Effect of surface-temperature rise on external thermal resistance of single-core and multi-core bundled cables in still air

A method is given in IEC Publication 287-2 for calculating the current rating of cables in still air. This method requires that the external thermal resistance T ₄ ^* of the cable be known, where T ₄ ^* is inversely proportional to h ( Delta theta _s,) ^q-1 and q=5/4. It is assumed that both the heat-dissipation coefficient h (W/m ² K ⁵4/) and q are independent of Delta theta _s, the temperature rise of the surface of the cable. It is shown that these assumptions are incorrect, both for a single core or cable and for multi-core bundled cables. Values of h and q-1 are presented for one-, two-, three- and four-core cables, with Delta theta _s, up to 100 K and core diameter up to 0.15 m.<>