Fig. 2,180.—Single phase line, used as basis of comparison in obtaining the relative weights of copper required by polyphase systems, as indicated in figs. 2,181 to 2,188.
In the case represented in fig. 2,180, if the total drop on the line be 100 volts, the generated voltage must be 1,100 volts, and the resistance of each line must be 50 ÷ 1,000 = .05 ohms. Calculated on this basis, a two phase four wire system is equivalent to two single phase systems and gives no economy of copper in power transmission over the ordinary single phase two wire system. This is the case also with any of the other two phase systems, except the two phase three wire system.
Figs. 2,181 to 2,188.—Circuit diagrams showing relative copper economy of various alternating current systems.
In this system two of the four wires of the four wire two phase system are replaced by one of full cross section.
The amount of copper required, when compared with the single phase system, will differ considerably according as the comparison is based on the highest voltage permissible for any given distribution, or on the minimum voltage for low pressure service.
If E be the greatest voltage that can be used on account of the insulation strain, or for any other reason, the pressure between the other conductors of the two phase three wire system must be reduced to E ÷ √2.
The weight of copper required under this condition is 145.7% that of the single phase copper.
On the basis of minimum voltage, the relative amount of copper required is 72.9% that of the single phase system.