While Voltage Unbalance is quantified by formulae in many standards, all lead to different estimates for a given set of three-phase source voltages. In this paper, we focus on the impact of this unbalance assessment in estimating their severity for Adjustable Speed Drives (ASDs). The true unbalance in system voltages, as defined by the ratio of negative to positive sequence is taken as the benchmark to assess the deviation arising out of various definitions. An estimate of Voltage Unbalance of a large sample of voltage sets through various definitions is compared with the true unbalance. It is shown that this indicates maximum possible errors that may arise when using various definitions. The Total Harmonic Current (THC) and Capacitor Heat i ng Factor (CHF) are then used as indices to assess the severity of a particular voltage combination and it is shown that definitions of unbalance do not lead to clear correlation to the THC and CHF. It is further shown that the quantum of negative sequence is a good measure to assess the severity of an unbalanced supply system for the Adjustable Speed Drive topology considered. © 2018 IEEE.