What is the maximum metric designator for flexible metal conduit (FMC) that should not be used unless permitted?

The maximum metric designator for flexible metal conduit (FMC) that should be used cautiously is indeed associated with the size of the conduit. Flexible metal conduit is available in various sizes, and the designators signify the internal diameter of the conduit that corresponds with certain applications.

The choice of 16 as the maximum metric designator indicates that while this conduit size can be used in specific circumstances, it is typically limited to applications where it is explicitly permitted. This recommendation is grounded in best practices that prioritize safety and code compliance. FMC is designed for flexibility and protection, but larger sizes, such as 16, may present challenges in terms of handling, installation, and compatibility with other systems.

When the conduit size increases, the weight and rigidity can lead to structural concerns, making it essential to consider the installation environment and type of electrical conductors being housed. The constraints ensure that contractors and electricians can maintain the integrity and functionality of electrical systems without compromising safety.

Other size designators, while commonly used, do not have the same limitations as the 16 metric size, which is why it is highlighted for careful consideration in certain applications.