DIRECT FEEDING SYSTEMS. 63 
cuit if possible rather than shut down part of the system. 
The present tendency is to make the various sections of 
feeders and trolley wire separable rather than separate, so 
that they can be cut apart when absolutely necessary, but 
not long before that crisis. 
Long lines, interurban and thelike, may often be best 
treated indirectly through substations, but when direct 
feeding is employed, it is ordinarily best to use a very sub- 
stantial trolley wire, not smaller than No. oo, installed in 
separable but not disconnected sections, and supplied with 
current by separate feeders, which may be linked if local 
conditions require. If large power units are to be em- 
ployed, requiring large currents, it is better to use a very 
large trolley wire than to install a principal feeder, since 
with large currents the larger the contact surface of the 
working conductor the better, and the conductivity of the 
trolley wire can be relieved if insufficient by connecting 
each section to its feeder in several places instead of one. 
There is no reason however why, on large work such asis 
found in converting steam roads to electric, the working 
conductor may not have a cross section equivalent to No. 
0000 wire or more which enables comparatively long sections 
between feeders to be employed with advantage. For ex- 
ample, suppose a No. oooo trolley wire carrying a current 
of 200 amperes per section received equally from the two 
adjacent feeders. 'This condition would be met by a train 
requiring one hundred kilowatts to drive and located mid- 
way between two feeders. Allowing no more than two 
per cent loss, i. e., about ten volts in the trolley wire be- 
tween feeder junction and load and substituting the above 
oo ICEL , the 
distance between feeders should be about 4000 ft. Inas- 
much as the average drop produced by the moving train, 
with a maximum of two per cent midway between feeders, 
would be but one per cent, it would generally be advisable 
to increase this amount. Allowing an average drop of two 
per cent in the trolley wire, i. e., a maximum of four per 
values in the fundamental equation ¢. 7. =