I'd like to know if people are utilizing 310.15 (B)(5)(a) or 310.15 (B)(5)(c) when sizing feeders for panelboards. The example below would be a 480V 3 phase 4 wire feeder feeding a 400A panelboard from the main building switchboard.

As an example, we've utilized #500 KCMIL for a 400A panelboard for years. Utilizing the 310.15(B)(16) chart the #500s are good for 380A in the 75°C column. I'm using the 75° column as that is what the panelboards are rated for. We then utilize 240.4(B) to upsize the breaker to a 400A. In the past we've always viewed the load on the neutral as carrying the unbalanced load which would fall under 310.15 (B)(5)(a) and not count as a current carrying conductor.

In today's world when you read the 310.15 (B)(5)(c) in regards to nonlinear loads, I question shouldn't the majority of loads fall into this category. In a typical office building the majority of the receptacle loads are going to be nonlinear loads. LED lighting is going to be a nonlinear loads, almost every mechanical motor is going to have a VFD (even though they don't typically have a neutral). From what I've been able to research in regards to 310.15 (B)(5)(c), it was established during a time that there were minimal regulations regarding harmonics. Since this time though, industry standards have been established minimizing if not eliminating harmonics from nonlinear loads. The worst offender on our projects is lighting which typically has a THD of 20% only results in less than 20A on a 400A panel with a conductor good for 380A.

All reference's are based on NEC 2017.