494 FARM MOTORS 



are laid out in wiring construction and groups of lamps 

 are fed from these centers Fig. 371. Feed wires are run 

 from the generators to these centers and a constant po- 

 tential is kept in them b^ regulation at the generator. 

 Sets of mains are run from these centers, and then sub- 

 mains are led off from these mains to supply subcenters 

 of distribution. To these subcenterc 

 lead wires to the lamps are connected. 

 In this system of wiring it does not 

 matter if there is a fall of potential oi 

 20 per cent, between lamps and genera- 

 tors, for the fall is alike in all. For 

 example, a voltmeter across the brushes 

 of a generator shows 225 volts, one at, 

 the main center of distribution shows 

 only 218 volts, one at the subcenters 

 shows only 212 volts and one across the 

 terminals of the lamp shows only 210 

 volts. But since there has been the 

 same number of divisions and subdivis- 

 ions the P.D. of all of the lamps is the same. 



665. Calculations for incandescent wiring. To find the size of 

 wire for carrying a certain current, let 

 C. M. = circular mil area of wire, 



K = 10.79 resistance I mil foot of copper wire. 

 L = length of circuit in feet, 

 C = current in amperes, 

 E = volts drop on the line. 

 In the formula, 



KXLXC I0.79XLXC 



T~ -E~~ 



After obtaining the circular mil area, this must be compared with 

 a wire table to get the number of wire to use. 



Example. Fifty 55-watt no- volt lamps are connected in parallel 

 to a center of distribution located 100 feet from a dynamo which 

 generates 112 P.D. By measurement the potential at the point of 

 distribution is no volts. What size wire is required for the feeder? 



FIG. 371 PARALLEL 

 CIRCUIT WIRING 



