Cable Geometry and the Calculation of Current-Carrying Capacity

The main purpose of this article is to express the calculation of current-carrying capacity in simple formulas. The allowable current for underground cables is usually limited by the maximum permissible temperature of the insulation. The temperature rise is of course a function of the ability of the cable system to dissipate the heat generated. The chief difficulty in the calculation of current-carrying capacity is the determination of the thermal resistances of the path through which the heat must flow. The main part of this paper deals with the errors in the standard formulas for calculating the thermal resistance and geometric properties between the conductors and the sheath. A graphical method of correcting the errors is obtained in terms of what is called the ``geometric factor,'' the results are tabulated for 2, 3 and 4-conductor cables throughout the range of practical sizes and an empirical formula is given. The check between the results of the graphical correction method and the published experimental data on this subject is very satisfactory, and emphasizes the errors in the standard formulas. The thermal resistance between the sheath and the duct is mentioned briefly, and an approximate method of finding the resistance between the duct and the region at base temperature is outlined. The previous work is then combined into a simple formula giving the allowable current for n-conductor cables, there being any number of similar cables in the duct bank.