Electricity is dangerous and can be hazardous. In doubt? Call a qualified electrician.
The easiest way to fit an extra socket to an existing ring circuit is by adding a spur. A spur comes off the ring circuit via a junction box, or more usually via an existing socket - which ever is easier. See also Adding a spur to a ring circuit from a socket.
To install a spur from a junction box, you require a 30amp junction box. Locate the cable of the ring main as close to the new socket as possible. Lift the floorboards and locate the cable that runs closest to your new socket.
A new length of cable (2.5mm2) should run between the new spur socket and the planned location of the junction box. Wire the new socket up first, Red to Live(L), Black to Neutral (N), the bare wire should be fitted to Earth (E or 
) with some green/yellow sleeving covering any exposed bare wire. After carefully checking all connections, screw the face plate of the new socket to its mounting box.
The next stage is fitting the junction box. We are not dealing with the existing cable, at this point just fitting the box. The box needs to be attached to a platform. Make a small plinth that sits between the joists. It is good practice not to attach the box to the joist as you will then be working side on in a confined space. Attach the junction box to the plinth making it ready for connection.
SWITCH OFF ELECTRICITY AT THE MAINS
Next cut the power supply by switching off the main switch on the consumer unit. Ensure the power has been cut by testing one of the existing sockets on the ring main. Once you are sure the power is off, remove the insulation from the part of the cable which will fit into the junction box.
Try to avoid cutting the existing cable completely if possible, it is better to skin the insulation of the conductors (red, black etc) so that the conductors can then be placed in the screw terminals. Undo the screws in the terminals completely and remove them. Place the skinned parts of the wires in the terminal slots, ensuring that the earth wire uses the centre one.
Once complete, put the screws back in the terminals but do not tighten them. Place the ends of the new cable into their corresponding holes by colour, i.e. Red to red, black to black, green/yellow to green/yellow, tightening the screws as you go. Finally, ensure all the screws are tight and replace the cover of the junction box and screw its lid down. Use cable grips to grasp each of the wires going to the junction box to keep it secure.
Finally, restore power and test the new socket to ensure it works.
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