For finished installations, boxes for wiring devices must provide what type of support?

The requirement for substantial support for wiring device boxes reflects the need for safety, reliability, and long-term performance in electrical installations. Substantial support ensures that these boxes can securely hold the wiring devices, such as switches, outlets, and connectors, that will be installed within them. Wiring devices must remain fixed in place over time, even as they experience regular use or potential stresses, like plugging and unplugging of cords or devices.

Using substantial support helps prevent issues such as the boxes becoming loose, which could lead to damaged wires, poor connections, or even create a fire hazard. This is particularly important in finished installations where aesthetics and safety must be balanced effectively without compromising the integrity of the electrical system.