Motors, Part 12

Motors, Part 12If a continuous duty motor is larger than 1HP, these requirements apply:

  • If using a separate overload device, size it at 115% of the nameplate rating unless it’s marked with a service factor of 115% or greater or is marked with a temperature rise 40°C or greater; then size it at 125% [430.32(A)(1)].
  • If using a thermal protector integral with the motor, ensure the ultimate trip current doesn’t exceed the required percentage of the motor full-load current (FLC). The required percentage is 170% if the FLC is 9A or less, 156% if the FLC is 9.1 to 20A, and 140% if the FLC is greater than 20A [430.32(A)(2)].
  • If the motor is larger than 1500HP, you must protect it with a protective device having embedded temperature detectors. The device must cause interruption of current to the motor when the motor attains a temperature rise greater than that marked on the nameplate (in an ambient temperature of 40°C) [430.32(A)(4)].

Nothing stops you from adding additional protection or tightening the temperature bands, if doing so makes sense from an engineering perspective and won’t cause undue interruption in operations.

Section 430.32 also provides requirements for motors of one horsepower or less [430.32(B) and (D)]. One difference in this horsepower range is you don’t apply FLC percentages. And if the motor isn’t automatically started and isn’t permanently installed, you can provide overload protection via the short-circuit and ground-fault protective device (e.g., branch circuit breaker) [430.32(D)(2)].

« Part 11 | Part 13 » | Source: Mark Lamendola | Mindconnection

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