and is my wiring assumption correct?
I should add there is no 120/240v switch on my PSU. So i assume it just figures it out on it's own? From single to double phase without a switch seems weird to me.
Correct the psu will recognize the voltage. That plug will work fine. Yes the two flats are hot and the round one is ground. No neutral for 220. This is for US. I don’t know about other countries. Red and black are hot. Green is ground. Make sure you get a 2 pole 20amp breaker and use at least 12Ga wire for 20 Amps. One S7 will not pull 20 amps but I haven’t seen a 220 breaker smaller than 20amp and you don’t want to run a breaker that is larger than what the wire can handle.