Current rating of cables as affected by mutual heating in air or conduit

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. 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. research on the effect of mutual heating of a group of conduits for different numbers of conduits and various geometric groupings. The authors report on research carried out by the Electrical Testing Laboratories under the supervision of a committee from the Insulated Power Cable Engineers Association (IPCEA) with certain supplemental data from the laboratories of one of the subscribing companies. The research consisted of taking oneand four-inch standard conduits in groups of 1, 3, 6, and 9 with standard spacing between conduits, inserting in each conduit a twisted pair of number 19 American wire-gauge hotbed heater wires, and applying various amounts of heat (watts per foot) to obtain a curve of temperature rise of the conduit above ambient temperature. As it is important to know the effect on the heat dissipating properties of the conduits when conduits are installed close to a wall, near ceilings, or in completely covered runways, tests were made with conduits freely open to the air, with barriers of paper board about fourfeet high, spaced about two feet from the conduit, and with barrier and covers, the latter about two feet above the conduits. These three conditions simulated free dissipation of heat, side restriction but vertical dissipation, and an enclosure restricting heat dissipation. Details as to the test methods, taken from the report of the ETL are reported.