On 2013-11-27, John Rumm wrote:

On 26/11/2013 20:49, Bill wrote:
Rumm writes

So, the one technical question that I have. I would imagine that 4mm
singles should be adequate to link the DIN terminals to the RCBOs,
certainly OK for lighting, but should I be thinking of 6mm for some of
the others?

IME RCBOs will come with the required fly lead to connect back to the
neutral bus bar.

They do, I was asking in regard to the tails from the separate DIN
terminals, if I went down that route.

Ah, yes I see what you mean. In that situation then either use the same
or greater CSA as used for the circuit wires you are extending. In the
case of a ring, you ought to either extend both ends, or bring them in
on a thicker single wire which is man enough for the nominal current for
the protective device in question.

So arguably with a ring with a B32 MCB that has two 2.5mm^2 ends, you
would mathematically arrive at 5mm^2 (hence 6mm^2 in practice), however
a 4mm^2 would be acceptable since we know that it can take the full 32A
load.

Does that apply outside the CU too, though? Would it be acceptable to
run a 4 mm^2 cable from a CU up to a junction box upstairs (let's say
it's a good one with Wago lever blocks in it) & then make a ring of
sockets with 2.5 mm^2 starting & ending at the junction box?