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The old house says as is with its' existing supply. The NEW supply
terminates at the meter in the new garage which is about 20 metres
from the new house,they have put in an underground duct between new
house and new garage. My question was about the cable from the meter
to the ccu, what to use as meter tails normally are about 1 metre
long 16mm cable.

Who is the "they" that have provided the 20m duct? It reads as if it was
the electricity supply network operator - and that seems odd if they've put
the meter in the garage. As Colin said it would be better to get the
service cable run to the new house if possible and move the meter there.
The designer of the house electrical installation will then be able to use
all the conventional assuptions, as per the On-Site Guide (OSG).

However if the meter is to stay in the garage then your friend will need to
run a sub-main circuit through the duct to supply the house. In the garage
install a 100A DP switch-fuse (such as a Wylex 110) to act as main isolator.
Provide 25mm^2 meter tails from the incoming side of the switch fuse, less
than 2m in length, for the supplier to connect. The fusing at this point is
essential; the supplier's fuse cannot be used to protect your wiring further
than 2 or 3m beyond the meter. 25mm^2 is now the usual size for meter
tails, don't use anything smaller. I'm assuming throughout that this is a
single-phase supply, by the way.

To the outgoing side of the switch-fuse you connect the submain cable, which
should be 25mm^2 2-core or 3-core (see below) XPLE insulated SWA cable to BS
5467. A feed for a consumer unit in the garage (for lighting and power) can
be teed off at this point if desired. This tee should also be in 25mm^2
cable, unless it is verified by calculation that a smaller size would be OK.

At the house end the SWA cable will feed the consumer unit directly.

Also should the new house have a TT earth?

They should know by now whether the supplier is offering a PME earth
connection or not. Use PME if it's available, i.e. provide a 16mm^2 earth
wire from a main earth terminal (in the switch-fuse) for the supplier to
connect. Any metallic water or gas services in the garage must be earth
bonded at the point(s) of entry, to create an equipotential zone. The
minimum conductor size for main bonding is 10mm^2; I recommend that you use
16. Since you are not allowed to use a combined neutral and earth
coonductor on your side of the meter, the house installation will have to be
treated as TN-S, and bonded to create a separate equipotential zone.

We now come to the question of how to extend the PME earth to the house: is
it OK to use only the armour of the SWA (2-core cable), or do we need a
3-core cable to get a copper earth? I did some trial calculations, assuming
that the cable length is 25m, and Ze at the supply terminals is 0.35 ohm.
Using the armour of a 2-core cable there's no problem with thermal
protection of the cable armour, but if the submain fuse is 100A (I assumed a
BS 88 type) there may be a problem with disconnection time. The submain
adds about 0.11 ohm to Ze - 19 mohm for the phase conductor and 92 mohm for
the armour - bringing the fault current (for an earth fault at the house
end) down to only 500A. At this current a 100A BS 88 fuse would take up to
8 seconds to blow, which is too long (5 s max. allowed). A BS 1361
cartridge fuse would be worse (15 seconds); a BS 3036 re-wireable fuse,
curiously, is OK (3 seconds) if not abused. Thus if the PME facility is
used, the SWA cable would need to be 3-core unless the house max. demand is
such that a 63A fuse could be fitted, in which case 2-core is OK.

Finally, if PME is not available, then you'll need to provide earth
electrodes and RCDs (TT system). A good arrangement would be electrodes
comprising one or more 8ft earth rod(s) at both the garage and house ends,
connected in parallel via the armour of the cable (only 2-core SWA needed
here). Install a 300mA Type S (time delayed RCD) in the garage. This
should be ahead of the 100A switch-fuse and mounted in a plastic enclosure.
Then the house and garage consumer units would use parallel-fed 100mA and
30mA fast RCDs, like Fig. 3a(i) in the OSG, or RCBOs like Fig. 3a(iii). The
combined earth system resistance needs to be under 166 ohms to comply; I'd
hope to see under 50 ohms.

HTH
--
Andy