What is the cable size for 90 kw motor?

Depends upon the voltage. The formula for amperage or (wire size) is Watts / Voltage. If the voltage is 220 volts, then the amperage would be over 400 amps. This would require a large wire size to run it. If it were 440 volts, the amperage would be 1/2 or 200 amps. That would require a smaller wire size. As the voltage goes up, the amperage goes down. At a thousand volts, the amperage would only be 90 amps. A wire gauge or size of a #2 would carry 90 amps for small distances.