In uk.d-i-y, Owain wrote:

With regards to cable/circuit ratings, under the regulations *each*
downlighter will be assumed to be 100W minimum (or actual loading if
greater) and you should check that you will not be overloading the existing
circuit.

I'm pretty sure this is not the case: the intent of the Regs would
surely be to say "anywhere a mumpty might put in an ordinary bulb, you
have to allow for mumpty favouring 100W bulbs", which is fair enough.
But if you have a lighting point whch takes non-ordinary-bulbs, you
surely assess the load based on - at very worst - the biggest-load
bulb available for that fitting; so, 20W for diddy-capsule halogens,
50W for bigger-size reflector ones, 11W for PL11D flourescents, and
so on. No? Likewise if the transformer you fit limits the load (so
that even if mumpty replaces 20W bulbs with 50W ones there won't
be a sustained overload, as the transformer will shut down or have its
fuses blow), it's reasonable to rate the load according to the
maximum the transformer will draw, not some mythical 100W-per-ligthing-point...

But what do I know? I've never made the time to read the Regs themselves,
as they seem to be statements of abstract principle leavened with
detailed engineering calculations to allow a tight balance between
safety and economy across a huge variety of installations. For d-i-y
work, the On-Site guide is as Official as I've ever wanted to get,
supplemented by the "working domestic electrician" books like the
venerable Stumbles.

Stefek