To ensure the safety of the electrical systems in buildings, it is important to properly select the wires that supply electricity to various loads through the buildings. In this chapter, the types and sizes of conductors and conduits suitable for branch circuits and feeders are presented. The specific requirements to select the adequate sizes of conductors to serve building electrical loads are discussed. In particular, ampacity, short-circuit current tolerance, and voltage drop allowance are considered in the design of conductors. Moreover, specific design guidelines for branch circuits and feeders for both nonmotor and motor loads are described with illustrative examples.