No, not a mistake in this case.

The minimum current carrying capacity of the conductors in a ring circuit is 20A - but there are two of them.

30A  or 32A is the protective device.

Thank you geoffd .I am fixing some defects following EICR done by others. I noticed that they picked up a ring circuit on 16A  but did not on 20 A. So I thought I check but those 2 pages in Onsite guide confused me. I do not know this  A1 20A  is relevant in the book and do not see the point. Thank you

A1 is the standard ring final circuit - 30A/32A with 2.5mm² T&E cable.

It is not an A1 20A circuit. It is just that you are looking at the conductor size H2.3 section as well.

The two legs must be a minimum of 20A CCC (although it states that the minimum size (except MICC) must be 2.5mm²; this still relates to a BS3036 30A fuse being used).

I may have misunderstood, what exactly was the defect that was found? It cannot be the CPD rating, was it the cable CSA?

Hi David 

Sorry for the delay. The defect I found was a 20A MCB on a ring circuit..Thanks

Can you confirm it's a ring and not 2 radials.

Ozz, that is not a defect, and is perfectly permitted! There may be reasons why this is there, usually that some part of a ring is of a smaller cable size or there are too many sockets on a spur. I would point out that the onsite guide is completely useless for inspection, a very much higher standard of knowledge of the regs is required, and the onsite guide only contains a set of suggestions which may well be over-engineered but are "safe" without further consideration. A smaller Circuit Protective Device (CPD) is not in any way dangerous, although it could be nuisance, that is all.

Hi Jon,

It is a ring  as per test result. 

Has someone derated the circuit because the cables are surrounded by insulation?

David, This ring got 4 points served. 2x 1 KW wall heater,1 undersinK 10 L water heater( could you not confirm current as no access to) and 1 twin socket. I understand that you can design non Uk standard ring circuit but can you can guaranty no one will plug anything on