The model of thermal relay and the current level of thermal components are determined mainly according to the rated current of the motor. In general condition, the thermal relay of two -phase structure can be selected. The thermal relay of three-phase structure can be selected for the motor with poorer balance of the voltage of power grid, in severe working conditions or less unattended.

Precautions The setting current (that is, the action current ) of the thermal relay usually is equal to the rated current of the motor or 0.95 ~ 1.05 times the rated current of the motor. 

If the motor drives the impact load or the motor starts for longer time, the selected setting current of the thermal relay shall be higher than the rated current of the motor. But for the motor with poorer overload capacity, the selected setting current of the thermal relay shall be smaller than the rated current of the motor. The selection of model should be determined according to the connection mode of the motor, when the stator winding is in star connection, it is possible to select the commonly used thermal relay. If the winding is in delta connection, then the thermal relay with open -phase protection shall be selected. 

By the way, it is pointed that: According to the IEC standard, there are two types of thermal relay: 

For Type 1 thermal relay, its setting current is determined according to the full- load current of the motor,  it is generally equal to or slightly greater than the rated current of the motor. Under the setting current, the thermal relay does not act; during overload, the thermal relay acts according to the overload margin in the corresponding period of time. The thermal relays produced in China belong to type 1. The protection characteristics and selection for the thermal relay above mentioned is aimed at type 

For Type 2 thermal relay, its setting current is the final action current, namely the action current of the thermal relay is taken as the base. Under the setting current, the thermal relay shall act within the prescribed time. When the current drops to a certain percentage of the setting current, the thermal relay shall not act.