Reliable marine power cables are imperative for the cost-effective operation of marine energy conversion systems. There is considerable experience with marine power cables under static and dynamic load conditions but the loading regimes for floating marine energy converters are not well understood, due to a lack of field experience. This paper aims to assess mechanical load conditions and failure modes for a dynamic power cable that is connected to a floating wave energy converter. The applied approach combines experimental tank test data with numerical modelling and site-specific wave characteristics to identify maximum load points and to quantify the fatigue life. The effect of varying wave parameters on maximum loads and fatigue cycles is investigated and results are presented for two common umbilical configurations: catenary and lazy wave. In situations with limited field experience, the presented approach provides a tool to determine if critical components are fit for purpose and to assess the expected level of reliability prior to deployment. The cable conductor's fatigue life is estimated for the lazy wave configuration and highlights component fatigue failure as a major concern that must be addressed in floating marine energy applications.