To improve the power conversion efficiency of a three-phase dual-active-bridge (3P-DAB) converter for low-voltage direct current distribution systems, a practical design methodology is proposed. Commonly, the 3P-DAB converter is designed by only specification, such as the rated power, input/output voltage, and frequency. However, using the conventional design method, the efficiency is low under heavy load conditions due to the high conduction loss. In this article, based on the practical power loss analysis, the effective design methodology of the coupling inductance in the 3P-DAB converter is proposed, focusing on power conversion efficiency. It can mitigate the drawbacks of the common design by reducing the peak and rms phase current and the insulated-gate bipolar transistors turn-off current. Experimental results verify the validity of the proposed design methodology and controller using a 25-kW prototype 3P-DAB converter.