brown core of the light cable is connected to the fourth terminal, while its blue core is connected to the main circuit neutral cores; the earth runs to the earth terminal. The brown core of the switch cable is connected to the main circuit live cores, while its blue core is connected to the light cable brown core.

The current flows from the main circuit down the brown core to the switch, then (if the switch is on and the circuit therefore unbroken) back up the blue core and along the brown core to the light, before finally returning through the light cable blue core to the neutral terminal in the box. Because the switch cable blue core becomes the feed to the light when the switch is turned on, it must carry a warning 'flag' of brown PVC electrical sleeving.

At the light unit itself, the current flows from the brown core of the cable into the brown (live) core of the flex, then back up the blue (neutral) core of the flex to the cable's blue core. In a pendant light fitting and some wall lights, the connections are made in the rose or base of the unit. But in flush-mounted fittings, the connections must be made in a heat-proof conduit box set in the ceiling or wall.

The colours of the live and neutral cores in cable have changed. Take special care when working on any installation that combines old and new.

In lighting circuits, a warning flag of brown (formerly red) PVC electrical sleeving is used to indicate that one or more cores apart from the brown live core may also at times serve as the live feed.

With loop-in wiring, the main circuit cable from the consumer unit loops in and out of multi-terminal ceiling roses, from which individual cables run to the light switches. This type of wiring system does away with the need for junction boxes and separate cables to the lights.

All the necessary connections are made within the rose. There's a separate terminal in the rose for the switch cable neutral core, which passes current directly to the live core of the pendant flex.

A lighting circuit is protected by a 5- or 6-amp fuse or circuit breaker. This fuse size allows bulbs with a maximum combined wattage of 1150 watts (5 amps x 230 volts) or 1380 watts (6 amps x 230 volts) to be run off the circuit.

In practice, it's best always to restrict the number of outlets per circuit to about eight - which means that even with a 100 watt bulb at every outlet, the total wattage is comfortably within the maximum. But if you're running high-wattage floodlights or a big multi-bulb chandelier, you'll need even fewer outlets if you want to avoid overloading the circuit.

Lighting circuits are generally run in 1mm2 two-core-and-earth cable, but particularly long circuits can use 1.5mm2 cable to compensate for the drop in voltage experienced on long cable runs.

Lights in a hall or on a landing are often controlled by two-way switching, which lets you switch the light on and off both from upstairs and downstairs. In this situation, the switch connected directly to the light (either through a junction box or loop-in rose) is linked to a second switch using special three-core-and-earth cable.

If you're planning to make changes to a power or lighting circuit in a bathroom, you must inform your local authority's Building Control Department first. Any additions or alterations to circuits in a bathroom must be protected by a 30mA RCD.