170 ELEMENTS OF ELECTRICAL ENGINEERING. 



rated full-load terminal voltage gives the rated kilowatts, but in alternating-current 

 apparatus only at non-inductive load. 



80. Thus, in machines in which the full-load voltage differs from the no-load volt- 

 age, the full-load current should refer to the former. 



If P rating of an electric generator and E= full-load terminal voltage, the full- 

 load current is : 



p 

 1= in a continuous-current machine or single-phase alternator. 



p 



1 = in a three-phase alternator. 



Vi 



P 



1= in a quarter-phase alternator. 



81. Constant-current machines, such as series arc-light generators, should be 

 rated in kilowatts based on terminal volts and amperes at full load. 



82. The rating of a fuse or circuit breaker should be the current- strength which it 

 will continually carry. In addition thereto, the current-strength at which it will open 

 the circuit should be specified. 



CLASSIFICATION OF VOLTAGES AND FREQUENCIES. 



83. In direct-current, low-voltage generators, the following average terminal volt- 

 ages are in general use and are recommended : 



125 volts. 250 volts. 550 to 600 volts. 



84. In direct- current and alternating- current low- voltage circuits, the following 

 average terminal voltages are in general use and are recommended : 



1 10 volts. 220 volts. 



In direct-current power circuits, for railway and other service, 500 volts may be 

 considered as standard. 



85. In alternating-current, constant-potential, primary-distribution circuits, an 

 average e.m.f. of 2,200 volts, with step-down transformers of ratios i/io and 1/20, is 

 in general use, and is recommended : 



86. In alternating-current, constant-potential, high-pressure circuits, at the receiv- 

 ing end, the following voltages are in general use, and are recommended : 



6,000. 10,000. 15,000. 20,000. 30,000. 40,000. 60,000. 



87. In alternating-current generators, or generating systems, a range of terminal 

 voltage should be provided from no-load voltage to 10 per cent, in excess thereof, to 

 cover drop in transmission. If a greater range than ten per cent, is specified, the 

 generator should be considered as special. 



88. In alternating-currrent circuits, the following approximate frequencies are 

 recommended as desirable : 



25 cycles per second. 60 cycles per second. 120 cycles per second.* 

 These frequencies are already in extensive use and it is deemed advisable to adhere 

 to them as closely as possible. 



*The frequency of 120 cycles per second may be considered as covering the already 

 existing commercial frequencies between 120 cycles per second and 140 cycles per 

 second. 



