A 30amp junction box should be used on a ring or radial circuit feeding sockets only.
Socket ring main diagram.
Its simplicity makes it a great arrangement for that use.
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.
For a radial circuit or spur off a ring main there will only be one cable.
Ring main and spurs.
In the diagram here in and out are the existing ring cables and spur is the new cable leading to the new socket outlet.
Junction boxes come rated for different uses by the amps they are allowed to carry.
Ring mains as they are called are a series of cables linked together starting and terminating at the same point in a distribution board.
Diagram of a possible configuration of ring final circuit.
For a radial circuit or spur off a ring main there will only be one cable.
Rear view of double socket showing ring main in and ring main out wiring.
Consumer unit fuse box is at bottom left.
Consumer unit wiring diagram.
The main purpose of a spur is adding an extra socket on to an existing ring main circuit.
Make sure that all three wires are secured in each terminal and avoid damaging the wires when replacing the socket.
The wiring for a junction box can be seen here.
The ring is fed from a fuse or circuit breaker in the consumer unit.
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.
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.
As before brown is live blue is neutral and the earth wire is bare copper with a green yellow sleeve.
Connect the cables as shown in the diagram for a ring main circuit.