ELECTRIC DISTRIBUTION AND WIRING. 291 



expense of inserting fusible cut-outs at each point where wires of 

 unequal size are joined, as required by the insurance rules, may 

 be considerable. 



Rule /. When it is desired to reduce the size of a pair of 

 street mains (or service wires) in steps so as to secure the greatest 

 economy of copper, the size of each portion of the mains is deter- 

 mined as follows. Having given the total drop to be allowed out 

 to the end of the line, calculate the factor s from the equation : 



__ 2 x 10. x a^ ^i 



total drop in volts 



in which a, b y c . . . are the lengths in feet of the respective por- 

 tions of the pair of mains, and i v i 2 , z' 3 are the currents in 

 amperes in the respective portions. The sectional areas of the 

 various portions of the mains in circular mils are then equal to 

 sVi v sVi v sVi z - respectively. 



Rule <?. When service wires of uniform size are to be used 

 for supplying current to a scattered group of lamps, the size of 

 the wire to give a prescribed drop out to the end lamp may be 

 determined as follows : Estimate the distance, Z, of the " center 

 of gravity " of the group of lamps by the formula : 



where /', /", l' n ', etc., are the distances in feet of the individual 

 lamps * from the service point and n is the total number of lamps 

 in the group. Then calculate the size of the wire that would be 

 required to supply the n lamps as a concentrated group at the 

 prescribed total drop and at the distance L from the service point. 



Case II. When the lamps of a group are always turned on 

 and off together the variation of voltage from lamp to lamp can 



* If the lamps are arranged in subgroups it is easier to take I' as the product of 

 the distance of the first subgroup times the number of lamps in that group, I" as the 

 product of the distance of the second subgroup times the number of lamps in that 

 group, and so on. 



