Rating of bare overhead conductors for continuous currents

The maximum continuous rating of a short transmission line is often based on the maximum permissible operating temperature of the conductors under the combined effects of wind, sun and air temperature. The paper describes measurements made in still air and in four different wind tunnels to determine the heat transfer from stranded-copper, aluminium, aluminium-alloy and steel-cored-aluminium (s.c.a. or a.c.s.r.) conductors. For Reynolds numbers (Re) between 100 and 3000, the heat transfer was very close to that for smooth circular cylinders of equal overall diameters, but for 3000 < (Re) < 50000, it was higher, the difference increasing with surface roughness. The heat transfer decreased with decreasing angle of attack of the wind relative to the axis of the conductor. Turbulence generated by the windward conductors of a bundle resulted in higher heat transfers for the leeward conductors. The effective radial thermal conductivity may be calculated from the radial temperature gradient, and typical values are given. Measurements of the absorption of solar heat by conductors mounted in a special drum are also described. Equations are given for calculating the continuous rating of conductors exposed to various atmospheric conditions.