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The problem of evaluating power cable loading performance subject to variations of the thermal circuit parameters is formulated and solved by a fast sensitivity technique based on the finite element method. The technique provides useful sensitivity information which can be accessed and manipulated by the cable designer or operator to simulate, in a simple and efficient manner, various soil, ambient, loading and boundary variations. The sensitivity technique has been applied to a variety of practical cable systems including direct-buried and pipe-type cables as well as cables in duct banks. The technique reduces significantly the analysis and computations via identifying the important parameters which most affect the ampacity and it simplifies the thermal model by eliminating the low-sensitivity (non-important) parameters. Because the sensitivity values depend on the particular cable system and soil and boundary conditions and can vary significantly from one situation to another, the generalization of the results is not possible. Therefore, the paper describes the sensitivity technique and its direct implementation by illustrating its use for a variety of cable systems.