Line voltage may vary widely at an installation, especially if local or total line loading varies widely, which it often does. A certain range of such variation is acceptable, but wider variations can cause trouble. This issue discusses the problems that can be caused by improper voltage and the steps that can be taken to correct them.
Power companies in the United States commonly supply line voltages of 120, 208, 240 and 480 volts. (Power companies vary in how much they allow variation from these nominal voltages, but in general they hold to a range of plus or minus two or three percent.) On the other hand, motor nameplate voltages are 115, 200, 230 and 460 – the voltages at which motors are generally designed to perform best. The reason for these differences between rated line voltages and rated equipment voltages is to allow for a three to five percent voltage drop in the lines from the transformer to the equipment (e.g. from 240 to 230 volts, from 480 to 460 volts). Continue reading