[Illustration: FIG. 65.--Area of Electrical Distribution at Montreal.]
In some instances, however, transmission and distribution lines are joined without the intervention of a sub-station, where regulation of voltage can be accomplished, though this practice has little to recommend it aside from the savings in first cost of installation and subsequent cost of operation. These savings are more apparent than real if fairly constant pressure is to be maintained at the lamps, because what is gained by the omission of sub-stations will be offset, in part at least, by additional outlays on the lines if good regulation is to be maintained. This fact may be illustrated by reference to Figs. 66, 67, and 68, in each of which _D_ represents a generating station and _A_, _B_, and _C_ towns or cities where energy from the station is to be distributed. In the case of each figure it is assumed that the distance between the generating station and each of the cities or towns is such that distributing lines with a loss of, say, not more than two per cent in voltage at full load cannot be provided between the generating station and each city or town because of the cost of conductors. This being so, one or more centres of distribution must be located in each town, and the transmission lines must join the distribution lines at these centres either on poles or in sub-stations. If several of these towns are in the same general direction from the generating plant so as to be reached by the same transmission line, as _A_, _B_, and _C_ in Fig. 66, this one line will be all that is necessary with a sub-station in each town. Where sub-stations are not employed a separate transmission circuit must be provided between the generating plant and each town for reasons that will appear presently. The percentage of voltage variation in a transmission line under changing loads will be frequently from five to ten, and is thus far beyond the allowable variations at incandescent lamps. To give good lighting service the centre of distribution, where the transmission line joins the distribution circuits, must be maintained at very nearly constant voltage if no sub-station is located there. Regulation at a generating station will compensate for the changing loss of pressure in a line under varying loads so as to maintain a nearly constant voltage at any one point thereon. No plan of station regulation, however, can maintain constant voltages at several points on the same transmission line when there is a varying load at each. The result is that even though the several towns served are in the same general direction from the generating station, as in Fig. 67, yet each town should have its separate transmission line where no sub-stations in the towns are provided. In the case illustrated by Fig. 68, where the towns served are in very different directions from the generating station, there should be a separate transmission line to each, regardless of whether there is a sub-station or only a centre of distribution there.