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Circuit Protection for Hermetic Motors

Circuit-Protection-for-Hermetic-MotorsPart III of Article 440 provides the requirements for branch-circuit short-circuit and ground-fault protection. These requirements are intended to be used as a companion to the requirements in Article 240. That means your design basis will be Article 240, but you will refer to Article 440 for amendments and additions to the Article 240 requirements.

Remember, this is a motor circuit so the job of the overcurrent protection device is split. There’s a third function not covered in Part III; overload protection is covered in Part VI of Article 440. In non-motor circuits it’s common to refer to the fuse or breaker as the overcurrent protection device (OCPD). Using that same nomenclature with the branch circuit protective device (BCPD) of motor circuits can lead to confusion. Thus, the distinction between OCPD and BCPD.

For hermetic motors, the BCPD can have a rating or setting not exceeding 175% of the motor compressor rate-load current or the branch-circuit selection current (whichever is greater) [440.22(A)].

The BCPD must be capable of carrying the starting current of the equipment [440.22(B)]. So if aforementioned limit of 175% isn’t sufficient for the motor’s starting current you can increase the rating (or setting) of the BCPD to 225% of the motor compressor rate-load current or the branch-circuit selection current (whichever is greater) [440.22(A)].

The BCPD rating cannot exceed the manufacturer’s values marked on the equipment [440.22(C)].

Source: Mark Lamendola | Mindconnection