This paper describes the design of a microprocessor based instrument to monitor temperatures in underground electric power transmission cables. Algorithms applying modern estimation and identification technology are employed to obtain a complete temperature profile of the cable cross section from only a few measurements. Cable ratings are also calculated in real-time in order to provide for maximum capacity utilization based on pre-specified temperature operating limits. Based on development test results, the instrument should provide accurate temperature estimates plus the potential for significant increases in cable capacity utilization when comapred to present cable ratings.