ELECTRIC DISTRIBUTION AND WIRING. 297 



dollars per kilowatt-hour is - -- I^ph dollars per year. On 



1,000 s 



the other hand W= 0.00000303 x 2ls pounds, of which the cost 

 is O.ooooo6o6/.sr dollars, the interest on this cost is 0.0000000606 

 Isct dollars per year, and the quantity to be made a minimum by 



choosing s is - -I z ph -f O.OOOOOOo6o6/&/. Differentiating 



this expression with respect to s and placing the differential co- 

 efficient equal to zero gives : 



-- - -o / *ph + o.ooooooo6o6/r/ = o 

 1,000^* 



from which / cancels out, and we find : 



j = circular mils per ampere =597 -\j 

 or 



(46) 



The meanings of the symbols, s, 7, h, /, c and /, are specified 

 above. When the delivered current, /, is not constant, the average 

 value of the current must not be used, but the square-root-of-the- 

 average-value-of-the-square should be used in equation (46). 



Example i. The cost of power at the switch-board of an 

 electric-power station is 1.6 cents per kilowatt-hour (/ = 0.016), 

 the interest on invested capital is 5 per cent., and the annual 

 depreciation and taxes is three per cent, (t = 8), the cost of cop- 

 per wire is 1 6 cents per pound (c = 0.16), and a current of 200 

 amperes is delivered to a customer for 1,000 hours each year. 

 Considerations of economy would lead, under these conditions, 

 to the use of transmission wires 650 mils in diameter, whatever 

 the distance of the customer from the station may be. 



If the distance from the station to the consumer is 538 feet, 

 then the total length of wire is 1,076 feet, its resistance is 0.0275 

 ohm, and the voltage-drop with 200 amperes is 5.5 volts. That 



