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Domestic electrical circuits

Ring circuits

The most common form of power circuit for feeding socket outlets is the ring circuit or ring main. A cable starts at the supply point (circuit breaker or fuse in consumer unit) and goes round the house, connecting socket to socket and arriving back to the supply so that the whole circuit forms a continuous ring. The most Ring circuits are commonly used in British wiring with fused 13 A plugs to BS 1363. They are generally wired with 2.5 mm2 cable and protected by a 30 A fuse, an older 30 A circuit breaker, or a European harmonised 32 A circuit breaker. Sometimes 4 mm2 cable is used if very long cable runs (to help reduce volt-drop) or derating factors such as thermal insulation are involved.

Ring circuit

Radial circuits

A radial power circuit feeds a number of sockets or fused connection units, but unlike a ring circuit, its cable terminates at the last socket. The size of the cable and the fuse rating depending of the size of the floor area to be supplied by the circuit. In an area of up to 20sq m, the cable needs to be 2.5mm2 protected by 20A fuse or MCB. For larger area, up to 50sq m, 4mm2 cable should be used with 30A fuse (a re-wirable fuse is not allowed) or 32A MCCB. High powered appliances like showers and cookers must have their own radial circuit.

Radial circuit

Light circuits

Domestic lighting circuits are of the radial kind, but there are two systems currently in use. The junction-box system incorporates a junction box for each light. The boxes are situated conveniently on the single supply cable. A cable runs from each junction box to the ceiling rose, and another from the box to the light switch. The loop-in system simply has a single cable that runs from ceiling rose to ceiling rose terminating at the last one on the circuit. In this system all the connections are made at the ceiling rose. A single circuit of 1.5mm2 cable is able to serve the equivalent of eleven 100W light fittings. If more lights are needed then another lighting circuit should be used. Lighting circuit must be protected by 5A fuse or 6A MCB.

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