ELECTRIC DISTRIBUTION AND WIRING. 287 



namely, (a) the case in which current is delivered at one point 

 to a motor or to a group of lamps, constituting what is called a 

 concentrated load ; and (b) the case in which current is delivered 

 to a scattered group of lamps or motors, constituting what is 

 called a distributed load. 



The problem of determining the size of wire required to deliver 

 a specified amount of power, P, to a concentrated load at a speci- 

 fied voltage, E (at the lamps), with a specified percentage drop, 

 pEjiQOy in the wires, is solved as follows : (i) The current, 7, is 

 equal to PjE. Sometimes the current is given directly, as when 

 a specified number of lamps is to be supplied with a specified cur- 

 rent per lamp. (2) Given the percentage drop, /, the actual drop 

 in volts is known. (3) This actual drop is equal to RI, so that 

 the resistance, R, of the two line-wires is thus determined. (4) 

 Knowing the value of R and the length, 2/, of the wire in feet, the 

 diameter, d, of the wire in mils may be calculated from the equa- 

 tion R= 10.8 x 2ljd 2 . 



The result of these separate steps may be most conveniently 

 represented in the formula 



2,i6o// / \ * 



d 2 = circular mils = (44K* 



ph 



in which d is the diameter in mils of copper wires required to 

 deliver 7 amperes to a concentrated load distant / feet from the 

 center of distribution, / is the drop of voltage expressed in per 

 cent; thus p = 6 for 6 per cent, and E is the voltage at the 

 lamps. 



It is preferable to avoid the specification of voltage-drop as a 

 percentage ; and in fact the drop is usually specified in volts. 



* In laying out the wiring for a complete electric-lighting system it is a great sav- 

 ing of time to make use of what are called wiring charts, which give at a glance the 

 solution of equation (44)0 or (44)^ for any particular case. The construction and 

 use of such charts are fully explained in a small book, entitled The Wiring Com- 

 puter, by Carl Hering (The McGraw Publishing Company.) 



