Engineering and economic features of distribution systems supplying increasing load densities

A. H. Kehoe (communicated after adjournment): The formulas submitted to obtain the economical design of transformer spacing, transformer size and secondary conductor size, are based on a uniform density of load and a reasonable number of service points. These conditions are seldom even approximated in practise. The calculations assumed that the location of the transformers is such that the center of distribution of the particular load will be chosen. The fact usually ignored in such calculations is that any concentrated load equal to more than one-half of all the load being considered causes the electrical center of distribution to occur at the point of concentrated load. In practical distribution the majority of transformer locations will be at the concentrated loads and such an arrangement makes it uneconomical to space many of the distribution transformers at regular junction points on the mains. However, it allows a larger average size of distribution transformer and a smaller secondary conductor to be used than is possible with the same amount of load distributed uniformly over the territory. This condition also requires several different sizes of transformers to be used on the same system. It is fortunate that any proper secondary distribution-system costs for varying loads gives a flat “U” curve, as this, with concentrated loads mentioned above, makes it possible to use a small conductor and yet be able to add large future loads as they occur without having to change the mains.