PROBLEMS. 485 



Note. If the lamps above specified are all exactly alike it is evident that the cur- 

 rent in each lamp of the 300 set cannot be the same as the current in each lamp of the 

 200 set. It is usual, however, in wiring calculations to consider that lamps of a given 

 size and type take a definite amount of current irrespective of the slight variations of 

 voltage. 



150. The three-wire system of problem 148 supplies 300 lamps 

 (150 amperes) on one side and 100 lamps (50 amperes) on the 

 other side, and the total voltage of 2 3 1 volts at the generator is 

 equally divided by the balancer as explained in problem 149. 

 Find : (a) The voltage across the set of 300 lamps ; and (b) 

 the voltage across the set of 100 lamps. Ans. (a) 100. 1 volts; 

 (b) 122.1 volts. 



151. A span, 150 feet long, of hard-drawn copper wire, No. 8 Brown and Sharpe 

 gauge, is to be strung at a temperature of 75 F. at a place where the winter tempera- 

 ture sinks to 20 F. The maximum tension of the wire is to be 164 pounds. 

 Find : (a) The sag at 20 F.; (b] the sag at 75 F., and (c) the tension at 75 F. 

 Ans. (a] 0.86 foot ; (b] 2.86 feet ; (c) 49 pounds. 



152. A group of ten lamps, each taking one-half an ampere, is 

 ten feet distant from 1 1 5 -volt mains, (a) Find the size of wire 

 required in order to give a drop of 5 volts, (b) What size of 

 wire (rubber insulation) would be required according to the table 

 of safe carrying capacity given in Art. 116, Chap. IX.? Ans. 

 (a) 14.7 mils diameter; (&) No. 16 Brown and Sharpe gauge 

 (51 mils diameter). 



Note. It is evident that a wire 14.7 mils in diameter would be excessively heated 

 by a current of 5 amperes. Usually the size of wire for supplying lamps near to the 

 generator or center of distribution is determined by the table of safe carrying capacity. 



The insurance rules of the National Electrical Code forbid the use of wire smaller 

 than No. 14 Brown and Sharpe gauge for house wiring. 



153. A pair of street mains leading out from a central station 

 delivers 50 amperes of current to a consumer at a distance of 200 

 feet from the station, 75 amperes to a second consumer at a distance 

 of 350 feet from the station, and 40 amperes to a third consumer at 

 a distance of 600 feet from the station. The station voltage is 

 1 1 5 volts and the voltage at the distant end of mains is 1 1 o volts. 

 Find : (a) The size, weight, and cost of mains of uniform size ; 

 and (b) the size of each section of the mains, and their total weight 



