RzB wrote:

"RzB" wrote in message
...

Yes - I won't skimp - I would like to keep the Vd to a
mimimum. What would you think is a good goal to aim for?

The old 2.5% limit is a good guide.

The best option at the house end, discrimination wise) is an HRC fuse
(cartridge fuse) in a separate switch-fuse unit. (Tee into the meter
tails using a service connector block.)

Hmmm - Lost me here... Are you saying that I shouldn't be
taking the workshop circuit through the house CU?
Why wouldn't this just be one of the circuits off an MCB
on the house CU?

It's better not to, especially as you're really looking at a 50 or 60/63
amp distribution circuit for the workshop. The problem with cascading
MCBs can be lack of discrimination on s/c faults - both MCBs may trip.
(This may not matter to you though, as you've got a separate feed for
lighting.) You also need to consider whether the house CU and indeed
the supply is OK for ~45 A of extra load. (Any electric showers or
space heating in the house?)

Design current - ignoring startup currents - see...
http://www.gillandroy.com/diy/workshop/electrics

Workshop is attached to house... and distance between
house CU and workshop CU is about 25M.

House is PME. Cable running through loft so might get
to 40degC ambient in the summer - yes/no? Also cavity
wall insulation will enclose cable when passing through
wall fo a couple of inches... No grouping.. Otherwise -
cable just clipped to wall/rafters?

After calcs, I was considering using 16mm T & E from
house to Workshop. But I'm not sure what CPC size comes
with standard 16mm T&E?

CPC is 6 mm^2. Yes, 16 mm^2 for the cable sounds about right, it'll
give you plenty of current rating in hand and keeps the voltage drop
down to 2%. A separate parallel 10 or 16 mm^2 CPC would be prudent,
although I haven't done the calculations to see whether it is strictly
necessary - this depends on your choice of overcurrent device at the
house end in any case.

--
Andy