National Electrical Installation Standards

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Friday, November 17, 2017

Question:

Example: A group of 3 vfd's of the same rating 1 feeder/branch ckt to supply all 3 How is ampacity determined? Assume pu vfd input current = 1 430.24: 1.25 x 1 + 1 + 1 = 3.25 or 430.122: 1.25 x (1 + 1 + 1) = 3.75

Thank you, Arthur Brower

A

Answer:

Hey Arthur thanks for your question. Parts I through IX of Article 430 apply unless something in Part X changes those rules. Your first example is correct, but only if the input current rating is 1 ampere.

Conductors supplying three adjustable speed drives would be sized at 125% of the largest plus 100% of the other two as stated in 430.24 using the drives rated input current as stated in 430.122(A). So if the adjustable speed drives are all the same size and have input current rating of 10 the minimum feeder conductor ampacity would be 32.5 amperes [10 x 1.25 = 12.5 + 10 + 10 = 32.5]

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