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The Equivalent External Thermal Resistance (ETR) of power cable installations is the principal quantity which determines cable ampacity. Variations of ETR with respect to various cable designs and surrounding parameters are of major interest to cable engineers, especially when the effect of varying many independent parameters (individually or simultaneously), on the cable ampacity, is to be determined. On the basis of accurate analysis of a variety of cable systems, this paper describes a simple technique to evaluate the effects of cable and ambient parameters variations on the computed cable ampacity. The technique utilizes coefficients of ETR of multiple cable systems and their observed linearized characteristics. The paper describes the technique and illustrates its implementation on several cable system examples.