External thermal resistance of aerial bundled cables

The calculation of the current carrying capacity of cables in air by the IEC method requires that the external thermal resistance be known, determined from the temperature-dependent heat dissipation coefficient, which includes both convection and radiation losses, and the temperature rise of the surface of the cable. To extend the method to twisted bundled cables, still-air and wind tunnel tests have been performed to obtain convective heat transfer data for a single core and for bundles of two, three and four cores. It is shown that the choice of an effective diameter for the bundle affects the correlation between the Nusselt number and the Rayleigh or Reynolds number, and it is suggested that the diameter of the circle which circumscribes the bundle be used