It’s easy to get confused over the requirements for motor overload protection (Part III of Article 430). To prevent that confusion, read the first sentence of 430.31 before you begin. Now read it again, starting with “due to”.
That’s the key. Motor circuit overloads don’t protect against faults and short-circuits, as does the breaker for a lighting circuit.
The branch circuit overcurrent protection device (OCPD), which may be a circuit breaker or fuse, protects against the high currents arising from faults or short-circuits. In a non-motor circuit, it also protects against circuit overloads. But with motor circuits, this really isn’t possible if the OCPD is also going to permit the motor to start.
So in a motor circuit, overload protection is handled separately. If you keep that in mind, you can implement the requirements without getting a massive headache.
The requirements vary according to the application. First, you need to determine if the motor is continuous duty (likely to run more than 3 hours). If so, then you must look at the applicable requirements depending upon whether the motor is:
- Larger than 1HP [430.32] and less than 1500HP [430.32(A)(4)].
- Larger than 1500HP [430.32(A)(4)].
- Smaller than 1HP [430.32(B)].
If the motor is larger than 1HP, certain requirements apply. An additional requirement applies if the motor is larger than 1500HP. The requirements address using a separate overload device or, alternatively, using one that’s integral with the motor.
We’ll look at these requirements in Part 12.