DNO earth cable size

davebodger · Apr 3, 2025

All help appreciated with this one.
I am currently training for C&G 2365 level 2.
Here is a photo of the MET in my flat.
As you can see, it is TN-S.
But how do I know what CSA the cable from the sheath is ???
Because it's double-stranded I'm having a hard time judging.
Looks like something between 4 and 6 mm2 to me but I could easily be wrong.
It is an old installation, probably 1960's to 1980's is my guess.
Oh, and it's a looped supply which also goes to the flat above me.

sunray · Apr 3, 2025

Well the Red, Yellow,Blue, Green colours appear to date it to 7th edition 1916-1924

It's hard to judge from a photo but they look like 7 strands each, comparing to the screw head they don't look big so I'l make a stab at 7/0.029" which is something like 2.8mm² each or 5.6mm² for the pair

EDIT: (0.029 / 2 * 25.4)² * 22 =2.984mm²
So ASSUMING it is 7/0.029" the pair is 5.97mm² Do you have a caliper to be able to measure on of the 14 strands?

davebodger · Apr 3, 2025

sunray said:
Well the Red, Yellow,Blue, Green colours appear to date it to 7th edition 1916-1924

It's hard to judge from a photo but they look like 7 strands each, comparing to the screw head they don't look big so I'l make a stab at 7/0.029" which is something like 2.8mm² each or 5.6mm² for the pair

EDIT: (0.029 / 2 * 25.4)² * 22 =2.984mm²
So ASSUMING it is 7/0.029" the pair is 5.97mm² Do you have a caliper to be able to measure on of the 14 strands?

Thanks, good idea. There are no free strands though but I'll try and tease one out.
It does worry me a little that it's that small when we are supposed to fit a much thicker cable from the MET to the CU.

littlespark · Apr 3, 2025

BS7671 is only concerned with the "internal" earth from the CU to the MET.... The size of what the DNO supplies is nowt to do with us, really. (as long as its there)

I wouldnt go "teasing" a core out to find out the size.... you'de be removing the earth from either your own installation or your neighbours.

It might not be a "looped" supply if you and the neighbour are off different phases

davebodger · Apr 3, 2025

littlespark said:
BS7671 is only concerned with the "internal" earth from the CU to the MET.... The size of what the DNO supplies is nowt to do with us, really. (as long as its there)

I wouldnt go "teasing" a core out to find out the size.... you'de be removing the earth from either your own installation or your neighbours.

It might not be a "looped" supply if you and the neighbour are off different phases

Thanks, more good points. I can't easily check the phase and to be honest I really don't need to know.
The earth wire going upstairs to my neighbour is half the size (i.e. it's one half of the stranded wire).
There is only one flat above me.
These small blocks were all built around the beginning of the 1920's.
This one shows signs of VIR (all cut back now) and it's my guess it was rewired in black & red PVC cable around 1970. It's all run in singles in metal conduit that's used as CPC.
The DNO head looks original. Definitely been there a long time.
It has a Wylex grey metal CU with 4 circuits, type 2 MCBs and an AC RCD DP main switch. Rated for 63 Amps and with single-insulated 16mm tails but sadly with what looks like a 4mm earth.
The nice man who fitted my smart meter recently fitted double-insulated tails from the cut-out to the meter and also put in a 100A isolator after the meter, so that's handy.
I feel a CU replacement is in my future.

DefyG · Apr 3, 2025

You dont need to tease one out, there is enough slack in the conductor to open up the lay of the twist to be able to put a caliper or micrometer on it.