National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, October 24, 2017

Question:

When doing a dwelling unit calculation, article 220.50 for motors will send you to 430.24, 430.25, 430.26. with these article you are to incorporate 25% of the largest motor. I have found different code calculation practice strategies in which some say if the A/C is omitted per 220.60 you would not use the A/C as the largest motor. Usually the A/C is the largest motor in a house calculation, could you tell me which would be the correct way to calculate this and where to find it in the code if you are to not use the A/C if omitted from the calculation. The way I read it the A/C motor is still installed on the service feeder so it should be able to be used as the largest motor.

Thank you, Markco

A

Answer:

Hey Markco thanks for your question. Those sections are not very well correlated and there likely are some inconsistent practices being done. If the AC is not the larger load, and is omitted as stated in 220.60, it is somewhat logical that it does not get used to apply the 125% as the largest motor. There is no wording that says you can omit it for one calculation but need to apply it in another. Keep in mind that the 125 % will be included in the branch circuit calculation as 220.60 only applies to the total load of a feeder or service.

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