288 ELEMENTS OF ELECTRICAL ENGINEERING. 



In this case the sectional area of the wire in circular mils is given 

 by the formula : 



_72 i -i 21.6/7 



a z = circular mils = - (44)^ 



in which e is the drop expressed in volts, and d, I and / have the 

 same meanings as in equation (44)^. 



In a great many cases the size of wires to be used to deliver 

 current to a uniformly distributed load with a specified drop of 

 voltage is determined by equations (44), by assuming a concen- 

 trated load equal to the given distributed load and located near 

 the center of the given distributed load. 



119. Wiring calculations for constant-voltage systems with dis- 

 tributed loads. When a number of widely distributed lamps is 

 supplied with current by one pair of service wires, or when a 

 number of widely distributed customers is supplied by one pair 

 of street mains, we have what is called a distributed load. The 

 problem of determining the size of street mains to supply a num- 

 ber of scattered customers is the same as the problem of deter- 

 mining the size of service wires to supply a number of scattered 

 lamps. In the first case the voltage-drop between the center of 

 distribution and the various service points is the important con- 

 sideration, and in the second case the voltage-drop between a ser- 

 vice point and the individual lamps is the important consideration ; 

 a service point is understood to be the point of attachment of a pair 

 of service wires to the street mains. 



In a distributed load two kinds of variation of voltage occur, 

 namely, (a) the variation of voltage from lamp to lamp when the 

 number of lamps in operation is fixed, and (b) the variation of 

 voltage at any given lamp as the number of lamps in operation 

 is increased or decreased. 



Concerning the first type of variation it may be stated in gen- 

 eral that the lamp voltage is less and less the more remote the 

 lamp is from the service point, the most remote lamp having 

 always the lowest voltage. 



