Thermal Analysis of Power Cables in Free Air: Evaluation and Improvement of the IEC Standard Ampacity Calculations

The thermal behavior of cables installed in free air depends upon physical parameters, such as surface emissivity, heat dissipation coefficients for radiation and natural convection, as well as induced heating from neighboring heat sources that depend on the configuration in which the cables are grouped. The IEC standard method for rating cables installed in free air considers all of these physical properties implicitly and only for particular conditions. In this paper, the IEC standard method for rating power cables installed in free air is evaluated against finite-element method simulations and laboratory experiments. A scientifically sound and accurate thermal-electric circuit for the calculation of the steady-state temperature of cables in air is derived from first thermodynamic principles. The model parameters are computed explicitly from the physical properties of the cable, cable grouping, and environment. Through numerous finite-element simulations, as well as laboratory experiments, the accuracy of the proposed method has been established.