"Mike Hall" wrote
| That's good news, thanks. Can someone who knows better please square
| the above statement with what I was trying to calculate using the On
| Site Guide:
| Now getting to the bit I am a little muddy on!
| The cable is now protected for 6A, however as it will be under
| insulation the On Site Guide says it needs derating by 0.5. Using
| the formula they give this means that although the MCB is 6A the
| cable needs to be rated for 12A, (right so far??)

Yes.

| So I look down the table of 'conventional circuits' and find that the
| next biggest radial circuit above 10A is 15A which is wired with
| 2.5mm^2 T&E.
| Have I gone wrong somewhere with my understanding of the derating
| process?

Yes. You don't change the circuit arrangement when you derate a cable. You
still run a 6A lighting circuit, you just have to use cable rated at 12A in
the applicable method of mounting.

A quick glance at Table 4.7 in Whitfield's electricians guide shows that
1.0mm in wall in thermal insulation has a rating of 11A which is not big
enough, so you will have to use 1.5mm at 14A. There should be no problem
using that with most lighting fittings.

Incidentally, if you were to use a 15A 2.5mm radial conventional circuit,
you would apply derating of 0.5 so your cable would have to be rated at 30A
in insulation. 2.5mm is only rated at 18.5A in insulation (compared to 27A
clipped direct), 4.0mm is rated at 25A in insulation, so you would have to
use 6.0mm at 32A for that radial circuit.

Whitfield has an example where that 15A radial circuit has not only thermal
insulation but ambient temperature and grouping factors apply, and ends up
with a 15A circuit run in 10mm cable, which would be impractical because the
switch terminals would be unlikely to accept the cable.

Owain