Procedure for Thermal Unit Selection

Determine motor data:

1. Full-load current rating

2. Service factor

   NOTE: If motor full-load current (FLC) is not known, a tentative thermal unit selection could be made, based on horsepower and voltage. Refer to Thermal Unit Selection on an Approximate Basis—Based on Horsepower and Voltage.

Motor and controller in same ambient temperature:

1. All starter classes, except Class 8198:

   1. For 1.15 to 1.25 service factor motors use 100% of motor FLC for thermal unit selection.

   2. For 1.0 service factor motors use 90% of motor FLC for thermal unit selection.

2. Class 8198 only:

   1. For 1.0 service factor motors use 100% of motor FLC for thermal unit selection.

   2. For 1.15 to 1.25 service factor motors use 110% of motor FLC for thermal unit selection.

Motor and controller in different ambient temperatures:

1. Multiply motor FLC by the multiplier in the table Selection of Thermal Units for Special Applications. Use the resultant full-load current for thermal unit selection.

Locate the proper selection table from Thermal Unit Selection .

1. The proper thermal unit number will be found adjacent to the right of the range of full-load currents in which the motor FLC or resultant full-load current falls.

See Trip Current Rating Calculation for calculation of trip current rating.