Motors, Part 17

Motors, Part 17A fundamental way in which motor circuits differ from other circuits is that the job of overload protection is split away from the jobs of overcurrent and fault protection. In other circuits, all three jobs are handled by the overcurrent protection device (OCPD).

However, in certain motor applications you can combine all three functions in the OCPD just as you would for any other circuit [430.33]. If the application is a motor used for a condition of service that is inherently one of these three:

  1. Intermittent
  2. Periodic
  3. Varying duty

Then permission to combine the three functions comes with these two caveats:

  1. Your definition of these three conditions must follow Table 430.22(E)
  2. The OCPD rating or setting shouldn’t exceed what’s specified in Table 430.52

Subsection 430.33 ends with a potentially confusing statement. It says that you must consider a motor application to be for continuous use unless the nature of the apparatus it drives precludes the motor from operating under load continuously under any condition of use.

What does this really mean? Another way of stating it is that if it’s possible for the motor, as installed, to be operated continuously, then you must consider it to be a continuous duty motor for the purposes of load calculations. This requirement is why the word “inherently” is in the first sentence of this subsection. That ending statement is simply emphasizing that requirement.

Table 430.22(E) gives examples of qualifying applications such as valve operators, passenger elevators, drawbridges, coal-handlers, and ore-handlers.

« Part 16 | Part 18 » | Source: Mark Lamendola | Mindconnection

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