On what size pipe is the pipe named for its outer diameter (OD)?

The correct answer is associated with the nominal sizes of pipe where the designation is based on the outer diameter (OD) rather than the nominal pipe size (NPS), which refers to the inner diameter. In the context of piping systems, pipes that have an outer diameter equal to or larger than 14 inches are typically designated by their outer diameter measurement. This naming convention is important because it facilitates a clearer understanding of the pipe's dimensions and aids in the compatibility for fittings and jointing in high-pressure applications.

For pipes smaller than 14 inches, the designation generally relies on the NPS, which does not correspond directly to the outer diameter due to wall thickness variations. Therefore, the distinction becomes significant as pipe sizes increase, leading to the shift in nomenclature at the 14-inch mark, emphasizing the outer diameter. Understanding this classification is essential for professionals working with high-pressure piping systems, as it affects material selection, installation processes, and safety considerations.