1, Check the label on AC, It will have Wattage and Current. Calculate current = Wattage/Voltage. Usually 16A single phase MCB is ok for 1.5T.
If ur 1.5T AC is tripping the MCB, then ur AC has a fault. It could be less gas also. It means ur AC is drawing more than rated current and MCB is tripping to protect ur AC.
DOnt go for a higher rating MCB else u will cause damage to AC