"Normal operation" describes the full (or rated) load conditions of each system component. For motors, it includes the amps initially drawn at start-up, i.e. inrush current, as well as the full- or rated-load amps drawn while running. The magnitude of inrush current for a particular application depends on the type of starter used (e.g. wye-delta). Normal operating conditions determine wire and transformer sizing. They're also used in conjunction with "fault conditions" to select overcurrent protection devices such as circuit breakers and fuses. Rating factors are applied, based on the type and number of connected loads, to assure that the devices selected adequately protect the motor as it starts and while it's running.

Let's look at an example. Suppose a 500-ton chiller has a 480-volt motor that draws 400 amps at rated load conditions. The electrical distribution system includes a wye-delta starter powered by a 1,500-kVA transformer. Operating "normally," the chiller motor draws about 800 amps during the 4 seconds it takes to start; then 400 amps or less at running speed.

The size of the interconnecting wires between the transformer and starter reflects the type and rated amperage draw of the load, i.e. the chiller motor. Sizing the wires on this basis assures that they can carry the inrush current at start-up without overheating.

Continue on to Overload Operation, n.