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.
The 32A ring and 20A cable is based on someone else's calculations about load bunching, and is a well understood and time-tested design.
However, with no risk at all, the same assumptions could be made and scaled for any similar layout, so a 64A device and 40A cable or a 16A device and 10A cable for example, keeping the ratio of MCB to cable rating.
So a 25a MCB should allow you to use cable rated at (25/32)*20A ~ 16amps.
The 32A ring and 20A cable is based on someone else's calculations about load bunching, and is a well understood and time-tested design.
However, with no risk at all, the same assumptions could be made and scaled for any similar layout, so a 64A device and 40A cable or a 16A device and 10A cable for example, keeping the ratio of MCB to cable rating.
So a 25a MCB should allow you to use cable rated at (25/32)*20A ~ 16amps.