What must each outlet box have in completed installations?

Each outlet box in a completed installation must have at least one electrical device installed to ensure functionality and compliance with electrical codes. An outlet box serves as a housing unit for various electrical devices, which can include switches, receptacles, or other functional components. Having an installed device ensures that the outlet is ready for use and conforms to safety regulations.

An empty box would not serve any practical purpose in a completed installation, and while a cover plate is necessary for safety and aesthetics, it does not fulfill the requirement for the box to be functional. Thus, the inclusion of at least one device alongside the option of having a cover plate is essential for proper installation, validating the choice that states any of the necessary components, including a device or cover plate, must be present in a completed outlet box setup.