Official wire sizes?

[Geoff1946]

strip my current 16mm2 and 35mm2 insulation off, join both copper wires to give 51mm2. I’m wondering how I would re insulate that copper cable properly,

Oh I see what you mean. No. That would be a messy bodge up. Terminate both cables independently, as I previously described each with proper lugs and then bolt them onto the same terminal stud.
It's debatable whether it's worth doing now you know the old cable is only 16mm. Paralleled cables are usually the same size to ensure equal load shareing.
I think the best suggestion of all is a piece of welder cable. It's really tough stuff, should be available locally, and the supplier will probably have the correct terminations too.

 

[Will C]

Oh I see what you mean. No. That would be a messy bodge up. Terminate both cables independently, as I previously described each with proper lugs and then bolt them onto the same terminal stud.
It's debatable whether it's worth doing now you know the old cable is only 16mm. Paralleled cables are usually the same size to ensure equal load shareing.
I think the best suggestion of all is a piece of welder cable. It's really tough stuff, should be available locally, and the supplier will probably have the correct terminations too.

Yeah I’m going to go down that route and get 5 metres of 50mm.
Thanks.