Two-phase, five-wire distribution: Its engineering and economic elements

In view of the present trend toward three-phase secondary distribution involving, in some cases, a change from a two-phase system, an analysis of the engineering and economic elements of a two-phase system may be of value. The two-phase five-wire secondary system is examined in the light of fundamental requirements, such as service continuity, safety, standard voltages, flexibility, low cost, etc., and compared particularly with the three-phase, four-wire star system. Many of the advantages of the two-phase, five-wire system result from the diametrical connection of the two phases, from the inherent balance thereby obtained, and from the greater power carried per wire. An important advantage of the two-phase, five-wire system lies in the fact that single-phase, two-wire and three-wire loads and two-phase loads can be supplied at standard voltages from combination lighting and power secondaries and that new loads can be flexibly supplied through all stages of load growth. There are marked advantages from a construction and operating point of view in having ordinarily only two transformers in banks which supply two-phase secondaries from either two-phase or three-phase primaries. The two-phase, five-wire system has certain advantages as to metering and a comparison of the first cost and the annual cost of two-phase and three-phase motor installations, with wiring, shows small differences. The inherent cost differential between two- and three-phase secondaries with several types of primary systems is shown to be of such a small magnitude that the cost of change over from one type of system to another may over shadow the theoretical savings. Accordingly, with a relatively small inherent cost differential between the existent system and one having certain, more or less, proved advantages and disadvantages, the central station engineer must produce extremely strong arguments leading out of his local situation in order to justify a change from the existing system.