System selection and equipment sizing represent a critical stage in the electrical design process, forming the bridge between load estimation and detailed system layout. This phase focuses on identifying the most appropriate electrical systems and components to meet the functional, operational, and safety requirements of the building, while ensuring compliance with applicable standards such as IEC 60364, IEC 60947, IEEE 141, and NFPA guidelines.

The design engineer must evaluate each system—transformers, generators, UPS units, capacitor banks, and central battery systems—not only in terms of capacity but also in terms of performance characteristics, installation environment, efficiency, maintainability, and future scalability. Equipment selection must be based on actual load profiles, starting currents, system autonomy requirements, redundancy needs, and available spatial and environmental constraints. Additionally, coordination with utility regulations and the mechanical and architectural design must be considered to ensure spatial compatibility and integration.

A well-structured sizing methodology begins by converting load estimates into electrical capacities (in kVA or kW), followed by selecting appropriately rated equipment. Derating factors—such as ambient temperature, altitude, harmonics, and installation methods—must be accounted for during this process. Selection decisions must also align with project-specific priorities, including energy efficiency, power quality, fault withstand capability, and operational continuity in critical environments such as healthcare, data centers, and high-rise developments.

This section provides a systematic framework for selecting and sizing key electrical systems. It includes load-based calculations, equipment configuration options, protective considerations, and international compliance criteria. The goal is to ensure that each selected system component can safely and efficiently perform under both normal and emergency operating conditions throughout the life cycle of the