Accurate demand load estimates on a particular branch circuit, feeder, or electric service are critical in order to determine the current ratings of these circuits, along with the actual and required number of branch circuits or feeders. Electric cable networks form the backbone of a building's electrical installations. Therefore, complete analysis of the building's electrical systems includes detailed analyses of the cable network, and assessment of the cable ampacities, ratings, and sizes. This assessment is necessary because cable current capacity depends on factors that are determined by actual usage conditions. This chapter provides a comprehensive overview of load calculation methods, steps required for accurate load estimates for facilities and buildings, and cable sizing, types, and construction. The power network configuration and characteristics are determined based on the requirements resulting from the building use, equipment, and installations. The electric load calculation methods to be served in a building, load demand, and load correction and derating factors are also detailed in this chapter. Several examples for load calculation to determine service entrance requirements for residential, commercial, and industrial facilities and occupancies are included. The specification of the type, size, and insulation type for conductors is an important part of electrical system design. Cable ampacity is limited by the insulation type, ambient temperature, and the operation conditions; conductor selection and size are determined by the carried currents, calculated from accurate demand load estimates. Finally, this chapter emphasizes the importance of the recommendations and requirements of codes or standards.