The lighting system in your home is supplied with power from a feed cable to all the lighting points and to the light switches. Lighting circuit wiring follows two general patterns: the loop-in system and junction-box system.
In the loop-in system, cable goes from the consumer unit to each of the roses of the lighting system in turn and is connected to them (or 'looped-in'). Separate cables connect the rose to the lamp holder, and a third cable connected inside the rose leads to the switch - the 'switch drop'. There may also be a branch line leading to another lighting point, but the number of terminals available restricts such branches to only one per rose.
In the junction box system, cable passed from the consumer unit travels via a series of 5-amp junction boxes to which each ceiling rose and wall switch is individually connected.
Your home may well have a combination of the two systems, particularly if lights have been added after the circuit was first installed. Both are, in fact, multi-outlet radial circuits. Unlike the cable of a ring circuit, the cable in the lighting system does not return to the consumer unit. Instead, after looping in and out of ceiling roses or junction boxes, the cable ends at the last rose.
Lighting circuits require 1 mm sq. PVC-insulated and sheathed two-core and earth cable. Each circuit is protected by a 5-amp fuse, so it can carry a load of up to 1200 watts - that means twelve 100-watt bulbs or their equivalent. That's why you'll often find that each floor of your home has a separate lighting circuit.
If you want to add an extra light to a circuit, you must first determine the current loading, and find out which lights are on which circuit. Switch on all the lights in the house, then switch off the mains power at the consumer unit. Remove one of the lighting fuses from the consumer unit and restore the power. Check which lights did not come on: these are all on one circuit. Repeat with the second lighting fuse, then the third, until you have plotted all the lighting circuits in your home.
Add up the total wattage on the circuit you wish to add to: lightbulbs of 100 watts and over should be taken at their face value; those bulbs with less than 100 watts, should also be counted as 100 watts. As long as the extra light does not take the load on the circuit above 1200 watts, you can add an extra light.
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