Second CU in domestic workshop - some advice please?

[lawsonium]

Hello,

If I wanted to put a second Consumer Unit in a domestic workshop with the following criteria, what cable would you advise for the extension.

Remote CU would be 50m away from the main board. The main board is not in a house but a separate garage, is a dedicated supply and has it's own earth.
Lights and sockets only, calculated to be no more than 10kW total (though never used at full capacity).
Switched fuse or RCD to protect the extension on the main board side of the cable run.
The workshop where the remote CU will be TT'd with it's own Earth.

My VD calculations suggest 16mm² SWA would be fine for the extension to the remote CU with this situation.
VD of 5.43V (2.4%) at a max of 43.5A
I don't think I can get away with 10mm² due to the VD and this having lighting circuits.

Second question is about the usage of 2 core SWA instead of 3 core?
I don't need to export the earth as the workshop will be TT'd and so I would just earth the armor at the main board. Thus the thought that 2 core is fine to use in this scenario (easier to work with and slightly cheaper).
Reason I ask is that online calculators suggest that the VD calculations are different between 2 core and 3 core SWA cables - i.e. there is greater VD with the 2 core than the 3 core SWA cables which doesn't make any sense to me but then, I'm not an expert. Thoughts?

 

[lawsonium]

Thank you everyone. I appreciate your concern and completely agree. Hence why I prefaced this by saying I intend to replace it all (at the same time as doing the extension). I see this as a single project. I'm aware that everything you see in the photos needs replacing.
I have two new consumer units waiting to go in. One here and one in the remote workshop.

The question I have at the moment, based on this conversation, is whether I need to change this TT earth configuration to TNC-S or not and if that will require the DNO before I or an electrician replaces the CU and installs the extension?
As it happens, the supply at the cottage is TT but it has a newer CU and a functioning main RCD.

Have spoken to my friend and her partner 'isn't taking on any more work at the moment' (might be code for 'not this kind of work though' lol).
I have two other local names to try tomorrow but being out in the countryside options are limited.

 

[pc1966]

You choice of earth is ultimately limited by the DNO.

Only the DNO can offer you a supply earth, and if it is available then it is usually very reliable.

Otherwise (and common in rural areas and in Europe) you are going to be TT and then need to have both a decent earth (no more then 200 ohms for up to a 100mA RCDs) and to have working RCD protection for all circuits.

Traditionally that was in the form of the incomer (in place of an isolation switch) but some go for all-RCBO boards. My own choice would be to have both the delay RCD incomer and RCBOs as then you have no single point of failure.

To get an earth below the 1-1.5 ohm or so needed to clear on faults to 32A MCBs is usually impractical unless you have major steel foundations in a reasonably damp soil. So then you depend on the RCD to clear any fault to earth.

If in doubt ask nearby farms, etc, who they use for any electrical work.

 

[lawsonium]

No major steel foundations but plenty wet ground lol. By the look of the supply, there is a supply earth available but just hasn't been used. Will have to call the supplier to confirm I guess.

If we make an assumption though that the supply will remain TT earthed, am I right in saying that it will be more important to have a new earth to TT the remote workshop rather than taking the earth from the existing TT earth?