SUBSTATIONS. 313 
At the 500 k.w. station the cost would be 1.8 cts. per kilo- 
watt hour. At 250 k.w. mean output the total yearly cost 
of power would be $32,850. Deriving the same yearly 
output from two 250 k.w. stations the cost would be 
$47,450, showing a yearly balance of $14,600 in favor of 
the single station, to offset the smaller cost of feeders and 
the greater efficiency of distribution possible with the two 
stations. It is, therefore, easy to get a rough idea of the 
relative cost of feeding a long line from a single central 
station and from a pair of stations symmetrically placed. 
Let A B (Fig. 65) be a line thirty miles in length and re- 
  
  
C 
ekl 
| b | 
S P : sl e B 
A D E 
F1c. 65. 
quiring a total average output of 250 k.w. with a capacity 
for 500 k.w. It is cheaper to feed it from a single station, 
C, at the middle of the line or from a pair of stations, D 
and E, each centrally located on a half of the main line? 
We may assume the current in either case to be 500 
amperes and the average drop fifty volts. With a single 
station, taking the average distance of transmission as half 
the extreme distance in either direction, the length of the 
transmission would be about 40,000 ft. 
Reverting now to our weight formula and writing 
3 X 14 = 42 as the constant we have 
4201, 
a 
And applying this formula to our data we find that for 
feeder copper for the given loss there would be required 
336 tons of wire and cable costing in the neighborhood of 
$94,000. On the other hand if two generating stations at 
D and E are employed the average distance of transmission 
would fall to about 20,000 ft., and since the weight of 
copper required varies directly as the square of the dis- 
tance, there will be required for the new state of things 
eighty-four tons of feeder copper costing about $23,500. 
W