The main purpose of a spur is adding an extra socket on to an existing ring main circuit.
Socket ring main diagram.
Junction boxes come rated for different uses by the amps they are allowed to carry.
As you can see it eliminates the need to wire a socket all the way back to the a consumer unit but the nearest ring main socket is used instead.
The wiring for a junction box can be seen here.
A 30amp junction box should be used on a ring or radial circuit feeding sockets only.
The ring is fed from a fuse or circuit breaker in the consumer unit.
For the rules that govern how tis can be dne please.
Diagram of a possible configuration of ring final circuit.
The ring starts at the consumer unit also known as fuse box distribution board or breaker box visits each socket in turn and then returns to the consumer unit.
Ring mains as they are called are a series of cables linked together starting and terminating at the same point in a distribution board.
For a radial circuit or spur off a ring main there will only be one cable.
Make sure that all three wires are secured in each terminal and avoid damaging the wires when replacing the socket.
The cable size is important it has to be either 2 5sq mm or 4 0sq mm twin core and earth these are rated in free air at 24amps or 32amps respectively.
In the diagram here in and out are the existing ring cables and spur is the new cable leading to the new socket outlet.
A separate ring main is usually installed on every floor of a given property to make sure things are kept safe and it is only when for example a spur socket additional socket is added on an upstairs ring main to feed a socket or light on a downstairs circuit that things can get tricky.
Consumer unit wiring diagram.
Ring main and spurs.
Connect the cables as shown in the diagram for a ring main circuit.
Rear view of double socket showing ring main in and ring main out wiring.
Consumer unit fuse box is at bottom left.
Connect the cables as shown in the diagram for a ring main circuit.