It is good idea to parallel ground wire with difference size ?, 50mm2 BC wire and 10mm2 BC wire, and bended together with twisted 10mm2 bc wire, with this way can I make 70mm2 BC wire ?

 

That might sort of work, don't know whether it is a good idea or not. I have not seen that done. Where there are electrical codes twisting two wires and using as one might not be permitted.

 

AFAIK there is nothing in the regs to say different, where are you located,? Jurisdiction?

 

AFAIK there is nothing in the regs to say different, where are you located,? Jurisdiction?

China / PRC, that project is in Beijing