Radial circuit cable size

[Steve Jones]

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.

Thanks,

Steve

[Steve Jones]

Or maybe point me to a good website.

[usenet]

These are the 'conventional circuits for BS1363 sockets' listed in one of the appendices of the IEE 'On-site Guide' so, yes, these are pretty certain to be allowed by the IEE regulations. However I believe the floor areas you have given above are somewhat out of date, if memory serves me correctly they are now 100sq m, 50sq m and 75sq m respectively. The floor areas are only recommendations really.

What are you trying to understand exactly? The above circuit layouts are practical and (unless taken to extremes in wierd ways) will conform to regulations regarding voltage drop, earth loop impedance, etc.

[Andrew Gabriel]

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.

[Andy Wade]

Paul Cook's /commentary/ - ISBN 0852962371 - is your friend: