A 20A lighting circuit sounds a bit on the high side,  though 16A driving ES lamp fittings used to be quite common in the days of big bulbs .  (1960s 250W filament lamp anyone ?)
Would be typical if the light fittings plugged in with 13A plugs, but then it is a socket circuit with switches, not a lighting cct. In any case not against the rules, just odd.

But, if it is 1.5mm it may be a bit undersized depending on cable grouping factors and so on,  (or if you prefer, a bit over-fused), but in practice most unlikley to be an issue - and it may have been done to avoid a need for C or D type breakers rated at a lower current.

Mike

A 20A lighting circuit sounds a bit on the high side,  though 16A driving ES lamp fittings used to be quite common in the days of big bulbs .  (1960s 250W filament lamp anyone ?)
Would be typical if the light fittings plugged in with 13A plugs, but then it is a socket circuit with switches, not a lighting cct. In any case not against the rules, just odd.

But, if it is 1.5mm it may be a bit undersized depending on cable grouping factors and so on,  (or if you prefer, a bit over-fused), but in practice most unlikley to be an issue - and it may have been done to avoid a need for C or D type breakers rated at a lower current.

Mike

Only 250W? Thats for girls!

1000W GES tungsten lamps in vitreous enamel reflectors! Thats proper factory lighting.

You didnt need heating either! Rows & Rows of 500W or 1000W fittings all beaming down their IR.

Ha!

Not my job but a colleagues, but over your self heating factory, I'll raise you a tale of metal parabolic reflector fittings on rusting jack chains that flickered when the hanger doors opened and they swayed in the wind.

Closer inspection revealed a single rubber cotton covered snaking down between the chainlinks and into each lamp-holder . OK, that means the chains must be - surely not?  -ah well that is why they flicker then  ... as the rust slides and makes and breaks the return path.

The past is sometimes a different world in terms of H and S, paperwork  and attitudes to  what is acceptable generally.

But you are right, I've never had to change a 1000W lamp, except theatrical and that's not real wiring, or at least the real wiring rules do not apply - and I'm pretty sure that I've never needed more than a half inch socket set to do up a terminal either,  so maybe I'm just a low power softie really ;-)

Mike