Questions: when are insulating bushings required on conduits?
Answer: Where raceways contain conductors 4 AWG and larger, the conductors must be protected from abrasion during and after installation by a fitting that provides a smooth, rounded insulating surface, such as an insulating bushing 300.4(f). For IMC and RMC, a bushing must be installed on conduit threads, regardless of conductor size, in accordance with 300.4(f), unless the design of the box, fitting, or enclosure is such as to afford equivalent protection, such as a field installed hub (Meyers hub) or the box has a built in hub (outdoor bell boxes).
