Which type of box shall not be used where conduits or connectors requiring locknuts or bushings are connected to the side of the box?

Round boxes are not suitable for use where conduits or connectors that require locknuts or bushings are connected to the side of the box. This is primarily because round boxes typically lack the necessary features to accommodate the secure attachment and grounding requirements imposed by the use of locknuts and bushings.

In contrast, square, octagonal, and rectangular boxes are designed to provide sufficient surface area and support for locknuts and bushings, ensuring a secure and stable installation. These boxes take into account the need for proper grounding and mechanical support that are critical in maintaining electrical safety and compliance with electrical codes. Thus, using a round box in such situations may lead to improper connections and potential safety hazards.