Nominal size is a term used for the comparative sizes of piping and tubing rather than the actual dimensions. Like 3/4″ or 1″. Nominal sizes of piping in sprinkler systems are not the actual sizes and are given in inches and fractions of an inch.For a given NPS (Nominal Pipe Size), the OD stays fixed and the wall thickness increases with schedule.ike 3/4″ or 1″. Nominal sizes of piping in sprinkler systems are not the actual sizes and are given in inches and fractions of an inch. For a given NPS (Nominal Pipe Size), the OD stays fixed and the wall thickness increases with schedule.