Rob G coughed up some electrons that declared:

Why on the earth does a TT earth require 16mm^2 cable ? Our own Wiki,
backed up by various other sources of information, indicates that the
ground resistance is in the order of 100 ohms, which gives maximum
fault current of 2.4 amps. Even if the ground resistance is half this
that is still only 5 A, so why the dirty great cable ?

I've found a reference that says that 2.5mm^2 cable has a resistance
of 7mOhms/m, so you would need to have an awful lot of metres of even
that before it has any influence on the total resistance.

Rob

I haven't got my books to hand (down at Bungalow) but you need a certain
minimum size for mechanical robustness, unless the wire is otherwise
protected.

You can't go sticking a bit of 2.5mm2 dangling outside, over the ground and
down a hole to a rod - it's too weedy and liable to get damaged.

The same logic is applied to supplimentary bonding conductors dangling
around pipes, but as my memory is ****e, you'll have to wait until I get my
books or someone else with books/better memory can comment.

From memory 4mm2 or 6mm2 is likely to be the minimum acceptable size
depending, purely from a mechanical POV.

But on the argument regarding resistances I agree completely.

Cheers

Tim