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Hermetic Motors, Part 2

Hermetic-Motors,-Part-2Hermetic motors have nameplate requirements beyond those applicable to other motors [440.4]. One of the things the nameplate must provide is the rated-load current (RLC). It can, at the option of the manufacturer, be marked on the equipment the motor is in rather than the motor nameplate.

This RLC value is significant, because you must use it to determine the rating or ampacity of the disconnecting means, branch circuit conductors, controller, branch-circuit short-circuit and ground fault protection, and the separate motor overload protection [440.6(A)].

Contrast this to working with non-hermetic motors [430]. With these, you use the :

  • Full Load Amps (FLA) from the motor nameplate to size separate overload protection (motor and branch-circuit)  per Part III.
  • Full Load Current (FLC) from NEC tables to size the motor branch-circuit short-circuit and ground fault protection per Part IV.

RLC is the current resulting when the motor-compressor is operated at the rated load, voltage, and frequency of the equipment it serves [440.2]. The hermetic motor, remember, is part of the equipment. And the hermetic motor, being submersed in the refrigerant, has different cooling characteristics than a non-hermetic motor.

These differences are why you can’t you just use FLA and FLC for hermetic motors.

There are two exceptions to using the RLC:

  1. The equipment may already be marked with the branch-circuit selection current; use that to determine the required ratings or ampacities.
  2. If the equipment is cord-and-plug connected, use the nameplate marking.

« Part 1 | Part 3 » | Source: Mark Lamendola | Mindconnection