Ferrite toroids (or clamps) are widely used to reduce common-mode (CM) currents in power systems. The CM impedance of the ferrite depends on the frequency-dispersive permeability and permittivity of the ferrite, the geometry of the system, and the location of the ferrite in it. An analytical model was developed to predict the CM impedance of a wire harness above a return plane with a ferrite on it. The model is based on transmission line theory for a cable, a ferrite, and a return plane. The parameters of the model are calculated using a frequency-dependent quasistatic model for a ferrite toroid. This model accurately predicts the CM impedance of a mock harness within 3 dB up to 1 GHz. The proposed model is also applied to a real power system consisting of an inverter and a motor. Knowledge of the CM impedance of the system in the operating regime is critical to determining the impact of the ferrite on CM currents. The CM impedance is determined using the dual current clamp technique. The impact of the ferrite on the CM impedance and currents of the power inverter system was predicted within 3 dB, demonstrating the usefulness of the modelling approach for analysis of power systems.