Among the various supply voltage abnormalities, unbalance and distortion are some of the major threats to the dc bus capacitor of adjustable speed drives. The effect of supply voltage unbalance on the dc bus capacitor changes in the presence of supply voltage harmonics. Further, the value of grid impedance and the load on the drive influences the response of these supply voltage abnormalities on the dc bus capacitor. This article analyzes the impact of unbalance, distortion, and simultaneous presence of unbalance and distortion in the supply voltage, on the dc bus capacitor, along with the effect of grid impedance and the load on the drive. As a result of the analysis, the article proposes the derating of load required to keep the capacitor current within the safe value irrespective of the level of abnormalities and grid impedance. The effects are analyzed by simulations and validated by the hardware test setup. © 1972-2012 IEEE.