Something you have to factor in when designing or extending the LV
wiring side is the type of transformer used and how this relates to
the cable type/size required.

If you are using a conventional heavy magnetic transformer, then you
can use the standard voltage drop figures in the wiring regs to work
out what size cable you need, although as you said, you need a much
lower voltage drop on the 12V side than would be acceptable on the
mains side. (For any sizable runs, minimum size is more likely going
to be dictated by voltage drop than by current carrying capacity.)

If you try doing this with an electronic transformer, you will be
very disappointed with the result. Problem is these generate AC output
at somewhere between 20-60kHz, and not 50Hz. At that frequency, a
physical property called "skin effect" has come into play, which means
the electricity only travels along the outer skin of conductors, so the
bulk of the conductor material is not usable, and the effective cross
sectional area of the wire is very much less than the actual cross-
sectional area. This became very obvious to me when I swapped a
toriodal transformer for an electronic one, and light output dropped
to probably only 1/3rd of what it had been. I had to upgrade the wire
from 2.5mm to 6mm to get the light output back as it was.

I had a room with a central pendent with 3 x 60W and a pair of wall
lights each with 1 x 60W, and wanted to convert this all to 12V.
I decided to use 35W 12V capsules to replace all the 60W mains lamps.
The transformers are in a metal case in the room above, directly above
one of the wall lights, and for that light, I take the 1mm² cable
directly into the transformer. For the other wall light, at the top
of the run up the wall, I convert it to 2.5mm² to run across under
the floor and up into the transformer. For the central pendent (3 x 35W),
I initially also used a run of 2.5mm² but the voltage drop was too
high, so I changed it to a run of 6mm², and that's fine.

In my case, I wanted to use mains cabling so that I could remove the
12V lighting and go back to mains at some point in the future without
having to replace the cabling. If this had not been the case, I might
have considered other types of cabling suitable for LV high frequency,
such as flat stranded speaker wire (higher surface area to volume ratio,
so less skin effect) or silver plated stranded wire (silver being a
good conductor works very well plated onto copper for high frequencies).

--
Andrew Gabriel
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