According to electrical installation practices, what type of conduit should not be used for permanent exterior installations?

Flexible non-metallic conduit is not appropriate for permanent exterior installations due to its susceptibility to environmental factors. This type of conduit may not offer adequate protection against UV radiation, moisture, and temperature fluctuations that can cause deterioration over time. Unlike rigid PVC conduit or liquidtight flexible metal conduit, which are specifically designed for outdoor use and can withstand exposure to the elements, flexible non-metallic conduit lacks the durability and weather resistance needed for long-term installations in outdoor settings.

On the other hand, rigid PVC conduit is widely utilized in outdoor applications due to its robustness and resistance to moisture and UV rays. Liquidtight flexible metal conduit is also suitable for exterior installations, providing both flexibility and protection against moisture ingress. Flexible metallic conduit, while generally more robust than its non-metal counterpart, may still be used in certain outdoor conditions where it is protected from direct exposure to harsh environments. Thus, when considering a conduit for permanent exterior installations, flexible non-metallic conduit is less reliable compared to the alternatives.