What is the size of the opening of a switch (device) box for a single device?

The size of the opening of a switch box for a single device is 3 inches (75 mm). This standard measurement is established to ensure that there is adequate space for the device itself, such as a switch or receptacle, as well as the associated wiring. This size allows for proper installation of the device while providing enough depth and room for the connections without overcrowding, which could lead to safety hazards or difficulties during installation.

In residential electrical installations, adhering to established box sizes is crucial for both safety and functionality. A 3-inch opening strikes a balance between accommodating most single devices while maintaining a compact footprint, making it suitable for typical residential applications. Any box size larger or smaller may not provide the necessary support or could complicate the installation process, impacting the overall safety and compliance of the electrical system.