June 10, 2002
Automatically started motors, permanently installed motors, or motors not in
sight of the controller location that are rated one horsepower or less have essentially the
same requirements as for motors above one horsepower. A non-automatically started
motor rated at one horsepower or less is allowed to be protected by the branch circuit
short-circuit and ground-fault protective device, provided that it is within sight of the
controller and is not permanently installed.
Verify that an overload relay is properly sized for the associated motor and
then check the overload relay tripping time for the motor's rated locked rotor current.
The overload relay tripping time as a function of current should allow sufficient time for
the motor to start, accelerate, and reach full speed. The selected overload relay should
not actuate throughout the motor's operating current range, from starting current to long
term operation at full load current. If the overload relay size is not adequate to start the
motor or carry the load, the next higher size overload relay is permitted by the NEC,
provided that it does not exceed the following percentages of motor full load rating:
140 percent for motors with a marked service factor of not less than 1.15.
140 percent for motors marked with a temperature rise not over 40 °C (104 °F ).
130 percent for all other motors.
Overload protection can be provided as an integral part of the motor or
controller. If necessary for the design, the overload protection can be installed in a
separate enclosure, provided that the enclosure is accessible and is clearly marked
regarding its purpose.
Verify overload relay coordination by the following process:
Determine motor full-load amperes.
Select overload relay ampere rating in accordance with NEC Article 430, Part
III (2002 Edition) .
Evaluate overload relay time-current characteristic curves.
Evaluate overload relay performance in relation to motor locked rotor
amperes and starting time.
Evaluate overload relay performance in relation to motor locked rotor ampere
damage time for medium voltage motors.
Ensure overload relays comply with NEMA ICS 2 and UL 508, Industrial