In article ,
Steve Jones writes:
Before my post yesterday about ring or radial for garage sockets I
assumed that I needed either a 2.5mmsq ring circuit or a 6mmsq radial
circuit. But one answer I received suggested that a 2.5mmsq radial would
suffice.

Wanting to understand this better I have spent quite some time this
morning looking on the Internet. My new understanding is:

Ring Circuit, 2.5mmsq, 32amp MCB, maximum floor area of 100sq m

Radial Circuit, 2.5mmsq, 20amp MCB, maximum floor area of 20sq m

Radial Circuit, 4mmsq, 32amp MCB, maximum floor area of 60sq m


Can someone give a concise description of why this is so? How is this
calculated? I would love to understand this fully.

Current rating is based on cable capacity, and floor area is
based on max load available, how far that's likely to spread
based on typical usage, and how well the circuit type handles
speading loads.

The cable capacity is part of the regs, but the floor areas
are guidelines only, and you should adapt them if you know
the expected usage of the circuit is not going to be typical.
(They are the area of floor space for which the supply is
expected to feed appliances, and not for example the area
inside the ring circuit.)

There are additional regs which cover max cable lengths too.

--
Andrew Gabriel