The plugs are rated for 5A so lets say that; 2.2mm core is overkill. 10kV is 5mm, 65kV is 10mm, so something like sqrt(V) + 4mm. I guess 500 kV would be around

The plugs are rated for 5A so lets say that; 2.2mm core is overkill. 10kV is 5mm, 65kV is 10mm, so something like sqrt(V) + 4mm. I guess 500 kV would be around 30mm for each wire, which is starting to get unwieldy.