During the decades-long operation of commercial nuclear power plants (NPPs), the hundreds of miles of electrical cables for instrumentation, control, and power in each facility can be exposed to elevated temperatures. Several knowledge gaps have been identified related to cable lifetime expectations from initial qualification to extended NPP operation beyond sixty years. Among these knowledge gaps, diffusion limited oxidation (DLO) or oxygen permeability hindrance at the surface of electrical cable insulation due to overly rapid accelerated aging has been a focus. DLO can lead to heterogeneous oxidation of polymeric insulation producing erroneous lifetime prediction when using certain characterization techniques to track insulation degradation. In this study, the most common class of insulation material in electrical cables within NPP containment, cross-linked polyethylene (XLPE), was thermally aged at four different temperatures (121°C, 136°C, 150°C, and 165°C) for select exposure periods to determine thermally triggered DLO thresholds. Following aging, the thermally degraded insulation was assessed for DLO using total color difference. By better understanding DLO behavior in accelerated aging, the effects of aging artefacts on lifetime prediction can be better accounted for in cable aging management, including prioritization of cables for testing, repair and replacement.