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Protecting Motor Feeder Conductors from Short Circuits, Part 2

Protecting Motor Feeder Conductors from Short Circuits, Part 2In Part 1, we broke down the long sentence in the first paragraph of 430.62(A) to make it comprehensible. But there’s a second sentence and there are two exceptions. The requirements also continue in 430.62(B) and 430.63.

What else must you contend with when sizing the OCPD that protects motor feeders?

The second sentence of 430.62(A) says to consider one of the OCPDs to be the largest, even if two or more are the same rating or setting, when making the calculation required by the first sentence.

Now, what about those exceptions?

  • Exception 1 applies an additional provision if you’re using an instantaneous trip breaker or motor short-circuit protectors.
  • Exception 2 requires you to apply 430.94 if the feeder OCPD also protects a motor control center (MCC).

It’s worthwhile to review Informative Annex D, Example D8.

You can base the rating or setting of the feeder OCPD on the ampacity of the feeder conductors, if the ampacity of those conductors exceeds what’s required by 430.24 [430.62(B)].

What if a feeder supplies other loads, not just the motor load? In that case, the feeder OCPD must have a rating of at least the sum of the other loads plus 430.63] the rating permitted by:

  • 430.52 (single motor).
  • 440.22 (single hermetic motor).
  • 430.62 (two or more motors).

The exception to the above applies to feeders that supply an MCC. In that case, follow the requirements of 430.94.

« Part 1 | Source: Mark Lamendola | Mindconnection