Inverse time circuit breakers have both thermal and instantaneous trip features and are preset to trip
at standardized levels. This is the most common type of circuit breaker used in the building trades
for residential, commercial, and heavy construction.
C The thermal action of this circuit breaker responds to
heat.
C If a motor’s ventilation inlets and outlets are not
adequate to dissipate heat from the windings of the
motor, the heat will be detected by the thermal action
of the circuit breaker.
• If a short should occur, the magnetic action of the
circuit breaker will detect the instantaneous values of
current and trip the circuit breaker.
• The National Electrical Code requires inverse time
circuit breakers to be sized to a maximum of 250%
of the motor FLA.
The rating of an inverse time circuit
breaker can be multiplied by 3 and
this total amperage will start any
motor with less locked-rotor
amperage.