Before converting or modernizing a system from 240 Volts to 480 Volts to reduce equipment and wiring costs, take a careful look at all of your existing equipment and your planned electrical growth. This study may show that some parts are obsolete or inefficient and should be discarded. It may be most practical to make a complete changeover to the higher voltage. On the other hand, you may discover that some devices are suitable for continued use if served by appropriate step-down transformers. These devices can be replaced much later by 480-Volt units in the normal course of plant operations.
Most motors of recent manufacture can be reconnected at their terminals to operate at the higher voltage. Across-the-line motor starters, if rated for operation on a 480-Volt system, can be converted from 240 Volts by replacing the overload relay heaters by appropriate units rated for the lower motor current at the higher voltage. The operating coil can be replaced by another rated for the new voltage or the machine tool transformer in the controller reconnected or replaced. However, reduced-voltage starters require more attention. These require new auto transformer coils, resistors, or reactors (as appropriate). Part-winding and wye-delta reduced-voltage starters and motors will have to be replaced.
Likewise, any motor which is overloaded or inefficient or cannot be reconnected should be replaced. A motor which has been found to be underloaded might also be replaced because it is helping to lower the plant power factor and increasing the demand and system losses.
If the lighting system operates at 120 Volts, a new transformer for lighting circuits can be used to step the voltage down from the 480-Volt feeder. Often, this can be done by using a dry-type transformer at the input to the lighting panel.
Where resistance heating loads are involved, it may be possible to reconnect the 240-Volt resistance elements in series across the 480-Volt lines. It is necessary to check with the manufacturer to determine if the resistance elements must be insulated from the equipment on which they are mounted (secondary level of insulation) before being used at 480 Volts. Otherwise, step-down transformers will be required.
If circuit breakers and switches are rated for use at 480 Volts (as is often the case), they can continue to be used after the voltage changeover. If they are not rated for 480 Volts, they must be abandoned. Conductors, if insulated for 480 Volts (which is also very likely), can carry a larger kVA load than before, or operate with the original load with reduced losses and better voltage regulation. The load can be expanded to twice the original kVA and with only one-half of the original percentage voltage drop.
In fact, this reduction in the percentage voltage drop is a major reason for making the change to 480 Volts. The circuit carrying kVA capacity doubling gives strong economic incentive if the plant is expanding. With the doubled kVA load possible at the 480 Volt level, the current is what it was before, so the percentage voltage drop in the wires is just one-half as much as it originally was. However, this consideration of the voltage drop is limited to the wires themselves and not to the transformer. If a new transformer with twice the original kVA rating but the same percent impedance is used to supply the new load, the voltage drop in the transformer is the same as with the smaller load at the lower voltage. However, the overall voltage regulation is better.
