The two most common secondary (point of use) three-phase voltages for industrial plants are 240 Volts and 480 Volts. A majority of smaller industries and plants with small or medium size motor loads can use either of these systems.
This graph shows the relative economics of selecting a 480-Volt system over a 240-Volt system to handle the same total load. While the difference in dollars saved cannot be taken directly from these comparison charts, the relative proportions are shown here. For these two system voltages, the costs for the motors will be the same since these motors usually have windings which can be reconnected at the terminal box for either 460 Volts or 230 Volts. Unit substations cost nearly the same, with a small advantage favoring the higher voltage because of the smaller conductors and hardware used in their construction. Motor starters and supply system wiring are rated on a basis of current and are generally suitable for use at either voltage. This definitely favors the 480-Volt supply system. In the larger NEMA sizes, a given 460-Volt starter can control a motor with twice the horsepower rating at 460 Volts as it can at 230 Volts.
On this basis, the 480-Volt system is more economical. A 600-Volt system might show greater savings, but 550- or 575-Volt equipment is not as readily available as the 460-Volt equipment. That is why it is only used in certain industries where the 600-Volt systems are a tradition.
