National Electrical Installation Standards

Standards as High as Your Own

Monday, August 21, 2023


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


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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