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 T4* of the cable be known, where T4* is inversely proportional to h ( Delta thetas,)q-1 and q=5/4. It is assumed that both the heat-dissipation coefficient h (W/m2 K5/4) and q are independent of Delta thetas, 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 thetas, up to 100 K and core diameter up to 0.15 m.
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V.T. Morgan (1994) studied this question.