22 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
copper, for a large part of the load is liable at any time to 
be concentrated on almost any part of the network. 
T'wo conditions must at all events be fulfilled. First, 
each one of the lines, A D, A E, etc., must be able to 
carry its own proper load without exceeding a standard 
drop; and, second, the sum of the distributed loads must be 
carried at certain points, which can be approximately pre- 
judged, without exceeding a certain maximum drop. 
Tt must be noted that the conducting system of a rail- 
way differs from that of a lighting plant in having a much 
greater proportion of feeders to mains. In fact the working 
  
BIG.  I4. 
conductor of a railway is generally of quite limited carrying 
capacity. Practically, in laying out a network like that of 
Fig. 14, one'has to cut loose from lighting precedents and 
deal with a special problem. 
Following the first of the conditions just named, a 
convenient first step is to compute the conductors as iso- 
lated lines, on the assumption that Z,,Z,, 75, etc., are the 
currents due to the normal load on each line. This fur- 
nishes the skeleton, as it were, of the conducting system. 
‘This work can often be simplified by bearing in mind the 
main lines of traffic and treating as one their component 
conductors. For instance, in Fig. 14, if A be the station