6.3.3 - The radial circuit

Two types of radial circuit are permitted for socket outlets. In neither case is the number of sockets to be supplied specified, so the number will be subject to the constraints of load and diversity. The two standard circuits are:

1. - 20 A fuse or miniature circuit breaker protection with 2.5 mm² live and 1.5mm² protective conductors (or 1.5 mm² if m.i. cable) feeding a floor area of not more than 50 m². If the circuit feeds a kitchen or utility room, it must be remembered that a 3 kW device such as a washing machine or a tumble dryer takes 12.5 A at 240 V and that this leaves little capacity for the rest of the sockets.

2. - 32 A cartridge fuse to B888 or miniature circuit breaker feeding through 4 mm² live and 2.5 mm² protective conductors (or 2.5 mm² and 1.5 mm² if m.i. Cable) to supply a floor area no greater than 75m².

The arrangement of the circuits is shown in (Fig 6.6). 4mm² may seem to be a large cable size in a circuit feeding 13 A sockets. It must be remembered, however, that the 2.5 mm² ring circuit allows current to be fed both ways round the ring, so that two conductors are effectively in parallel, whereas the 4 mm² cable in a radial circuit must carry all the current.

Fig 6.6 - Radial circuits

Radial circuits can be especially economic in a long building where the completion of a ring to the far end could effectively double the length of cable used. As for ring circuits, danger can occur if flexible cords are too small in cross-section, or are too long, or if 3 A fuses are not used where appropriate.

The minimum cross-sectional area for flexible cords should be:

0.5mm² where the radial circuit is protected by a 16 A fuse,
0.75mm² for a 20 A fuse,
or 1.0mm² for a 30 A or 32 A fuse.