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.
Loop-In Ceiling Rose
A modern loop-in ceiling rose will have three terminal blocks to accommodate the two cut ends of the circuit cable plus the switch cable. The red (live) conductors of the circuit feed cable run to the central live terminal block and the neutral (black) conductors run to the neutral block on one side, while all the earth conductors run to a common earth terminal. The red (live) conductor of the switch cable is connected to the remaining live terminal in the central block. Power runs through this conductor to the switch and back to the ceiling rose through the black (neutral) conductor on the switch cable. This ‘switch-and-return’ wire is in fact live when the light is switched on, and should therefore be clearly labelled by having a piece of red insulating tape wrapped around it to distinguish it from the other black conductors (which are neutral).
The black ‘switch-and-return’ conductor is connected to the third terminal block in the rose. The earth conductor from the switch cable is attached to the earth terminal that is shared by all the other earth conductors, including the one belonging to the actual pendant light flex if it is a three-core flex. The brown (live) conductor from the light pendant is attached to the terminal on the switch block while the blue (neutral) conductor from the light pendant is attached to the neutral block. You can identify the last ceiling rose on a loop-in system by the fact that, because the circuit feed cable does not have to exit out of the rose to carry on to the next rose on a circuit, only one set of cable conductors is connected, with the switch cable and lamp flex attached.
Junction Box System
A ceiling rose in a junction-box system will have the live (red) conductor connected to one of the outer terminal blocks, the neutral (black) to the other, and the earth conductor to the earth terminal. The flex conductors of the lamp are wired to match: brown (live) to the live terminal block holding the red conductor and blue (neutral) to the neutral block holding the black conductor. If the flex is three-core (with an earth conductor in green/ yellow), this is connected to the earth terminal along with the cable earth.