GENERAL CONSIDERATIONS 273 



Distribution. Voltage The selection of the proper distributing 

 voltage is also an important matter. Where large territories 

 have to be served from high voltage transmission circuits, the 

 general practice seems to indicate that the most economical volt- 

 ages for such systems are in the neighborhood of from 22,000 to 

 33,000 volts. A second or even third transformation is, therefore, 

 necessary before the power can be used for motors or lighting. 



The distribution of alternating current for general commercial 

 purposes is accomplished almost universally by 2300 volt mains, 

 supplying step-down transformers located near groups of con- 

 sumers, whose premises are served by secondary mains at 115 to 

 230 volts. Single-phase circuits are quite generally used for 

 lighting service, while power service is, as a rule, given from two- 

 phase or three-phase mains. The former system is used chiefly 

 where this method of distribution was established in the early 

 period of the development, and where it is too extensive to warrant 

 a change to the three-phase system, which is standard for all new 

 installations where a polyphase supply is wanted for power service. 



For small- and medium-sized cities a three- wire, " delta "- 

 connected, 2300-volt system is very generally used for power dis- 

 tribution, while for larger cities there is a steady trend toward the 

 four-wire, " Y "-connected system operating at 2300-4000 volts. 

 There are numerous advantages with this system where feeders 

 are extended more than two miles from the point of supply, and 

 where adjacent towns within a radius of five miles may be served 

 without step-up transformers or substations. It is possible to 

 regulate the phases separately, and there is not so much of a 

 necessity for maintaining a carefully balanced load. Even for 

 secondary distribution the four- wire, three-phase system, oper- 

 ating at approximately 115-200 volts, is being generally used. 

 With this system lighting and motor service may be given for all 

 ordinary retail purposes from the same circuit, the principal dis- 

 advantages being that there are three phases to be kept balanced. 



Frequency. The subject of frequency for commercial power 

 and lighting systems, far from being settled, is discussed again with 

 every new installation. Frequency affects the operating charac- 

 teristics of circuits and apparatus, and also their cost. 



The frequencies most commonly employed in this country 

 are 25 and 60. In general it may be said that, where lighting 

 load is predominating, 60 cycles should preferably be selected; 



