286 ELEMENTS OF ELECTRICAL ENGINEERING. 



where R is the resistance of the two wires in ohms, W is the 

 weight in pounds of the two copper wires required to deliver P 

 watts of power to a receiving unit at E volts, p is the percentage 

 drop in the wires (actual drop pEj 100), and / is the distance in 

 feet from the center of distribution to the lamps. The constant 

 in equation (43) applies only to copper wires. 



Inasmuch as the weight of the copper is inversely propor- 

 tional to E 2 , according to equation (43), it is evident that a very 

 great saving in copper may be effected by using a high voltage 

 at the lamps. The permissible voltage at the lamps is limited, 

 however, (a) by the fact that incandescent lamps cannot be made 

 to operate satisfactorily at voltages higher than about 220 volts, 

 and (b) by the danger that is involved in the use of high voltages. 



The saving in copper by the use of high voltage, combined 

 with the practical necessity of low-voltage delivery, has led to the 

 use of the Edison three-wire system as explained in Art. 112. 

 In the alternating-current system of distribution power can be 

 transmitted at any desired high voltage and cheaply and efficiently 

 transformed near the place of consumption to any desired low 

 voltage. Therefore the alternating-current system permits of 

 very great economy of copper in the transmission lines and does 

 not involve any of the difficulties or dangers incident to the utili- 

 zation of high voltages at lamps and motors. 



When che voltage-drop in a transmission line is not limited 

 by the necessity of maintaining an approximately constant voltage 

 at the lamps, or other receiving units, the size of wire should be 

 determined on the basis of economic considerations as explained 

 in Art. 1 20 ; and it is to be particularly noted that the weight of 

 copper, demanded by economic considerations, for the delivery 

 of a given amount of power is not proportional to ^/E 2 but to 

 IjE. 



118. Wiring calculations inconstant-voltage systems fora motor 

 or for a concentrated group of lamps. Two important cases arise 

 in the laying out of wiring plans in a constant-voltage system, 



