Investigation of the seasonal variation of ground thermal conductivity on high voltage cable ampacity

Total cable loading depends on heat dissipation from the cables due to their current and the surrounding thermal environment. Cable currents, jacket temperatures, and ambient air temperatures were recorded by an underground thermocouple based data acquisition system. This allowed for the analysis of temperature effects on the cable duct from both sources. There is interaction between the adjacent cables. A step in current corresponds to a transient response it its jacket temperature. Daily ambient air extremes correlated well with the daily current and cable jacket temperature extremes. The thermal area of influence was reduced from what was previously thought, a circle of radius 10 ft, allowing for the tighter placement of adjacent underground ducts. The assumed value for thermal resistance used in the software modeling was verified. Seasonal trends were obtained and used to find a temperature delay that will be used to create an overload duration chart for emergency loads.