What component is threaded onto the terminal screw of back-wire devices?

The component that is threaded onto the terminal screw of back-wire devices is a clamping plate. This part is essential for securing the wire connections in back-wiring systems effectively. When the clamping plate is tightened onto the terminal screw, it compresses the wire against the conductive terminal, ensuring a reliable electrical connection while preventing the wire from being pulled out inadvertently.

The back-wiring mechanism is designed to make installation easier and faster while also providing safer connections by eliminating the need for twisting wires together. The clamping plate's role in this setup is critical, as it allows for good electrical contact and reduces the risk of loose connections that could lead to overheating or electrical failures.

Other components listed, such as the mounting strap or grounding terminal, do not serve the same purpose. The mounting strap provides a way to attach the device to the electrical box, and the grounding terminal is involved in the grounding of the device rather than securing the wire connections. The device box is where the whole assembly will reside but is not specifically threaded onto the terminal screw itself. Thus, the clamping plate is the right answer due to its functional importance in maintaining the integrity of back-wire connections.