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.
