72 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
W == 
Remembering that we are considering feed wire alone, since 
the trolley wire is fixed in location, we may assume a reason- 
able drop in voltage of, say, thirty volts. K above then 
becomes 3 
Forming the above summation we have at twenty am- 
peres per car, 
2W = (10 96 X 4895 
J-6.X-20 X. 25 
- 40X'20.X 0.) £8=—62871bs, 
Now at fifteen cents per pound this feeder copper would 
cost just about $1ooo. For any other point than e the 
cost will be greater by varying amounts and the increase 
is about the same for all points equidistant from e. As the 
weight of copper varies with the squares of the distances, 
the mean distance of the load with respect to weight of 
copper is determined by 
L2 C=21%2c=6170 where C== ¢= 400 
Hence I, = 3950 ft. nearly. 'T'his distance is the radius of 
the circle about which the station can be shifted without 
more than doubling the cost of copper noted above. 
That is, the station can be located anywhere within about 
three-quarters of a mile of the center of gravity of the 
system without increasing the cost of copper more than 
$1000. Such figures are necessarily approximate only, 
since in practice wires cannot be run in straight lines, but 
have to follow the streets, nevertheless they give valuable 
information. 
A brief examination of proposed sites for the power 
house will generally disclose that which is most advanta- 
geous with respect to coal and water, and a quick summa- 
tion as above will tell quite nearly whether the extra cop- 
per will cost too much or not. In the case before us we 
will assume the point / (Fig. 46) as best meeting all the re- 
quirements. As the distance ¢ ¢! is small compared with 
the displacement of /, we can let the extension question 
take care of itself and are ready to proceed to 
Step 4. The predetermination of the maximum or