What's the cable for a meter in 18kW boiler in 3phase connection?

In my opinion are missing a lot of information for a right sizing: if you really need our help, we need to know the type of machine (motor, electrical heater, etc.), rated voltage, estimated distance between them and power source, connection diagram (star or delta), voltage level on the power source, type of start (for motors it is important to know if is DOL, with variable frequency drive, with soft starter etc and with type of machine is motorized with this motor - oil pump, water pump, fan etc.) and also we need to know the the service type (continuous, intermittent with which periodicity, for evaluate the cooling possibility of the cable). Moreover, it is important the cable installation solution - in air, in conduit, underground etc. Otherwise, for be covered, select something between 2.5 mmsq (as indicated for 690V three phase) and 20mmsq for 48V mono-phase.

Your question was open ended. Different meanings could be deduced from it. For instance the voltage was not given and from my understanding, you have a machinery which is rated at 18Kw,3phase and you want to know the size of cable to connect it from the meter.Is is right? If my assumption is correct,you have to give the distance of the meter from the motor so that a cable of suitable size can be calculated taking into consideration voltage drop.