Why 1.5mm cable for lighting circuits?

[JohnW2]

On our certificate where it states "Maximum Zs permitted" it looks like they use a de-rated value, as they list 6.13 ohm for the lighting circuits, which equates to 230/30 x 0.8. So I think what they're doing is noting the max permissible Zs at ambient temperature so it can be directly compared with actual measured R1+R2 and Zs.

Indeed, but where I imagine your certificate would run into difficulties is if the Zs was greater than 0.8U0/Ia, but still 'OK' - since your certificate would then think/indicate that it was 'not OK' ... as EFLI reminded me yesterday, Appendix 14 says that if Zs is greater than 0.8U0/Ia, one may undertake a more detailed calculation taking temperatures into account - and that calculation could indicate that the Zs was actually acceptable.

Regarding voltage drop, is it not the convention that lighting circuits are assessed as if the total distributed load was a single point load at 50% of the length? I can't remember where I heard that, so may not be up to date.

That would make mathematical sense, although I don't recall having seen it stated as such. As for the magnitude of the load, the OSG is obviously a bit behind the times, since, IIRC, it wants one to consider each lampholder as representing a 100W load.

Kind Regards, John