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Wednesday, December 20, 2017

Question:

Charlie, Can you shed some light on NEC 430.6C  I was always taught to use 125% of table 430.250 using VFD hp rating to size the circuit conductors to a VFD.  430.6C appears to give you 2 avenues, #1 use the maximum current rating on the VFD name plate or #2 150% of table 430.250 if the value does not appear on the name plate.  The code commentary does not address this code section. Any help is much appreciated.

Thanks Matthew Varvel

A

Answer:

Hey Matthew thanks for your question. The motor nameplate is used for ampacity determinations of AC adjustable voltage motors as stated in 430.6(C). If the maximum operating current is not on the motor nameplate then 150 % of the values in Table 430.249 and Table 430.250 are used.

If an adjustable speed drive (could be a VFD) is used then the rated input current of the drive is used to determine conductor ampacity as stated in 430.122 and 430.120.

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CHARLIE TROUT: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Even though Charlie passed away in October of 2015, his work continues in spirit. NECA continues to maintain this question forum for its many subscribers in memory and recognition of all his significant contributions to making the NEC what it is today.

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