What must each outlet box have in completed installations?

In completed installations, every outlet box is required to have a cover to ensure safety and compliance with electrical codes. The cover serves multiple important functions: it protects the electrical connections within the box from dust, moisture, and accidental contact, which can prevent electrical shock and short circuits. Additionally, the cover helps to maintain the integrity of the outlet box's structure and minimizes the risk of objects entering the box that could interfere with the electrical components.

While connectors, clamps, and access panels can be important in specific contexts, they are not universally required for every outlet box in the same way that a cover is. Connectors and clamps are typically related to securing cables and maintaining strain relief, but the core requirement for an outlet box, in terms of ensuring safety and compliance, is the presence of a cover. Access panels, on the other hand, are only needed in specific situations, such as allowing for maintenance access to concealed wiring or devices. Hence, the requirement for a cover is a crucial and universal aspect of completed electrical installations.