ELECTRIC DISTRIBUTION AND WIRING. 



289 



Concerning the second type of variation it may be stated in 

 general that the voltage at every lamp falls off to some extent 

 when additional lamps are turned on, and rises when lamps already 

 in operation are turned off. The range of variation in voltage at 

 a given lamp is from a lowest value, when all the lamps are in 

 operation, to a value very nearly equal to the voltage at the ser- 

 vice point, when the given lamp, only, is in operation. Therefore, 

 the lamp that is most remote from the service point is subject to 

 the greatest range of variation of voltage as other lamps are 

 turned on and off. 



There are two clearly defined cases that arise in the laying out 

 of wires for distributed loads, namely, Case /., in which the lamps 

 supplied by a given pair of service wires are turned on and off sepa- 

 rately, and Case II. , in which all of the lamps supplied by a given 

 pair of service wires are turned on and off together. In the first 

 case the wiring must be laid out so as to keep the voltage varia- 

 tions of both types (a) and (&) within certain limits ; and in the 

 second case the wiring may be laid out with reference to the lim- 

 itation of voltage variations of the first type only, that is varia- 

 tions of voltage from lamp to lamp, inasmuch as voltage variations 

 of the second type (b) do not exist in Case II. 



Supply mains 



Fig. 163. 



Case I. An example of a distributed load is shown in Fig. 

 163. When all of the lamps are in operation the end lamp, L, has 

 the lowest voltage of any lamp in the group, and the voltage at 

 this lamp varies through the greatest range when other lamps are 



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