On 23 June, 23:27, John Rumm wrote:
On 23/06/2010 17:36, David Robinson wrote:

Excuse my ignorance - I've never run DC cable before!

and you are not about to either - these LV lights are AC ;-)

My physics teacher would be ashamed of me!

Though I remember using P = I^2 * R to calculate the power dissipated
in cables as heat - i.e. higher voltage = more efficient and lower
voltage = huge cables or lots of electrical energy turned into heat on
the way!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

Indeed *- if you run several lights from one cable the current can be
high, and they are very sensitive to voltage drop (in the sense that the
difference will be visible)

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?

Personally I usually star wire them in groups. Typically in 1.0mm
T&E[1], keeping each cable in the 1 - 3m sort of length range.

With six lights I would probably go for two 150W transformers. If
possible positioning the thing such that it can be extracted later if
required.

I assume this means the connector block between the transformer and
the wires needs to be very hefty?

I found this on-line calculator that seems useful (though from what
you've said 1% voltage drop at 12V is already far too much)...

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

I'm putting lights on top of (and underneath) kitchen cupboards. This
means they're physically grouped (2 left of window, 5 right of window,
9 across the back wall), and with transformers on top of the cupboards
they'll be easily accessible, with suitable wires run down in the wall
to the lights below.

I was going to run one wire down per run of units, but I see now it
makes more sense to run one wire down per light! (Even one per pair of
lights would be pushing for 1.5mm2 according to that calculator?!).
Not quite so neat, but normal people don't peer under the bottom of
cupboards anyway.

Cheers,
David.