Each rated voltage has an optimum power range, the lower and upper limits of which should not be exceeded if a favourable construction is to be produced. If the voltage is excessively high with respect to the power, the conductors become rather thin, and the number of turns increases drastically. In the opposite case, the winding has to be made of preformed copper, which increases the winding costs. For example 630kW is typically the limit for a 400V machine. The rated current of such a machine is about 1080 A, which is a value that can be considered a practical maximum for the rated current of a direct-on-line (DOL) induction motor. The starting current of such a machine may reach even 10 kA. According to the IEC 60034-1 standard, the minimum rated output is 100kW for a rated voltage UN =
1–3 kV, 150kW for UN = 3–6 kV, and 800kW for UN = 6–11 kV.
The available voltages are 460, 575, 2300, 4000, 4600, 6600 and 13200 volts. Since it is not possible to design all sizes of motors at all voltages and all speeds, NEMA identifies horsepower ranges available at various voltages, as well as speeds for all sizes. A table below gives horsepower assignments, and available speeds.