Basic lighting circuits

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Of all the DIY jobs you can do in your home, tackling electrics seems to hold the most fears for the do-it-yourselfer. The truth is, carrying out electrical work is quite straightforward, so long as you are clear what you are doing and work carefully and methodically. Modern electrical wiring equipment has never been so easy to install or maintain.

This leaflet explains how to recognise different lighting systems and how they work.

Always consider 'safety' before you start any DIY project. Look for the 'Be Safe' sign and read the safety advice.

Lighting circuits

Every lighting system needs a mains supply to power all the lighting points, and a switch that can interrupt the power supply to each point. There are two ways of doing this: the junction-box system and the loop-in system. Your house will most likely be wired in one or the other, though some homes have a combination of the two.

Junction-box system

This system consists of cable running from the consumer unit (the heart of the electrical supply and the point where all electrical circuits in your home stem from) to a series of junction boxes, one for each lighting point. Each junction box has a separate cable running to a light and another that runs to a switch.

Loop-in system

As the loop-in diagram shows, a special loop-in ceiling rose takes the place of the junction box. The cable from the consumer unit runs into each rose and out again, before going on to the next. The switch cable and the flex to the light are connected within the rose.

Combined system

(Loop in system with junction box addition) The loop-in system is more popular nowadays as it requires fewer connections and saves money on the cost of junction boxes. This said, lights located some distance from a loop-in circuit, or lights added after the circuit is installed, may be wired from junction-boxes.

The lighting circuit

Lighting circuits are multi-outlet radial circuits. In other words, the cable runs from the consumer unit, loops in and out of the ceiling roses or junction boxes, and ends at the last one. It doesn't return to the consumer unit like a ring circuit. Lighting circuits require 1mm2 PVC-insulated-and-sheathed two-core and earth cable. Each circuit is protected by a 5amp circuit fuse allowing up to twelve 100w bulbs or their equivalent on the circuit. The average house has two separate lighting circuits, one for the ground floor and another for upstairs.

Identifying connections

Before you start any work on the lighting circuit in your home it's important you can recognise which system you have. Use the diagrams below to help you.

Checking and existing circuit

Switch off the power at the consumer unit, remove the lighting circuit fuse and examine ceiling roses and light switches for any signs of deterioration. Pre-World War II wiring will be in rubber-insulated and sheathed cable. If the insulation appears dry and crumbly then it is no longer safe and needs to be replaced. If you find any signs at all that your lighting circuitry is out of date or you notice anything unusual then we recommend you consult a professional electrician.