Alistair Riddell wrote in message . org.uk...

The tables in the regs say that the current capacity of standard 1mm2
twin&earth enclosed in insulation is 11 Amps so you should have no problem
with normal lighting circuits.

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:
I've got, (or rather intend to have by the end of today), 9 bayonet
fittings on the upstairs lighting circuit, (3 x bed, landing, 2 x
bathroom, 3 x loft + bathroom fan). This equates to a design current
of well less than 6A, so 6A TypeB MCB should be fine.
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??)
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?