Marine power cables play a crucial role to utilize energy in marine areas, such as offshore wind, wave energy and tidal energy. Marine energy devices are typically grouped into arrays to increase the economic viability, and power cables transfer the energy from the devices to a central hub which is then transmitted ashore. Cables connected to moving devices may experience millions of load cycles per year, and thus they need to be flexible and designed for mechanical loads due to the movements of the cable. In this study, the focus is on the mechanical life of flexible cables connecting devices to hubs, and thus lowand medium voltage power cables is the focus. The reliability design method Variational Mode and Effect Analysis (VMEA) is applied that is based on identifying and quantifying different types of uncertainty sources, such as scatter, model uncertainties and statistical uncertainties. It implements a load-strength-approach that combines numerical simulations to assess the loads on the cable and experimental tests to assess the strength of the cable. The VMEA method is demonstrated for an evaluation of bending fatigue, and it has been found to be a useful tool to evaluate uncertainties in fatigue life for cables in WEC (Wave Energy Converter) systems during the design phase. The results give a firm foundation for evaluation of safety against fatigue and are also helpful for identifying weak spots in the reliability assessment that can motivate actions  in the improvement process. Uncertainties in terms of scatter, statistical uncertainty and model uncertainty have been evaluated with respect to the WaveEL 3.0, a WEC designed by the company Waves4Power, and deployed in Runde, Norway. A major contribution to the overall uncertainty is found to originate from the fatigue life model, both in terms of scatter and model uncertainty.