Abstract: Renovation of urban-substation projects frequently incorporates complex contextual environments and features constrained working conditions. Therefore, site-specific non-standard layout proposals to accommodate these conditions must be developed. In this study, a method was developed to simplify estimating voltage loss of evacuation lighting circuits housed within irregular layouts, while sustaining lighting quality. The method treats uniformly distributed loads as equivalent concentrated loads. The study derives a voltage loss calculation model optimized for DC systems, thus circumventing the traditional segment method’s intricate complexities and potential errors. The cable-layer evacuation lighting circuit of the 110 kV substation was thus selected as a case study and the proposed method was juxtaposed against the traditional segment method. The outcomes validated the proposed method’s superiority in accuracy and efficiency of the calculation. Subsequently, the study’s findings were programmed into a novel software application, the practical design exemplars illustrating the value of the proposed method in the evacuation lighting installations of substations.