Which mounting method cannot be used for standard sectional switch boxes?

The mounting method that cannot be used for standard sectional switch boxes is the one involving magnets. Standard sectional switch boxes are designed for secure mounting methods that ensure stability and integrity for electrical installations.

Screwing to wood backing, mounting to adjustable bar hangers, and nailing to brackets provide strong and reliable connections that can support the weight and electrical connections of the box, making them suitable for use in residential wiring. These methods ensure that the box remains in place over time and can withstand the stresses of use, such as wiring and installation of switches.

On the other hand, using magnets would not provide sufficient strength or stability for securely holding a switch box. Magnets can be affected by various factors like vibrations, temperature fluctuations, and accidental impacts, which could lead to the switch box becoming loose or detached. Therefore, magnetic attachment does not meet the standards required for a safe and durable installation in a residential context.