There are two types of radial circuit; 20amp circuits wired with 2.5mm2 cable and 30amp circuits wired with 4mm2. The principle of the radial circuit, is that the mains cable leaves the consumer unit and passes through each socket until it reaches and ends at the last socket. Alternatively, on a ring circuit the mains cable leaves the consumer unit passes through every socket and then returns to the consumer unit. The advantage of the ring circuit is that electricity can reach the sockets from two directions and so reduces the load on the cable. For other advantages see Types of electrical circuit.
The two diagrams show the difference between the radial and ring circuits. The top diagram shows the existing radial circuit, the bottom diagram shows the original radial circuit converted into a ring circuit. (The new part of the circuit is shown in orange).
Radial circuit converted to ring circuit
Ring circuits are wired with 2.5mm2 cable and always have a 30amp fuse/ 32 amp MCB. If your existing radial circuit is a 30amp circuit with 4mm2 cable you can simply complete the new part of the circuit using 2.5mm2 cable returning from the last socket to the consumer unit. If however your existing radial circuit is a 20amp circuit using 2.5mm2 cable, then you can complete the loop back to the consumer unit with 2.5mm2 cable but the fuse will have to be upgraded from a 20amp to a 30amp fuse.
The usual reason to convert a radial to a ring circuit is because the return stretch of cable can be used to add more sockets to the house.
Initially, plan the route of the return cable noting the locations of any extra sockets you require. To locate the position of the last socket on a radial circuit, first switch off the power by the main switch on the consumer unit.
SWITCH OFF ELECTRICITY AT THE MAINS
All of the sockets on the radial circuit will have two cables going into them, however the last position on the radial circuit will only have a single cable. Once this has been located the cable for the new part of the ring circuit should be connected to it.
Ensuring that the power supply is off, remove the conductors from the last point on the radial circuit. Twist together the conductors from the old cable with the new cable i.e. red to red, black to black and green/yellow to green/yellow, (if the earth wire is bare then it should have a green/yellow sleeve placed over the its bare part). Insert the twisted conductors back in their appropriate screw terminals on the back of the socket i.e. red to Live (L), black to Neutral (N) and green/yellow to Earth (E or ), and replace the socket.
Use the remaining length of new cable to return to the consumer unit. If you desire more sockets, leave a generous loop of cable at the new socket points. These points can be installed at a later time (See Installing a power socket).
At this point the power should still be OFF. The cover of the consumer unit should now be removed.DANGER: despite the fact that the electricity is switched off, the cables from the meter are still live, so be very careful.
Locate the fuseway/MCB with the single cable going out for the radial circuit you are dealing with.