Cable Used for Transmitting Electric Energy in Airplanes

A method is proposed for determining the current capacity and the temperature of operation of cables in airplanes. The cable selected should in combination with the power plant result in the smallest weight for each electrical unit converted into useful work. A general equation is given for selecting the optimum current which is expressed in terms of the weight of the cable, power plant, and fuel; the length and resistance of the circuit, and the operating voltage of the system. The method of selection indicates that the current capacity of a cable operated for short periods with-out requiring additional power-plant or fuel capacity is limited by the temperature the insulation will withstand. For open wiring requiring power-plant capacity bare copper conductors should operate from 50 to 65 degrees centigrade above ambient temperature, and insulated copper conductors 10 to 25 degrees centigrade lower. The temperature of the conductors in conduit will depend upon the installation. Measurements made upon two samples of cable in 0.5-inch conduit indicated that the conductors should operate at 45 to 80 degrees centigrade above ambient temperature. All bus bars and conduit should be enameled or painted to reduce their operating temperatures. Measurements upon an aluminum-alloy tube indicated a drop of 35 degrees centigrade in 100 degrees. The greatest reduction in weight of the electric system will be accomplished by increasing the voltage. Further reductions may be accomplished by using aluminum conductors.