STANDARDIZATION Rl'Ll- 



295 OVERLOAD CAPAI in; - KKCOMMENDED. The following overload capaci- 



are recommended: 



296 a. GENERATORS. Direct-current generators and alternating-current 

 generators, 25 per cent for two hou 



297 />. MOTORS. Direct-current motors, induction motors and synchronous 

 motors, not including railway and other motors intended for intermittent 



ce, 25 per cent for two hours, and 50 per cent for one minute. 



298 c. CONVERTERS. Synchronous converters, 25 per cent for two hours, 

 50 per cent for one-half hour. 



299 </. >RMERS AND RECTii it K-. Constant-potential transformers 

 and rectifiers, 25 per cent for two hours; except in transformers connected 

 to apparatus for which a different overload is guaranteed in which case the 

 same guarantees shall apply for the transformers as for the apparatus 

 connected thereto. 



300 e. EXCITERS. Exciters of alternators and other synchronous machines, 

 10 per cent more overload than is required for the excitation of the syn- 

 chronous machine at its guaranteed overload, and for the same period of 

 time. All exciters of alternating-current, single-phase or polyphase gen- 

 erators, should be able to give at their rated speed, sufficient voltage and 

 current to excite their alternators, at the rated speed, to the full-load ter- 

 minal voltage, at the rated output in kilovolt-amperes and with 50 per 

 cent power factor. 



301 /. A CONTINUOUS-SERVICE RHEOSTAT, such as an armature- or field- 

 regulating rheostat, should be capable of carrying without injury for two 

 hours, a current 25 per cent greater than that at which it is rated. It 

 should also be capable of carrying for one minute a current 50 per cent 

 greater than its rated load current, without injury. This excess of ca- 

 pacity is intended for testing purposes only, and this margin qf capacity 

 should not be relied upon in the selection of the rheostat. 



302 g. An INTERMITTENT SERVICE OR MOTOR-STARTING RHEOSTAT is used for 

 starting a motor from rest and accelerating it to rated speed. Under 

 ordinary conditions of service, and unless expressly stated otherwise, a 

 motor is assumed to start in fifteen seconds and with 150 per cent of rated 

 current strength. A motor-starter should be capable of starting the 

 motor under these conditions once every four minutes for one hour. 



303 (a) This TEST may be carried out either by starting the motor 

 at four-minute intervals, or by placing the starter at normal tem- 

 perature across the maximum voltage for which it is marked, and 

 moving the lever uniformly and gradually from the first to the 

 last position during a period of fifteen seconds, the current being 

 maintained substantially constant at said 50 per cent excess, by 

 introducing resistance in series or by other suitable means. 



304 (6) OTHER RHEOSTATS FOR INTERMITTENT-SERVICE are employed 

 under such special and varied conditions, that no general rules are 

 applicable to them. 



%. 



III. VOLTAGES AND FREQUENCIES. 



A. VOLTAGES. 



306 DIRECT-CURRENT GENERATORS. In direct-current, low-voltage gener- 

 ators, the following average terminal voltages are in general use and are 

 recommended: 



125 volts. 250 volts. 600 volts. 



306 LOW-VOLTAGE CIRCUITS. In direct-current low-voltage circuits, the 

 following terminal voltages are in general use and are recommended: 



115 volts. 230 volts. 550 volts. 



In alternating-current low-voltage circuits, the following terminal volt- 

 ages are in general use and are recommended. 



110 volts. 220 volts. 440 volts. 550 volts. 



