On 07/02/2019 17:57, John Rumm wrote:
On 07/02/2019 14:24, wrote:

My TNC-S supply enters in the attached garage, where the tails are
split to a CU (in the garage, feeding: the kitchen, cooker, utility
and a few other bits) and a 60A switched fuse which feeds a 16mm2
sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board
in the same location is connected to one of the MCBs in the CU via a
2.5mm2 cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend all
the cables back to it, but that requires a lot of disruption. I'm now
wondering whether I should simply replace the two wylex boards with a
single new (RCBO) CU in their present position and feed it from the
sub-main.

Yup, that sounds like a less disruptive option.

Â*From some quick checks the existing 16mm2 will be OK at 20m for volts
drop if I protect it with a 50A MCB. However, from a quick look (more
poking around required) the only earth connection seems to be the CPC
in the 16mm2 T&E so that will need improving and the disruption
involved won't be much less than extending all the circuits, although
it will be less work.

Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2 of
copper or equal or better amount of copper equivalent via some other means.

What do the relevant panel experts think?

You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

.... snipped
Thanks John, that was what I thought.
The existing 16mm2 T&E runs between joists (mostly just resting on the
ceiling, rather than being clipped) from the garage to the fuse boards
so it's going to be a (necessary) pain to lift carpets and boards to run
the green/yellow, but it would be more of a pain to replace it with SWA
- why would I consider doing that?

I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?