Radial Power Circuits

A radial power circuit is the name given to a circuit supplying a number of 13 A socket-outlets and fixed appliances using one cable which is not wired in the form of a ring but terminates at the last outlet.

The permitted number of socket-outlets (and fixed appliances) depends on the current rating of the circuit which can be either 20 A, wired in 2.5 mm² cable, or 30 A wired in 4 mm² cable. The number of socket-outlets and fixed appliances which can be served by a 20 A radial circuit depends also on the location of the circuit and whether the outlets are confined to one room.

Types of radial power circuit

There are, in effect, three types of radial power circuit. A 20 A circuit supplying more than one room; a 20 A circuit supplying one room only and a 30 A radial circuit supplying one or more rooms.

Two-room 20 A radial circuit

This circuit can have a maximum of two 13 A socket-outlets (or fixed appliances) each in a different room, hall, landing or other service area. It is wired in 2.5 mm² 2-core and earth cable starting at a 20 A fuseway used solely for that circuit.

One-room 20 A radial circuit

This circuit is also wired in 2.5 mm² cable from a 20 A circuit fuseway and may supply up to a maximum of six 13 A socket-outlets and fixed appliances. This is provided they are all in the one room of ar area not exceeding 30 m² (300 ft²) and that the room is not a kitchen and none o’ the appliances is a fixed water heater.

This is wired in 4 mm² 2-core and earth PVC sheathed cable from a 30 A fuseway. It may supply up to a maximum of six 13 A socket-outlets and fixed appliances but without the restrictions applying to a 20 A radial circuit. The socket-outlets and fixed appliances (if any) may be installed in any rooms and service areas and the fixed appliances may be of any type having individual loadings not in excess of 3 kW. This circuit is excellent as a supplementary to a ring circuit and particularly suitable for a fully equipped kitchen to lighten the load on the ring circuit; especially where the kitchen is also used as the home laundry.

The 15 A power plug system

Before the advent of the ring circuit and its associated 13 A fused plugs and socket- outlets, the ‘power system‘ consisted of 15 A round-pin plugs and socket-outlets, each wired from a separate circuit. A multi-way power fuseboard or splitter main switch and fuse unit containing 15 A fuses supplied these circuits. The system was open to abuse as it was usual to extend the circuits by adding 15 A socket-outlets resulting in two, three or more socket- outlets to a circuit.

Originally the plugs and sockets were all 2-pin and many of these still exist in some installations. This means that appliances connected to them are not earthed. Subsequently, the 3-pin plug and socket-outlet was introduced and these replaced existing 2-pin sockets but with no attempt to provide earthing. These also still exist in some installations and only when they are replaced by the modern 13 A socket-outlet is it noticed that they are not earthed.

The 5 A socket-outlet

The 5 A round pin plug and socket-outlet was also used extensively in dwellings, these too being in 2-pin and 3-pin versions. They were intended mainly for small appliances of up to 1000 W.

One 5 A socket-outlet was supplied from a 5 A circuit, two from a 10 A and three from a 15 A circuit but, in many instances, more than one 5 A socket-outlet was supplied from a 5 A circuit. The 2 A plug and socket-outlet was installed especially for table lamps, floor standards and other portable lighting and are still installed for this type of lighting.

As each socket has an assumed maximum load of 1/2 A (equivalent to 120 W on 240 V supplies) up to a maximum of ten 2 A socket-outlets may be connected to a 5 A circuit exclusively used for the socket- outlets. The 2 A socket-outlet is also supplied from a lighting circuit, but it must be assessed at 120 W when adding up the lighting load.

Round-pin plug adaptors

Because of the scarcity of 15 A sockets on the old radial wiring system, plug adaptors were used extensively on these circuits. Principally they were to enable 5 A and 2 A plugs to be used from the 15 A sockets but the adaptors also enabled two 15 A plugs to be used from one 15 A socket-outlet.

The use of multi-plug adaptors is bad practice as, apart from a risk of overloading a socket-outlet, long flexes have to be used which are themselves a hazard.

The radial system

The numerous 15 A circuits radiating from a central fuseboard are known collectively as the radial system. This system is now superseded by the ring circuit. It has no connection with the modern radial power circuit.

Circuits for conversion

Where only one 15 A socket-outlet is on a circuit this may be replaced by a 13 A type. Two may be replaced provided the circuit cable is 7-strand and that the fuse can be uprated to 20 A.

If there are more than two 15 A socket- outlets on a circuit, conversion is not permitted and the circuit should be scrapped. Because of the old wiring, attempts to change the sockets will almost certainly result in the insulation breaking off the conductor. The only satisfactory solution is to rewire by installing a ring circuit.

Conversion to 13 A socket-outlets

Most 15 A socket-outlets can be replaced by the modern 13 A socket-outlet. When replacing a 15 A socket-outlet by a 13 A type, check that there is an earth wire at the outlet. If not, it is necessary to run an earth conductor from the socket-outlets to the consumer unit using 2.5 mm² green PVC insulated cable. The 15 A 2-pin and 3-pin socket-outlets often will not have an earth.

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