With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.
It is impossible to design in good use of a ring final circuit
I agree you cannot control the plugged in loads (well, the total may be limited by mcb choice I suppose.)
However, there are things you can do that eliminate the cable overload risk altogether, though this is only worth it if you know it will be heavily used
1) you can make sure that no more than 13A of load is within 20% of the MCB end (so if the ring uses 50m of cable, the first 10m on each end should only have 1 socket each.)
, to avoid the double socket and adaptor thing as well, you could simply have no sockets that close to the CU, or wiring the ones that have to be close as direct spurs from the MCB rather than mid-ring.
Or
2) you could arrange it so the cable's current rating of the starting end of the ring is higher either routing the first bit of the ring as surface wiring or plastered direct, or doing the first 20% or so in 4mm cable.
It is impossible to design in good use of a ring final circuit
I agree you cannot control the plugged in loads (well, the total may be limited by mcb choice I suppose.)
However, there are things you can do that eliminate the cable overload risk altogether, though this is only worth it if you know it will be heavily used
1) you can make sure that no more than 13A of load is within 20% of the MCB end (so if the ring uses 50m of cable, the first 10m on each end should only have 1 socket each.)
, to avoid the double socket and adaptor thing as well, you could simply have no sockets that close to the CU, or wiring the ones that have to be close as direct spurs from the MCB rather than mid-ring.
Or
2) you could arrange it so the cable's current rating of the starting end of the ring is higher either routing the first bit of the ring as surface wiring or plastered direct, or doing the first 20% or so in 4mm cable.
