So we were recently given a past exam paper with a ring final cable calc question in!
This is what C&G say to do:
Ring final, so rule 433.1.204 applies, 2.5mm² is your starting point as cable has capacity of over 20A. However as we're in trunking (yes t&e in trunking!) with 2 other circuits, there is a Cg factor. Even though the breaker is 32A, It is calculated by dividing 20A by Cg to get 28.6A. This means 2.5mm² won't have the capacity so we have to go to 4mm².
They state Ib=In=32A but the factor is applied to 20A. I can't find where this particular method is specified, but it's a special case for ring finals.
At least I know what method they use for Ring Finals now.
