Tabulated Current - Trainee - Am I applying derating factors twice?

Hi,

Thanks for your patience with me - I am a trainee.

So I have got a 2.5 ring in a 20mm PVC conduit. 2 multicore cables enclosed means that according to table 4C1 on page 443 I have to apply Cg of 0.8 to the tabulated current.

So I know that must be > or at least = to In/Cg - (Appendix 4 reg 5.2 equation 5 page 426)
So my It is 32/0.8 - 40A. In the case of a ring then this is 20A per leg effectively for cable selection.

So far so good - I think anyway.

So off I go to table 4D5 on page 456. Selecting a 2.5mm for Ref method B - 23A. I get it.

But in table 4D5 the cable (in this case 2.5mm) has been already derated from 27A clipped direct to 23A for my reference method.

So am I?

A - applying the derating twice?
B - or is it that table 4D5 is just confirming that in a conduit the 2.5mm cable can carry the current I want (at 70deg)?

I think its B - but I have got in a twist with it.

Cheers,

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