View Single Post
  #2   Report Post  
Posted to uk.d-i-y
John Rumm John Rumm is offline
external usenet poster
 
Posts: 25,191
Default Question about sub-main sizing

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.

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.

http://wiki.diyfaq.org.uk/index.php/...e_requirements

http://wiki.diyfaq.org.uk/index.php/...potential_Zone

http://wiki.diyfaq.org.uk/index.php/..._Wire_Armoured

http://wiki.diyfaq.org.uk/index.php/...our_as_a_C PC


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/