Meter tails

When I've fitted tails, I've sized them to match the main cutout, e.g. 25mm 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 (16mm or maybe even 10mm), 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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Andrew Gabriel
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Andrew Gabriel wrote:

The REC do not seem to complain about much as long at the tails are connected to a meter!
The 3m rule in reg 434.2.1 would apply in the case of 16mm or 10mm tails.
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Adam



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On 04/08/2012 08:16, Andrew Gabriel wrote:

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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Andy

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snipped-for-privacy@arcade.demon.co.uk wrote:

Christ on a bike. That a lot of wasted copper. I know I have seen the link before:-)
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Adam



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