Fig. 2,189.—Twin circuit "aermotor" towers carrying 44,000 volt conductors (Southern Power Co.). These towers vary in height from 35 to 50 feet, and the circuits are transposed every 10 miles. The towers are assembled on the ground and erected by means of gin poles. They are normally spaced 500 feet apart with a sag of 5 feet 8 inches. The minimum distance between towers is 300 feet and the maximum 700 feet.
Figs. 2,187 and 2,188 are two examples of three phase four wire systems. The relative amount of copper required as compared with the single phase system depends on the cross section of the fourth wire. The arrangement shown in fig. 2,188, where the fourth wire is only half size, is used only for secondary distribution systems.
Fig. 2,190.—General Electric standard tower for high tension three phase transmission line.
Fig. 2,191.—General Electric transposition tower for high tension three phase transmission line.
Choice of Voltage.—In order to properly determine the voltage for a transmission system there are a number of conditions which must be considered in order that the economy of the entire installation shall be a maximum.
The nature of the diversely various factors which affect the problem makes a mathematical expression difficult and unsatisfactory.