Applying these principles to the 150-mile transmission, it is evident that an increase of the voltage to 60,000 will allow the weight of conductors to remain exactly where it was for the transmission of 100 miles, the rate of working and the line loss being equal for the two cases.
The only additional item of expense in the 150-mile transmission, on the basis of 60,000 volts, is the $35,000 for pole line. Allowing 15 per cent on the $35,000 to cover interest, depreciation, and maintenance, as before, makes a total yearly increase in the costs of transmission of $5,250 over that found for the transmission of 100 miles. This last sum amounts to 0.0175 cent per horse-power hour of the delivered energy.
The cost of transmission is thus raised to 0.307 + 0.0175 = 0.324 cent per horse-power hour of delivered energy on the 150-mile system with 60,000 volts.
Existing transmission lines not only illustrate the relations of the factors named above to the cost and weight of conductors, but also show marked variations of practice, corresponding to the opinions of different engineers. In order to bring out the facts on these points, the data of a number of transmission lines are here presented. On these lines the distance of transmission varies between 5 and 142 miles, the voltage from 5,000 to 50,000, and the maximum rate of work from a few hundred to some thousands of horse-power. For each transmission the single length and total weight of conductors, the voltage, and the capacity of the generating equipment that supplies the line is recorded. From these data the volts per mile of line, weight and cost of conductors per kilowatt capacity of generating equipment, and the weight of conductors per mile for each kilowatt of capacity in the generating equipment are calculated. In each case the length of line given is the distance from the generating to the receiving station. The capacity given for generating equipment in each case is that of the main dynamos, where their entire output goes to the transmission line in question, but where the dynamos supply energy for other purposes also, the rating of the transformers that feed only the particular transmission line is given as the capacity of generating equipment.