AS-B825-016 SCHNEIDER

General

All tables are based on the operation of the motor 

and controller in the same ambient temperature, 

40°C (104°F) or less. Always be certain the 

correct thermal units are installed in the starter 

before operating the motor. Each thermal unit 

shall be installed such that its catalog number is 

visible. On melting alloy thermal units the ratchet 

wheel must engage the pawl assembly.

Selection Procedure

1. Determine motor data:

a. Full load current rating

b. Service factor

NOTE: If motor full load current is not known, a 

tentative thermal unit selection could be made, 

based on horsepower and voltage. Refer to 

Page 22.

2. Motor and controller in same ambient 

temperature:

a. All starter classes, except Class 8198:

1. For 1.15 to 1.25 service factor motors use 

100% of motor full load current for thermal 

unit selection.

2. For 1.0 service factor motors use 90% of 

motor full load current for thermal unit 

selection.

b. Class 8198 only:

1. For 1.0 service factor motors use 100% of 

motor full load current for thermal unit 

selection.

2. For 1.15 to 1.25 service factor motors use 

110% of motor full load current for thermal 

unit selection.

3. Motor and controller in different ambient 

temperatures:

a. Multiply motor full load current by the 

multiplier in Table A. Use the resultant full 

load current for thermal unit selection.

4. Locate proper selection table from index, 

Pages 19 and 20.

a. The proper thermal unit number will be found 

adjacent, to the right of the range of full load 

currents in which the motor full load current 

or resultant full load current falls.

5. See Page 21 for calculation of trip current rating.

Slow Trip Thermal Unit Selection

To select Type SB slow trip thermal units, the 

selection table for a standard Type B thermal unit 

may be used with the following modifications: For 

continuous rated motors having service factors of 

1.15 to 1.25, select thermal units from the 

standard Type B table using 93% (102% for Class 

8198) of the full load current shown on the motor 

nameplate and then substitute an SB for the B in 

the thermal unit type number.

Example: A motor with a full load current of 14 

amps controlled by an 8536 SCG3 would require 

B22 thermal units for standard trip applications 

and SB19.5 thermal units for slow trip 

applications. The SB is selected by multiplying 14 

amps times 93% for 13.02 amps and using this 

value to select B19.5s. Then add the S prefix to 

arrive at SB19.5s.

For continuous rated motors having a service 

factor of 1.0, select thermal units in the same 

manner using 84% (93% for Class 8198) of full 

load current shown on the motor nameplate.

NOTE: SB thermal units are used on Size 0, 1, 2 

and only some Size 3 applications. Check thermal 

unit tables for current ranges.