I scanned the entire 2017 and 2020 NEC and nothing for voltage percentage.(Is this in reference to a specific NEC article? which one?)
NEC values are based on nominal voltages, subject to the industry standard +/- tolerances.Equipment voltage ratings are absolute numbers with no adjustments.
What I mean is, for example, we're never required to exceed nominal voltages by a set percentage.NEC values are based on nominal voltages, subject to the industry standard +/- tolerances.