A dimensionless designator for such traditional terms as "nominal diameter", "size", and "nominal size". Corresponds to actual outside diameter only in sizes 14 inches and over.
Nominal Pipe Size. Designates a method of identifying the size of steel pipe without compromising the actual diameter data. Thus nominal 1 inch pipe with the actual external diameter of 1.315 inch (33.40 mm) becomes NPS 1; nominal 14 inch pipe with internal diameter of 13.250 inch (336.55 mm) becomes NPS 14.
National Pipe Straight threads used in steel drums. Most steel drums have both a 2" and a 3/4" NPS opening.