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Monday, August 21, 2023

Question:

Good morning, Code Friends. The Informational Note under the definition of “Nonlinear Loads” lists: Electronic/Electric-Discharge Lighting, Adjustable-Speed Drive Systems, and similar equipment may be nonlinear loads.

My question is, what portion of an ASD (or VFD) system should be considered nonlinear? According to 310.15(E) we may have to consider the neutral a current-carry conductor if this is the case... Best, Chris
A

Answer:

Hello Chris, thanks for participating in the CQD. The NEC® does not specify an exact value or percentage when considering nonlinear loads and neutral current as it applies to conductor ampacity adjustments. Section 310.15(E)(3) for 3-phase, 4-wire wye circuits states: “where a major portion of the load consists of nonlinear loads” which is subjective, leaving it in up to the installer/contractor to determine if there is a cause of concern when it comes to harmonic distortion, neutral currents and ampacity adjustments. There are diagnostic test instruments capable of performing sophisticated power measurements including harmonic distortion caused by nonlinear loads. Performing this analysis on a circuit or feeder would be one way of supporting your installation method to the AHJ.

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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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IMPORTANT NOTICE: Unless the question requests a response based on a specific edition, all answers are based on the latest edition of NFPA 70® National Electrical Code®.

This correspondence is not a formal interpretation of the NEC® and any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, the NEC Correlating Committee any Code-making panel or other electrical technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services. 

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