What calculations determine the choice of the overload relay trip class to protect a motor
The motor trip current does not determine the choice of trip class. Trip class specifies the length of time it will take for the relay to open in an overload condition. Classes 5, 10, 20 &amp; 30 are the most common. Class 5, 10, 20 &amp; 30 overload relays will trip within 5, 10, 20 &amp; 30 seconds respectively at 600% of motor full load amps. Class 5 is usually used for motors requiring extremely fast tripping. Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.. Class 20 is usually sufficient for general purpose applications. Class 30 is usually required for high inertial loads to help prevent nuisance tripping.

Class 20 is used for all general purpose motors that have no special trip class instruction requirements provided by the manufacturer. If the motor requires a faster trip class the manufacturer's instructions should state that. If the motor has been designed to be compatible with a slower trip class, that should also be stated by the manufacturer. If you encounter a motor that has smaller dimensions that what is standard for the power and speed rating, you should check with the manufacturer to make sure that it doesn't need a faster trip class. If you encounter an application that has a high inertia and class 20 overloads trip when the motor is started, check with the manufacturer to see if class 30 overloads are permissible. If they are not, the application may require a special or oversized motor.

This information is based on NEMA Trip Classes. If IEC has a similar system, it may be different.