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

[ceeeee]

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,

 

[PS Electrics]

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,

It is neither in the sense it isn't the same rating factor applied twice.

Table 4C1 is grouping and 4D5 is current carrying capacity for different Installation methods, they are 2 separate things. You will see in the side note to Grouping that Item 1 can be used with Refence Methods A to F.