Hi, I'm an electrical engr. And I want to know how to calculate the rating in KW of a 20Ton Package ACU to decide the size of cable to use.thank you!

Look at the power requirements on the unit. The real driver in the system would be the condenser pump motor. This is off the top of my head, but I think a 20 ton unit is approx. 70Kw. (1 ton = 3.5Kw).

It depends on the voltage of the unit, the amp draw. Every unit is different. I have a 17 ton unit the uses 480VAC, 3ph @ 60 amps. My wire size will be about 6 or 4 AWG (4 is oversized alot) per NEC2005 codes.

If you have the spec sheet or manual of the ac unit this will tell you what power service to provide, in which you can size your wires and disconnect needed.