Procedures for calculating the temperature rise of pipe cable and buried cables for sinusoidal and rectangular loss cycles [includes discussion]

It has been customary in calculating the temperature rise of cables in duct to take account of the nature of the load cycle, since this has the effect of reducing the temperature rise from that which would result if the load were maintained continuously at a constant value. With the advent of pipe-cable systems and the increased use of directly buried cables in the United States, the problem naturally arises as to how to calculate the effect of the load cycle in these cases as well. This matter is discussed at some length in an AlEE conunittee report (ibid., vol. 70, pt. I, 1951, pages 45-52). While this report presents the results of many calculations of the effect of various loss cycles on a number of typical cable systems, it does not offer a simplified procedure for making such calculations. This paper presents such a procedure. The procedure involves merely the determination of the internal thermal resistance of the cable system in conventional manner, and adding to it the effective external thermal resistance as given in a table as a function of cable diameter and loss factor. By the use of simple nomographs, the procedure is made to include the effects of changing the earth constants, depth of burial, or frequency of the loss cycle. A rigorous straightforward method of solution of the temperature rise under sinusoidal loss cycles is also given for use where additional accuracy is required.