Stuart Peacock asks this question, which concerns the sizing of tails and earth supplying consumer units of new installations. It has been answered by the NICEIC (Theme – Cables, Wiring & Wiring Accessories):
Question: I have always used 25mm tails with a 16mm earth to supply the consumer unit of a new installation. I've now recently started working for a new company that uses 16mm twin and earth cable (T&E), along with a 10mm earth! The main fuse is 80A.
I wouldn't use this approach, but is this company right to do so?
Answer: If the loading and the protection for the cable is adequate, then there is no problem.
For a 100A supply, ideally a 25mm tails and a 16mm earth would be great. However, if the earth cable size has been adequately calculated, rather than taking from the tables in the regulations, then again it would be compliant.
Note that it is the responsibility of the designer of the installation to verify that, by design, the requirements of the regulations are complied with.
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