If one machinery have the boiler of 18k watt then which cable in mm we need to used from the meter if it is 3phase connection?

It's hard to size the cable because you can't calculate the current when you don't know what the voltage is.

I am not sure what you meen when you say ' a meter'.. but i assume that you are talking about about kw meter.., if so,, then you use wiring from the CT's surrounding the pwr. cables..

meter means unit uses reading meter.