272 



ELECTRICAL EQUIPMENT 



The factors determining the proper transmission voltage are, 

 as a rule, of an economical nature, and, while no fixed formula 

 for determining the voltage can be given, in general it may be 

 said that the most economical voltage is the one for which the 

 annual cost of the energy loss added to the annual cost for depre- 

 ciation and interest on the first cost, becomes a minimum. In 

 determining the value of the energy loss, a mean value for a 

 number of years should evidently be taken, and the value should 



150,000 



'125,000 



100,000 







75,000 



50,000 



25,000 



50 100 150 200 



Transmission Distance in Miles 



250 



FIG. 150. Approximate Voltages for Power Transmission of 

 Various Lengths. 



s 



be based on the cost for which the power can be produced. The 

 interest and depreciation as well as operating charges should only 

 be applied to such items that will vary with changes in the volt- 

 age, such as the line conductors and tower line, the generating 

 and substation buildings, transformers, switching equipment and 

 lightning arresters. 



An approximate average scale of voltages for transmission 

 lines up to 250 miles in length is given in Fig. 150. 



