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 reasoning behind a ring final circuit is that the total load and therefore the OCPD can be greater than the cable rating becuase the currrent will be divided between the two legs of the circuit.
The total load and the OCPD may not however be twice the cable rating because that would require that the total current be divided equaly between the two legs of the circuit.
The original design was a 30 amp rewireable fuse, selected because that was already a standard size for electric cookers, and because a 30 amp fuse was the largest that would discriminate fairly reliably with a 60 amp suppliers cut out fuse.
The cable originaly used was a then standard size.
The design then evolved a bit to allow the slightly smaller metric cable 2.5mm and to permit use of 32 amp MCBs rather than 30 amp fuses. It is all bit empirical, but has a good record in practice.
The reasoning behind a ring final circuit is that the total load and therefore the OCPD can be greater than the cable rating becuase the currrent will be divided between the two legs of the circuit.
The total load and the OCPD may not however be twice the cable rating because that would require that the total current be divided equaly between the two legs of the circuit.
The original design was a 30 amp rewireable fuse, selected because that was already a standard size for electric cookers, and because a 30 amp fuse was the largest that would discriminate fairly reliably with a 60 amp suppliers cut out fuse.
The cable originaly used was a then standard size.
The design then evolved a bit to allow the slightly smaller metric cable 2.5mm and to permit use of 32 amp MCBs rather than 30 amp fuses. It is all bit empirical, but has a good record in practice.
