Is this a mistake in the Onsite guide?

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

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. 

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