s 
112 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
supplied with first class station equipment. Plain, substan- 
tial, brick power house and stack. Coal $3.00 per long ton 
delivered in coal bins. Interest and depreciation are 
grouped together at ten per cent per annum. 
Taking into account labor at ordinary rates, and 
assuming a service of from eighteen to twenty hours per 
day, one can compute the cost of power with tolerable 
exactness. ‘The results are shown in Fig. 64. Three 
curves are given showing the cost per kilowatt hour for 
average outputs of fifty, sixty and seventy per cent of the 
total nominal working capacity. Ordinary care is sup- 
posed to be exercised in keeping unnecessary machines out 
of service. 
These results are higher than those often claimed, but 
they check quite closely with several independent estimates 
made by different engineers and also with results obtained 
from actual practice in modern plants, making the neces- 
sary corrections for cost of fuel, etc. 
A casual inspection of Fig. 64 shows several important 
facts very plainly. First, under 500 k.w. capacity the cost 
of power per kilowatt increases very rapidly as the station 
decreases in size. For example, following curve a, the 
cost per kilowatt hour in a station of 500 k.w. is 1.8 cts., 
rising to 2.6 cts. in a 250 k.w. station. 
Second, above 1000 k.w. output the cost decreases quite 
slowly with the output, and above 2000 k.w. it would be- 
come almost uniform. 
Hence, generally, when a plant is of such magnitude 
that sub or auxiliary stations of 1000 k.w. or more can be 
employed, the cost per kilowatt hour at such stations will 
vary little from the cost at the main station and most of 
the saving in feeder copper will be pure gain. On the 
other hand if substations need but two or three hundred 
kilowatt capacity they can be operated only at a cost suffi- 
cient to balance a large expenditure for copper. To take 
a concrete case, compare the cost of power from a 500 k.w. 
station with that from two 250 k.w. substations, as given 
by curve a, assuming the day’s run to be twenty hours.