ELEMENTS OF ELECTRICAL ENGINEERING. 



is, if current is to be supplied to the customer at 1 1 o volts, the 

 size of wire required on the basis of a 5 per cent, drop of voltage 

 is the same as the size of wire required to give an economic 

 balance between the loss of power and the cost of copper under 

 the specified conditions. If the distance is greater than 538 feet, 

 then considerations of economy would give a smaller wire than 

 would be required by a 5 per cent, drop in voltage ; and, if the 

 distance is less than 538 feet, then considerations of economy 

 would give a larger wire than would be required by a 5 per cent, 

 drop in voltage. 



Example 2. Cost of power, rate of interest and cost of cop- 

 per being the same as in example I, it is required to find the 

 most economical size of wire for carrying 100 amperes for 400 

 hours each year and 300 amperes for 600 hours. The average 

 square of the current is 



(ioo 2 x 400) 4- (300 2 x 600) 



-^= 58,000 amperes 2 

 400 + 600 



and the square-root-of-average-square is 241 amperes. There- 

 fore using h = 400 4- 600 hours and I 241 amperes in equa- 

 tion (46), we have s 509,200 circular mils, or the diameter of 

 the wire must be 7 1 3 mils. 



Dependence of total weight of copper on voltage of delivery and 

 distance of customer from station. A given amount of power, P t 

 is to be delivered to a customer at a distance, /, from the station 

 and at a voltage, E. The current is PIE so that from equation 

 (46) we have : 



P 



which, substituted in the formula W= 0.00000303 x 2,1s, gives : 



W= 0.003618 P-^^ (47) 



which shows that the amount of copper required by economic 



