Procedure for Calculating the Trip Current Rating

1. Use the selection table for the specific controller involved.

2. Find the minimum motor full-load current listed for the thermal unit.

3. Multiply that current by 1.25 (1.15 for Class 8198). The result is the trip current rating.

Example 1:

Determine the thermal unit selection and trip current rating for thermal units in a Class 8536 Type SCG3 Size 1 magnetic starter used to control a three-phase, 1.15 service factor motor with a full-load current of 17.0 Amperes, where the motor and controller are both located in a 40^oC (104^oF) ambient temperature.

1. From thermal unit Table 13, the proper selection is B32.

2. The minimum motor full-load current is 16. 0 Amperes.

3. Trip current rating is 16.0 x 1.25= 20.0 Amperes.

• Protection Level: Is the relationship between trip current rating and full-load current. Protection level, in percent, is the trip current rating divided by the motor full-load current times 100. In the example above, the protection level for the B32 thermal unit is: 20.0/17.0 x 100=118%.

  National Electrical Code, Section 430-32, allows a maximum protection level of 125% for the motor in the above example.

• Minimum Trip Current: Also called ultimate current, may vary from the trip current rating value, since ratings are established under standardized test conditions. Factors which influence variations include:

  • The number of thermal units installed

  • Enclosure size

  • Proximity to heat producing devices

  • Size of conductors installed

  • Ambient (room) temperature, and others

Except for ambient temperature-compensated overload relays, an ambient temperature higher than 40^oC would lower the trip current, and a lower temperature would increase it. This variation is not a factor in selecting thermal units for the average application, since most motor ratings are based on an ambient temperature of 40 °C, and motor capacity varies with temperature in about the same proportion as the change in trip current. Temperature-compensated relays maintain a nearly constant trip current over a wide range of ambient temperature, and are intended for use where the relay, because of its location, cannot sense changes in the motor ambient temperature.