Most of the 208Y/120-volt system capacity in the U. S. is found in underground secondary networks supplying high-density commercial areas of cities. The use of this type of distribution system is very extensive and probably will continue to be so for many years to come. Some utilities also provide 208Y/120-volt service to large isolated commercial loads outside of secondary network areas, such as shopping centers and schools.

The first automatic a-c network system as we are familiar with it today was put into operation in New York City in 1922 more than 50 yr ago. At that time the voltage level was 199Y/115-volts. The network system had its inception under circumstances requiring highly dependable electric service, flexibility, and economy to serve a mixture of power and lighting loads. It was natural that its earliest applications were in the business sections of large cities supplanting the 3-wire, Edison d-c networks which had become unwieldy and uneconomical. The increased use of the secondary network was rapid. Today it is used in about 90% of the cities in the country having a population of 100,000 or more and in about 1/3 of all cities with a population between 25,000 and 100,000.

A study of some of the AlEE papers presented in 1924 reveals that a basic part of the philosophy in selecting the four-wire, 199Y/115-volt system was to permit the supply of service to standard single-phase and three-phase utilization equipment available at that time. Principal load components were 115-volt incandescent lighting and 220-volt, three-phase motors. The electrical "stiffness" of the network system resulted in generally acceptable performance of the 220-volt motors, whereas this would not necessarily be true for 199Y/115-volt radial systems.

This paper reviews the changes in voltage levels since the early 1920s as relates to standard C84.1 which was expected to be dated in 1976.