In a large electric system such as the national Grid, it is necessary to control the voltage because there is an inverse relationship between voltage and current. As electric usage rises, such as on a hot day when everybody’s air conditioner is running, the current draw on the system rises. This current rise causes the voltage to drop. The drop in voltage, if severe enough, can cause problems and even damage to electric equipment that requires a steady voltage source.
This inverse relationship, if uncorrected, can also feed upon itself. As the current rises, the voltage drops. If the voltage drops, the equipment drawing the current are forced to draw even more current which causes further voltage drop. And the vicious cycle can continue until damage is caused by excessive current draw. Conductors are sized by how much current they can handle.
Because of the problems this can cause, voltage regulation is used on the national Grid system. This is accomplished through the use of standalone voltage regulators or load tap changers that are incorporated into a power transformer. These devices will raise or lower the voltage each time it changes by 5/8 of a percent in order to maintain a steady voltage as current draw rises and falls.