I do not know what exactly an M.V cable is. When you say different size I am guessing you mean different gauge wire. Each gauge (size) and metal composition of wire will have its own resistance per meter. For example if you had two wires, one with 0,1 ohm of resistance and another with 0.01 ohm of resistance and you connected them in parallel from a power supply to the same load the one with the 0.01 ohms of resistance would carry approximately 90% of the current and the 0.1 ohm wire would carry approximately 10% of the current. If I have not properly answered your question please clarify and ask again. If I don't know one of the other guys will help us out.
I think given the voltage and current in question the biggest issue is safety in the event of a failure and ensuring nothing caught fire. In the case of adding an extra 3x70mm2 cable in parallel with the existing cable, what would happen in the event of one of the parallel pair breaking or becoming disconnected? You'd end up with all the current either through the old 3x120mm2 cable or even worse through the 3x70mm2 cable, neither of which may be able to cope with the significantly increased current in this failure condition and could pose a fire hazard.
Best Regards,
Rachael
Disclaimer: I'm not an expert when it comes to such voltages and currents. I deal with much lower voltage and current in my day job!
