Abstract :
THE current rating of a cable is determined by its surroundings in addition to its inherent characteristics. The cable may be exposed in air, drawn into a conduit in the air, buried directly in the ground, or drawn into a duct in the ground. It is rarely necessary to consider a single cable, that is, it is customary to install a number of cables in a group. Thus, instead of a single cable dissipating its heat to the surroundings, the problem becomes more complex because of the effect of the heat being dissipated from each cable on all of the cables in the group. Therefore, the current rating for a single cable must be modified. For cables drawn into duct systems, duct heating constants obtained from experimental data are added to the thermal resistance of a single cable.1–3 In the case of buried cables, there is a similar term added to the thermal resistance of a single cable for each additional cable. For cables in air or in conduit in air, no experimental data have been available to show the influence the grouping of the cables or the conduits have on the current rating of the cable. Correction factors for the rating of a single cable in air or when installed in a conduit have been estimated theoretically, but lack of confirmatory data made assumed values of doubtful accuracy.
