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Protecting Motor Feeder Conductors from Short Circuits, Part 1

Protecting Motor Feeder Conductors from Short Circuits, Part 1You protect motors from overloads due to excessive motor loads (e.g., a scrap grinder gets too much scrap fed into the intake chute all at once) by the use of motor overload protection devices such as thermal strips in the motor starter.

But to protect the supply conductors from overcurrents caused by short circuits or grounds, you use an entirely different method. You use an overcurrent protective device (OCPD) in the supply circuit. The sizing requirement for the OCPD that protects motor feeders consists of two paragraphs in 430.62(A). The first paragraph consists of a single sentence that runs on for nearly 100 words.

It should come as no surprise that people have a very hard time wrapping their heads around this. The first step to solving this puzzle is to tease out from this dense text the core requirement. Begin by summing up:

  1. The largest rating or setting of the branch circuit OCPD for any motor supplied by that feeder.
  2. The sum of the full-load currents of the other motors on that feeder.

This OCPD can’t exceed this sum.

A qualifier to this appears mid-sentence, in brackets. The largest setting/rating is based on the maximum permitted value for the specific type of OCPD. Refer to 440.22(A) for hermetic motors, or 430.52 for all other motors.

And the requirement begins by saying this is for motor loads that are fixed and consist of conductor sizes based on 430.24.

Part 2 » | Source: Mark Lamendola | Mindconnection