This paper investigates inverter time ratings and short-time electrical and thermal limitations in peak duty intermittent-load motor drives. An inverter electro-thermal model is developed and validated to complement prior motor thermal models. The inverter model is used to assess short-time capabilities of IGBT inverters. The results show that the inverter is the thermal bottleneck for short-term ratings in a drive system. A large thermal mass heat sink is shown to increase time ratings. The limit is the semiconductor heat capacity, which is invariant for any duration above a fraction of a second. Analyses are extended to inverter characterization for repetitive duty. For aperiodic duty, an energy-based time rating estimation process is discussed and case studies based on automotive duty and fault duty are presented.