For S9, the power consumption is 1323W (13.5TH/s batch).. say 1400W. At 220V supply, load current is 6.4A plus 10% at the wall for efficiency.. so around 7.1A.

Wire size should be 1.5mm² as per NEC wire sizing for 7.1A load. That makes your wire undersize for your load if it is only 0.75mm² that is why it gets hot.

Better get thicker wire size to match your load current and that is 1.5mm² to be safe.