Which type of conduit is least durable based on the NEC standards?

The least durable type of conduit based on NEC standards is Schedule 40 PVC. This type of conduit is made from plastic and is primarily used for indoor installations and in areas where it won't be subject to heavy physical damage. While PVC conduit is resistant to corrosion and is lightweight, it lacks the mechanical strength and impact resistance that metal conduits possess. This makes it more susceptible to damage from external forces, such as being struck or crushed, particularly in environments where physical protection is essential.

Rigid Metal Conduit and EMT conduit, being composed of metal, offer much greater durability and can withstand harsher environments without significant risk of damage. Flexible metal conduit, while not as strong as rigid options, still provides a level of protection and flexibility for certain applications, making it more durable than Schedule 40 PVC. Therefore, in terms of durability according to NEC standards, Schedule 40 PVC is the least robust option.