What is "cable derating"?

[ColJack]

Completely over the top. 1.0mm cable is the norm for lights on a 6 amp breaker even if it is buried in loft insulation.

Irrelavent as to what "the norm" is in your opinion.. the OP asked about de-rating, not "rule of thumb" circuit design.
If you need to use 1.5mm because the calculations say so then you use it..
I was pulling a rough example out of the air.
As I'm a commercial / industrial sparky then it's not uncommon to have to use 50m+ for some lighting runs bunched in conduit/trunking etc..

 

[JohnW2]

Irrelavent as to what "the norm" is in your opinion. ... If you need to use 1.5mm because the calculations say so then you use it.

Agreed. However, particularly given that this is a DIY forum, I presume winston was trying to say that it would be extremely uncommon for the calculations to indicate the need for 1.5mm² cable in anything approaching a standard domestic installation (in relation to either CCC or VD).

As I'm a commercial / industrial sparky then it's not uncommon to have to use 50m+ for some lighting runs bunched in conduit/trunking etc..

Fair enough - but, as above, that is a world apart from nearly all DIY.

Kind Regards, John

 

[EFLImpudence]

Plus - there are/have been a lot of posters who, for some strange reason, come here and say they will of course use 1.5mm² for the lights as if it has been instilled in them.

 

[JohnW2]

Plus - there are/have been a lot of posters who, for some strange reason, come here and say they will of course use 1.5mm² for the lights as if it has been instilled in them.

Indeed. Mind you, as far as I can make out, an appreciable proportion of electricians also do that, so it has presumably been 'instilled' in them as well.

Kind Regards, John

 

[ban-all-sheds]

As I wondered, maybe they just CBA to carry 1mm on the van for the sake of a trivial cost saving.