While Part VI of Article 430 (covered in our previous two issues) provides the requirements for motor control circuits, Part VII provides the requirements for the motor controllers themselves.
The stated intention of Part VII is to require suitable controllers for all motors [430.81]. To satisfy this intention, Part VII addresses several aspects, such as:
- Controller design
- Number of motors per controller
- Adjustable speed motors
Part VII addresses some other issues, also. For example, Parts (A) and (B) of 430.81 address stationary motors of 1/8HP or less and portable motors of 1/3HP or less, respectively. Logically, these requirements belong in another section. But the NEC has used up the available sections and renumbering would present a major rework of Article 430. These requirements basically say you can unplug these loads as a means of controlling them.
Let’s look at the controller design requirements. First, the controller must be capable of:
- Starting the motor
- Stopping the motor
- Interrupting the locked-rotor current of the motor
If the motor controller is an autotransformer starter, it must meet several requirements. For example, it must not be able to rest in any position (e.g., start) that would defeat the overload device [430.82(B)].