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Motor Controllers, Part 4

Motor Controllers, Part 4How do you determine the correct rating for a motor controller? Depending upon the application, you can use one of four methods.

The first method applies unless your application is a “small motor” (undefined in the NEC), stationary motor of 2 hp or less, or a torque motor. This method permits three types of devices to be a motor controller [430.83(A)]:

  1. Branch-circuit inverse time circuit breaker.
  2. Molded case switch.
  3. All other types of controllers.

If the controller isn’t one of the first two listed above, it must have a horsepower rating (at the application voltage) equal to or greater than that of the motor.

The second method concerns “small motors.” The NEC doesn’t define “small motor” in Article 100 or 430.

To understand what’s meant, we start by looking at the requirement. It says, “Devices as specified in 430.81(A) and (B) shall be permitted as a controller” [430.83(B)]. Turning to 430.81(A) and (B), we see “Stationary Motor of 1/8 Horsepower or Less” and “Portable Motor of 1/3 Horsepower or Less” respectively.

The stationary motors in question are for are things like clocks [430.81(A)]. If the load is normally left running and can’t be damaged by overload or failure to start, its branch-circuit disconnect (e.g., breaker) can server as its controller.

The motors in 430.81(A) are for things like portable pumps. You can plug them in.

« Part 3 | Part 5 » | Source: Mark Lamendola | Mindconnection