For continuous-duty applications, electric motors are conventionally designed to experience a hot-spot temperature below their insulation thermal class. When frequent overloads are required, a safety temperature-margin is considered at the machine's design stage, so that the insulation is not thermally overstressed. This procedure leads to an increment of the machine's size, with a consequent power density reduction. Indeed, lower thermal loadings are obtained by decreasing the winding current density by e.g. increasing the equivalent cross-sectional area of conductors. For some applications, it is possible to avoid the 'over-engineering', but the reliability level might be compromised. This work addresses the subject of thermal overload capability in electrical machines, combining elements of thermal analysis, reliability and physics of failure. A novel design methodology is introduced and validated by means of thermal accelerated lifetime tests on winding specimens. The proposed approach is used for redesigning a brushless DC motor achieving excellent results in terms of power density boost.