When I've fitted tails, I've sized them to match the main cutout,
e.g. 25mm² for 100A cutout. Even when tails are split at a
service connection block and load on one side can't get anywhere
near that, I maintain full size for the main cutout.
Looking at a couple of cases where an electrician has added a
second CU (e.g. for a shower or shed), they've used tails which
are smaller (16mm² or maybe even 10mm²), presumably on the
basis that the load can't exceed this.
Is this reduced conductor size OK, or could the REC complain
that the tails are undersized for the 100A cutout?
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The determining factor is whether the main fuse is providing overload
and fault current protection, or only the latter.
For a typical house CU the sum of all the MCB or fuse ratings will
exceed the DNO's main fuse rating ("diversity has been applied to a
circuit feeding a group of final circuits") so overload is possible and
the tails must be sized to give a current rating at least equal to the
main fuse rating (in practice usually 25 mm^2).
OTOH 'auxiliary' CUs for showers etc. don't usually present a potential
overload situation (no diversity for the group of final circuits need be
applied) so, in principle, the tails only need to be sized for the MD of
the auxiliary installation and to meet the adiabatic fault current
requirement [434.5.2] in conjunction with the characteristics of the
main fuse. In practice use 16 mm^2 as this will be uncontroversial.
Typical service connector blocks only have two ways that accept 25 mm^2,
all the other ways being 16 max.
Many years ago a man from Eastern Electricity told me they have a
minimum tails requirement of 16 mm^2, regardless of other
considerations, but this was in the context of direct connection to the
meter, not via a Henley block.
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