Nominal Pipe Size (NPS) is a North American set of standardsizes for pipes used for high or low pressure and temperature. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter), in which sizes are measured in millimetres.The term NB (nominal bore) is also frequently used interchangeably with NPS. According to ASME B36.10 and ASME B 36.19.