<p>This paper introduces an optimal design of the hybrid active filter combining a single-tuned passive filter and an active filter to cost-effectively improve the permissible loading capacity of power transformers operating in harmonic-rich systems. The proposed design method aims to minimize the harmonic loss factor (<i>F</i>_HL), the filter’s investment cost, and the fundamental frequency loss in the active filter part. Its constraints include the individual and total harmonic distortion limitations specified by the IEEE standard 519, the desired displacement power factor interval, and the passive filter’s quality factor range. To evaluate the performance of the proposed optimal design, particle swarm optimization and genetic algorithm methods are applied to two harmonic distortion scenarios within an industrial two-bus distribution system. Simulation results show that the proposed filter design significantly reduces harmonic distortions and the <i>F</i>_HL index, improving transformer loading capacity by rates of up to approximately 168%. In addition to that, the parametric analyses obtained in the Matlab/Simulink environment confirm the robustness of the design across varying loading and short-circuit impedance conditions.</p>