This work provides a detailed analysis of the process of designing and creating a bill of quantities, highlighting potential errors in the selection of equipment for electrical installations in a residential-commercial building. The focus is placed on the importance of ensuring quality at every step, from the preparation of technical documentation and calculations to the selection of materials and equipment with precise specifications. The creation of the bill of quantities is presented as a crucial part of the process, covering methods for calculating material quantities and installation costs. Potential errors, such as improper equipment selection or miscommunication between the designer and the contractor, which may lead to delays and increased costs, are analyzed. The importance of precision in the bill of quantities is also emphasized to avoid unforeseen expenses and enable efficient project execution. This paper provides practical guidelines for designers and contractors, emphasizing the importance of adhering to legislative frameworks and technical regulations to ensure quality. The results and methods presented in this paper can be applied to future projects, reducing the risk of errors and ensuring efficient implementation within specified time and financial constraints. All calculations are supported by tables authored by Mario Lisjak.