Is there a formula or a simple rule to work out current drawn from supply for a certain kilowatt rated air conditioner

I think you mean to ask: how to convert watts to current draw in amperes, right? The basic formula is:

Power (watts) = current (amps) times voltage.

So for example a 1000 watt appliance operating at 110 volts draws 1000/110 = 9.1 amps.

Usually the Air conditioning KW rating is what it produces in cooling power therefore isn't convertible unless you have an idea of what sort of system you are working with. For instance if it is an old system this would draw a lot of current from the supply to produce it's KW of cooling, whereas a more modern VRV system would use the incoming electrical power more efficiently and thereby produce more KW's of cooling. As a rule of thumb you could roughly estimate about a quarter of the cooling KW would be approximately equal to the electrical load in KW.

16KW cooling - 4KW power needed to run if VRV

4KW / 230-volts = 17.40 Amps

and if VRV system a 20Amp C-type mcb would be fine in practise although the manufacturers often say slightly higher.

Stretchyboy