ECONOMICS OF TELEPHOXK l^ELAY MM'l.K ATIOXS 



231 



adding generators and batteries in parallel the needed capacity can he 

 attained. Beyond the range, basically heaviei' machines ;u-e lUH'ded. llw 

 resulting installed price of such power plants has the chaiacler shown in 

 Fig. G. Since prices and installation ciiarges change from year to year, 

 the vakies shown here are gi\'en in (qualitative form as illustrations — 

 they must of course be evaluated as concisely as possil)le when considei- 

 ing new central office designs. As seen in the figure, the power jjlant price 

 xai'ies in two wa^'s: in fairly big jumps as basic plant size is changed, and 

 ill smaller luit rather uniform manner as the basic size varies within its 

 lower and upper limits. 



From Fig. 6 it is seen that for each basic size of plant the rate of in- 

 crease in price with small increases in capacity is about the same for all 

 plants. The average rate for this incremental power capacity, correspond- 

 ing to the expense if relati\ely small power changes are made in any one 

 l)asic type of power plant will be called a. As the power requirements 

 of the office change be.vond a certain point, it is necessary to install 

 basically larger or smaller equipment with correspondingly different base 

 prices. At the points of maximum capacity, there is an abrupt change in 

 the basic price of the plant, amounting often to many thousands of 

 dollars. These abrupt increments, designated .4, represent a lump sum 

 of mone}' that can be saved whenever the power plant size can be re- 



80 120 160 



CAPACITY IN KILOWATTS 



Fig. 6 — Installed value of i)0\ver plants. 



