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(this is a draft outline - feel free to add topics you would like to see covered)
This article covers the ways in which you can safely extend existing socket circuits to provide more outlets.
How many sockets
You the user decide this. Just bear in mind that numerous socket additions have been made, only to find later that more are wanted. The amount of domestic electronics in use is continuing to increase, so what's adequate today probably won't be later.
Ring & Radial Circuits
British socket circuits are always ring or radial. A ring circuit makes a complete loop from CU (fusebox) to each socket and back to the CU. A radial only has a single run of cable from CU to the sockets, no return cable.
Extending radial circuits
Radials can be extended from any point in the radial circuit. See cable size section.
Extending ring circuits
New sockets can be added as part of the ring, or as a spur. Its very much recommended to add them as part of the ring where this is practical. This means breaking the existing ring, and often adding a bit more cable.
A relatively easy way to add new sockets as part of a ring is to cut the existing cable run and fit 2 new sockets, one to each end of the cable, then link the 2 with a new piece of cable. Extensions can also be included in in a ring from a single existing socket position. See this technique.
Sometimes running a single piece of cable from an existing point in the ciruit to a new socket is the only practical option. This is called a non-fused spur
A non-fused spur should feed only one single or one twin socket-outlet, or one Fused Connection Unit. Three- and four-socket 'power strips' should be connected by a Fused Connection Unit or incorporate a 13A cartridge fuse.
The total number of non-fused spurs must not exceed the total number of socket outlets and items of stationary equipment that are directly connected to the circuit
Splitting a ring
Occasionally it can be worthwhile to split a ring circuit. By breaking the circuit at some point, the one circuit then becomes 2x radial circuits. At the fusebox the 2 tails are separated, each getting its own fuse. Splitting the circuit like this changes the fuse rating permissible for each half, in most cases this makes it not worthwhile, but where a ring was run in 4 mm^2 you can get more total ampacity by splitting it into 2 radials.
Splitting a ring requires careful checking of voltage drops at the end of the new radials and of the current carrying capacities of the cables if they are installed in insulation.
2.5 mm^2 the most popular socket circuit cable. Its sufficiently rated for 30/32A ring circuits as long as its not buried in 100mm or more of insulation. Its also good for 20A fused radials as long as its not buried in 100mm or more of insulation.
BS 3036 fuses (rewireable fuses) are not permitted on 30A 4mm radials due to the correction factor applied to the cable when using these type of fuses.
1.5 mm^2 cable is sometimes used on radial circuit where the circuit is fused at 15A and the cable won't be buried in 100mm or more of insulation. Where the cable is buried, 1.5 mm^2 can only be fused up to 10A. 1.5 mm^2 is typically used where an old immersion heater feed is reused to supply sockets.
Its ok to use larger cable than required if its able to fit into the terminals of the accessories in use.
New sockets circuits normally require an RCD, though there are exceptions. Adding sockets to existing sockets does not require fitting an RCD. The RCD goes in the CU or fusebox - but some old fuseboxes can't accommodate an RCD.
Cable joins in a circuit can be of 2 types:
- accessible, meaning it can be reached readily using a tool
- inaccessible, eg buried under tiling
Accessible joints are usually made using screw connections, either in the socket itself, or a junction box, or screwblock strip contained within an electrical backbox (for fire protection.)
Inaccessible joints may not use screw connections, as they can come loose in time. Permitted connection methods are:
Of these, crimping is by far the simplest and the least prone to problems in inexperienced hands.
The other approach to cable joins is to replace it with a new piece of cable that's long enough. Sometimes this is practical.