Derek Doormer wrote:

Just about to re-wire the lighting circuit, with the Loop-in system, the
cable from switch to light has the live switch (which I know is the black
which usually has red tape attached) but ive also read of people using
another colour (Yellow and Blue I think) and Ive also noticed Twin Red core.
So whats the best combination to use??

Most people use ordinary red & black for the switch 'drops', with, as
you say, red marking. Sleeving is much better than tape by the way:
easier to apply and doesn't fall off. T&E cable with two red cores is
available but never seems to have gained widespread acceptance -
probably because it sometimes helps to know which wire is the live feed
and which is the switched live. (If one of the red cores had a thin
white stripe or similar it might have caught on more.) There's also the
argument about only needing one type of cable of course.

The other colours (yellow & blue) only come into play with 2-way or
2-way and intermediate switching, where you need 3-core & earth cable
between the two-way ends (via any intermediate switches on the way). So
far as BS 7671 is concerned all cores are red - i.e. should be marked
with red sleeving. There's no fixed 'correct' use of the colours in the
3-core cable: I use red for the permanently live side, yellow between
the commons of the two-way switches and blue for the switched live side,
but many others seem to do it with the red & yellow the other way round.

You are now at liberty to use the new colours. The translation is as
follows:

Red ----- Brown
Yellow -- Black
Blue ---- Grey
Black --- Blue

I've also read that 1mm or 1.5mm cable is used, is it common practice
to use 1.5mm for the circuit cable and 1mm for the switching cable?

No, best to stick to one size throughout. 1mm^2 is almost always OK for
6A circuits. For 10A or 16A circuits you may need 1.5mm^2. Check your
cable installation conditions and grouping factors against the tables in
the OSG.

--
Andy