DIRECT FEEDING SYSTEMS. i 
average load is no easy matter, yet upon it depends the 
proper design of the conducting system. It is not diffi- 
cult to estimate with a fair degree of accuracy the actual 
power which must be supplied to drive a car of assumed 
weight over a certain line at a given speed. But what the 
real weight of the loaded car will be, and what the condi- 
tion of the line will be is a case at best for educated guess- 
ing. Roughly speaking the power required at the car 
wheel for a speed of eight miles per hour is .4 h. p. per 
ton, plus .4 h. p. per ton for each per cent of grade. More 
exactly 
P—=W.(435 45 C) 
Wherein G is the per cent grade, W the weight of car and 
contents in tons and P the total horse power. Thisassumesa 
straight track and a tractive effort of twenty pounds per ton 
on the level. But there are alwayssome curves, the speed is 
often above eight miles per hour ana at low speeds the 
motors are somewhat less efficient than at high speeds. Al- 
lowing a complete efficiency of two-thirds from trolley to 
car wheel and assuming a pressure at the car of about 500 
volts we shall not go far wrong in reckoning 11{ amperes 
per ton of car plus 174 amperes per ton for each per cent of 
grade. 'This average indicates an average of about fifteen 
amperes per car. ‘The average current taken while the 
car is under full headway will frequently exceed this 
amount, but an allowance of fifteen amperes average 
throughout the hours of running will generally be nearly 
right for a road such as that under consideration. With 
long double truck cars the average current will rise to 
about twenty-five amperes. 
Now the maximum current must be considered. On 
large systems it may be no more than twice the average. 
Very often it will be three times the average on such roads 
as we are discussing. As the number of cars becomes 
smaller this ratio increases. With one or two cars it is nc 
uncommon thing to find maximum currents of four or five 
times the average. Still larger ratios would be common if 
the same speed were maintained on grades as on the level