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ELEMENTS OF ELECTRICAL ENGINEERING. 



116. Safe carrying capacity. An electric wire rises in tem- 

 perature until it gives off heat to its surroundings as fast as heat 

 is generated in it by the current. Therefore the rise of tempera- 

 ture for a given current, or the current corresponding to a pre- 

 scribed rise of temperature depends upon the facility with which 

 the wire gives off heat ; and this facility depends greatly upon the 

 degree of ventilation of the region in which the wire is placed, 

 and upon the nature of the adjacent materials, whether they be 

 wood, or plaster, or stone, or metal. Wires covered by wooden 

 mouldings or enclosed in the narrow air space inside of the walls 

 of a building are located most unfavorably with regard to rise 

 of temperature because they cannot give off heat readily, and be- 

 cause any great rise of temperature involves a serious risk of fire. 



TABLE OF CARRYING CAPACITY OF WIRES. 



(From National Electrical Code.) 



For insulated aluminum wire the safe carrying capacity is eighty-four per cent, of 

 that given in the following tables for copper wire with the same kind of insulation. 



The lower limit is specified for rubber covered wires to prevent gradual deteriora- 

 tion of the rubber by the heat of the wires, but not from fear of igniting the insulation. 

 The question of voltage-drop is not taken into consideration in the above tables. 



