Roger Mills wrote:

- Voltage drop: [...]

So what are we aiming at here - how high can it be?

4% (9.2 V) maximum between the meter and all points of utilisation.

- Disconnection time: [...]

Sounds like we're *well* in.

That's where you want to be.

- CPC sizing: [...]

What would be the effect on the calculations if it were a 40A or 45A BS1361
fuse rather than 30A?

The first question here is why would you want a 40+ A feed when your
stated load in the garage is only about 20 A? Gut feel says that for
40+ A the submain would need to be in at least 10 mm^2 and that 6 is
likely to be marginal at best.

Anyway, the current to blow the 45 A fuse in 5 s is 240 A. We
calculated 230 A earth fault current (at 70 deg. conductor temperature)
so calculations fail at the first hurdle in that case, unless you can
get positive confirmation that the supply is TN-C-S (the DNO should be
able to tell you). Even it did just meet 5 s disconnection, the
required CPC would be 6 mm^2, so that T&E cable would be at serious
risk. (BS 7671 doesn't give any data for a 40 A '1361 fuse, so I can't
comment on that.)

An option to overcome the Zs issue would be to fit a 100 mA Type S
(time-delayed) RCD at the house end of the feed.

To use 6 mm^2 T&E on a 45 A circuit the installation conditions would
have to be 'clipped direct' at all points - no grouping and no thermal
insulation allowable. This cable goes through the loft, so that might
not be practicable in any case.

By now you should see why (in June) I said stick to a 30/32 A circuit.

Others have suggested going out through the wall with the T&E, and joining
it to the SWA using an IP65 box mounted on the outside of the wall. That
might be easier in some ways, but I'm not sure how best to seal the T&E
cable entry into the box. Any suggestions? Do I drill the back of the box,
and align the hole with where the cable comes through the wall, and seal
round the cable with silicone sealant? Or what?

You'll never seal it completely against porous brick. Drill a drain
hole in the bottom of the box.

Incidentally, there's a restriction on wiring in a bathroom that you
need to consider. Wiring which is in Zones 0, 1 or 2 and is not in
metal conduit or is not done in a metal-clad cable such as SWA is only
allowed to supply equipment in those bathroom Zones. This applies even
if the wiring is on the surface or in the normal safe zones - see Reg.
601-07-01. However it doesn't apply if the cable is buried 50 mm deep
in the wall. So if your old shower cable is over the bath (Zones 1 & 2)
you'll have to bury it more than 50 mm in the wall in order to be able
divert it for another purpose. The Zones finish at 3 m above the floor,
so there's a get-out if you have high ceilings. For the region
alongside a bath (where a shower pull-switch might previously have been
located) Zone 2 finishes at 2.25 m above the floor and 601-07-01 then
ceases to apply.

Someone suggested using a different type of CU in the garage - so that if I
were to do something with a power tool which caused the RCD to trip, it
wouldn't also take out the lights. Is that what is meant by a 'split load'
CU? Any suggestions as to exactly what I would need (make, model, etc.)?

Yes, it would be split load. This was covered earlier in the thread and
I made some suggestions then. Again:

Yes, for TT a main switch and two RCBOs would be a good option, or main
switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B
and 16 A Type B might tend to trip on motor starting currents, or the
inrush current of a 230 - 115 V tool transformer. (Alternatively keep
the 16 A circuit for sockets only and provide separate motor circuits
for any machines.) You can buy all the MK consumer unit parts
separately from a wholesaler, including TLC if you want to buy on-line.

--
Andy