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In this paper, a procedure is presented for the life estimation of high-voltage cables subjected to electrothermal stress due to applied voltage and load cycles. Thermal transients that affect cable insulation as a consequence of cyclic current variations are modeled by means of the well-known CIGRE two-loop thermal network analog. The effect of the relevant cyclically varying electrothermal stress is accounted for via the cumulative damage law of Miner. The life fractions lost during each step of the load cycle are evaluated by resorting to a proper combined electrothermal life model holding for cable insulation. The model is considered within its due probabilistic framework for associating life with residual reliability and failure probability. The procedure is applied to high-voltage ac EPR- and XLPE-insulated cables, subjected to stepwise-constant daily load cycles. The application shows that life is very sensitive to load cycles, as well as to thermal transients and to the synergism between electrical and thermal stress. Thus, none of these factors should be neglected for an accurate estimate of life expectancy of high-voltage cables in real service conditions.