the recomended wire needed would be 10mm twin and earth shower cable the old reference used to be 6mm twin and earth
then comes your wire distance
50a mcb id be more concerned about the wire if you ever had a shorted break in it as the shorted break would become so hot around that area
saying that ive only come across a break in the wire usually with 1.0mm 1.5mm and 2.5mm wire and not many of them either even when they are new on the roll
a few times ive come across a wire that was actually live due to some idiot wiring in from another meter usually in flats where you have shut that mains off using the rooms mcb board but a sodding wire has been live seen a few bright flashes of blue when cut wires off due to some idiots brilliant idea of saving money
the most that would be on together is the kettle and welder.ive got to ask will you be getting a plasma cutter and air tank at all
and have you got lights in there as well ?
so whats possible is lights on mig welder and grinder all at same time
plasma cutter air container and kettle on at same time we all do this from time to time but normally its use plasma cutter then allow air chamber to fill up whilst a brew is being made