Utility Voltage Levels
Anyone receiving power from an electric utility will see the nominal incoming voltage level (e.g. 120V) change over the course of a day to a small or large degree. There are many factors contributing to the amount of voltage level fluctuation observed including: 1) location on the local distribution line, 2) proximity to large electricity consumers, 3) proximity to utility voltage regulating equipment, 4) seasonal variations in overall system voltage levels, 5) load factor on local transmission and distribution system, etc.
Voltage levels are often highest during the nighttime hours and weekends when the electrical demand is minimal and are lowest weekday afternoons when the demand for electricity peaks. Most electric utilities in the U.S. try to maintain the voltage level within plus or minus 5% of the nominal voltage level (e.g. 480V +/-5%), however for short periods the voltage level may be as much as 6% high or 13% low (according to ANSI Standard C84.1). On the nominal 480V system, this would translate to incoming voltage ranging from 509V (480V +6%) to 420V (480V-13%). Larger deviations from nominal voltage are also permissible on a momentary basis or may simply be unavoidable.