This article covers the ways in which you can safely extend existing socket circuits to provide more outlets.
How many sockets
The choice is entirely yours. 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
Extending ring circuits
New sockets can be added as part of the ring, or on 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 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 much more total ampacity by splitting it into 2 radials. One 30A or 32A 4mm ring can become 2x 32A radials.
When splitting a ring, regs require checking the voltage drops at the end of the new radials. Also check the current carrying capacities of the cables if they are buried in insulation, as the insulation leaves them able to handle less current.
If you can find a convenient point about mid-way on the ring and run two cables new back to the consumer unit, you can split the ring into two rings.
2.5 mm^2 is the most popular socket circuit cable. Its sufficiently rated for 30/32A ring circuits and 20A radials when installed as per reference methods A, B, C, 100 and 102.
4 mm^2 is used for 30/32A radial circuits, and also for 30/32A ring circuits where the cable will be in contact with insulation or a large circuit is needed and voltage drop is a limiting factor. Trying to get 3x 4 mm^2 wires into socket terminals can be a challenge!
- Info on maximum ring circuit lengths and allowed installation methods is here
- 4mm 30/32A radials may only be installed as per reference methods B & C
- BS 3036 rewireable fuses aren't permitted on 30A 4mm radials or 20A 2.5mm radials due to the correction factor applied to the cable when using these type of fuses.
- 6mm 30/32A radials need to be used if the cable is installed as methods A, B, 100 and 102. A BS 3036 rewirable fuse may be used on a 30A 6mm radial if installed as method C.
1.5 mm^2 cable is sometimes used on radial circuits and is typically used where an old immersion heater feed is reused to supply sockets. This circuit should never be used with a BS3036 rewirable fuse greater than 6A. A 15A BS1361 fuse, 16A BS-88 fuse or a 16A MCB may be used provided the cable is installed as per reference methods 100, 102, B or C given here.
New sockets circuits require an RCD now. Adding new sockets to existing sockets also requires an RCD. The RCD goes in the CU or fusebox - but many old fuseboxes can't accommodate an RCD. If that's the case, current regulations require you to fit a new CU.
See main article Electrical connection
Cable connections in a socket circuit can be of 2 types:
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.
See main article Electrical connection
Ensure disused cables are disconnected at both ends.