Emissivity and Its Effect on the Current-Carrying Capacity of Stranded Aluminum Conductors [includes discussion]

An apparatus for the measurement of the total hemispherical emissivity of stranded conductor is described. A measurement is made by suspending the specimen in the center of an evacuated chamber, heating the specimen, and taking a cooling curve. New stranded aluminum conductors have emissivity values of about 20 to 25%; old, weathered conductors may have emissivities of 90% or higher. Data tending to confirm the values of emissivity obtained are presented in the form of actual temperature rises of several sizes of stranded conductor under controlled conditions. It is shown that the values of convected heat loss obtained by subtracting calculated radiated heat loss from the measured I2R input to the conductor check very closely for both new and blackened conductors.