10 sq mm T+E distribution circuit supplying CU

[timhoward]

Can someone sanity check my thinking please....

An 80 amp PME service head, tails to meter, 25 sq mm tails into an enclosed fused isolator with 60 amp fuse in it.
16 sq mm from cut out into a MET. Gas and Water bonding connected to the MET.
16 sq mm main earth from MET to the fused isolator.

Then 10 sq mm T+E glanded to the fused isolator and running off to a CU some distance away, cable route unknown and undiscoverable.
This means 4 sq mm CPC for this dist circuit. Let's assume the CPC meets the adiabatic equation.

My doubt's are:
1) Is there any nuance of PME earthing requirements that I've forgotten that renders this non-compliant? (noting the bonding is connected upstream).
2) I can't prove the 10 sq mm T+E is capable of carrying 60 amps as that would only apply if clipped direct and I can't verify the cable route
3) Similarly I can't prove if it requires RCD protection to cover impact protection or not, if it did it would also lead to selectivity issues.

Specific questions
1) This isn't an EICR, but if it was would you LIM this or assume the best and C3 it?
2) Would you be happy to move the fused isolator and extend the T+E by 3 meters and mechanically protect it to where a new service head is going. Or would you dig heels in an say a new SWA dist circuit is needed.

Many thanks!

 

[pc1966]

The use of 10mm T&E seems a bad choice for all of the reasons you have identified. It might just have been OK at some past time if you know expected max load is in the 45A region (cooker diversity + one 32A RFC, etc), but that is certainly not something you can assume going forwards if electric showers, etc, get fitted.

A 60A fuse and 4mm CPC is only meeting adiabatic for Zs below 0.35 ohms (OSG Table B5 (ii) for info) so that is another factor to check at the CU, and of course PME bonding needs 10mm minimum. Either of these would merit a C2 from the BPG #4

I personally would not modify a circuit I felt was not meeting the regulations, that is a step beyond simply repairing a failed accessory, etc. Going to 16mm 3C SWA or similar (16mm 2C + CPC if easier to wrangle in tight spaces, etc) would solve all of those issue.

 

[timhoward]

Thanks a lot @pc1966. Yes, my gut reaction was to groan when I saw it.

and of course PME bonding needs 10mm minimum.

I was initially slightly confused about this, but as the bonding is connected to the MET, before the fused isolator and before the T+E, I don't think I can cite that particular reason.

The situation is that a keen and talented DIYer has already arranged a DNO visit. It was two houses, both with their own supply. It's now now one house and the DNO is coming in near future to replace one head with an 100amp supply, and decommission the 2nd one described above.
I feel that at least the one that is moving should have a new distribution circuit.

 

[pc1966]

I was initially slightly confused about this, but as the bonding is connected to the MET, before the fused isolator and before the T+E, I don't think I can cite that particular reason.

OK in that case it is not an issue, but adiabatic might be.

The situation is that a keen and talented DIYer has already arranged a DNO visit. It was two houses, both with their own supply. It's now now one house and the DNO is coming in near future to replace one head with an 100amp supply, and decommission the 2nd one described above.
I feel that at least the one that is moving should have a new distribution circuit.

If they are getting a 100A supply and at any time in the future want to make use of the extra capacity then they ought to be looking at 25mm SWA.

Do you know how convoluted the route is?

 

[westward10]

I wouldn't be happy with a 10.0 on a 60A fuse (I assume a 1361) especially not knowing the installation method.