Appendix 15 in BS7671 is probably the most useful practical description in the whole book. If you want to know what you can do and what goes where it’s all here.
Ring Socket Circuits…
Spread your sockets around evenly to avoid overloading any part of the ring.
Don’t put immersion heaters, storage heaters on the ring.
Also cookers, ovens and hobs exceeding 2kW should be on their own circuit.
As a guide, one ring to cover 100 m sq.
Unfused spurs – one single or one twin socket only.
Fused spurs – as many sockets as you want but remember the maximum 13A fuse in the spur!
Using 1.5mm sq cable is acceptable after a fused spur but best practice is to keep with 2.5 mm sq.
Radial Socket Circuits…
20A radial 2.5 mm sq.
30A / 32A radial 4.0 mm sq.
Unfused spurs off a 20A radial. As many as you want using 2.5 mm sq.
Unfused spurs off a 30/32 A radial. One single or one twin using 2.5 mm sq. As many as you want using 4.0 mm sq.
Fused spurs – as many sockets as you want but remember the maximum 13A fuse in the spur!
Using 1.5mm sq cable is acceptable after a fused spur but best practice is to keep with 2.5 mm sq.
Best practice: Keep the number of spurs as low as possible. Multiple spurs off a radial circuit could be a sign of poor design.
Regulations to bear in mind…
411 Automatic Disconnection of Supply, disconnection times, maximum Zs values
411.3.3 Additional Protection ‘RCDs’
433.1.1 (Design current ‘Ib’, protective device rating ‘In’ and current carrying capacity of cable ‘Iz’) relationships
433.1.204 Ring circuit conductor sizes, 2.5 mm sq T&E etc
553.1 to 553.1.7 Plugs and socket outlets
Also Part ‘M’ of the building regulations concerning mounting heights of sockets.
(other regulations may also apply, see BS7671)