Pipe Sizes, NB, DN and NPS - Are they all the same?

First some common definitions of the two words in the title:
Nominal: As expected or planned; normal | Bore: Diameter of a tube / pipe 

So, putting those together we would expect “Nominal Bore” to mean the expected diameter of a tube – but within the realms of pipe support systems, it is often a cause for errors.

Fig. 1 - Pipe Diagram

U-bolts, pipe clips, pipe clamps, hangers and of course our modular pipe support system – the 1205 series, all have one thing in common – you need to know the size of fitting to fit your pipe. And this is where the “Nominal Bore” phrase comes into play, and where it sometimes goes away.

For pipes, the bore is the hollow centre, and Nominal is used in the sense of “in name only” or approximate. Nominal bore is actually the approximate internal measurement across the diameter of the mouth of a pipe – NOT necessarily the expected diameter.

As we know, pipes are for carrying fluids and gasses and therefore the internal diameter is critical for allowing us to calculate capacity, flow rates etc. However, regulations govern the size of a pipe’s outside dimensions to ensure it is manufactured in standard sizes for easy fitting. But because pipes vary in material, weight, and the way they are formed, internal dimensions of pipes that share the same outer dimension may vary. For this reason, any measurement of that internal dimension can only be “nominal.”



It is worth remembering also that nominal bore can have different names. In the inch system, nominal bore is abbreviated as NB; in metric systems, it is “diameter nominal” (DN), or Nominal Pipe Size (NPS) in North America. So, how does all this help us when purchasing a pipe support system? Quite often people assume that the nominal bore relates to the inside diameter of a pipe support, or perhaps the outside diameter of the pipe being supported, but it is neither – it is the approximate inside diameter of the pipe.

For example, a support that is quoted as 3” nb means it is for a PIPE with a 3″ nominal bore, and that PIPE has a fixed outside diameter of 3.5”. The wall thickness (Schedule) can vary hence the inside diameter is 3” nominal (around 3” up or down dependent upon the wall thickness).

There is an added complication. because of how pipes have traditionally been made, the fact that the Nominal Bore size is less than the pipe outside diameter only holds up to a 12″ pipe (300 NB). Above that size, the NB is the SAME as the outside diameter!

So, in summary, watch out for nominal bore, it is NOT the expected bore of the pipe, but the approximate inside diameter based on a standard outside diameter. You should be fully aware of the outside diameter of the pipe to ensure you get the correct supports.

 

Nominal Bore Dimensions

Nominal Bore (metric / NB / DN) Nominal Bore (Imperial) Pipe Outside Diameter (metric)
10mm
3/8"
17.2 mm
15mm
1/2"
21.3mm
20mm
3/4"
26.9mm
25mm
1"
33.7mm
32mm
1 1/4"
42.4mm
40mm
1 1/2"
48.3mm
50mm
1 2"
60.3mm
65mm
2 1/2"
76.1mm
80mm
3"
88.9mm
100mm
4"
114.3mm
125mm
5"
139.7mm
150mm
6"
168.3mm
175mm
7"
193.7mm
200mm
8"
219.1mm
225mm
9"
244.5mm
250mm
10"
273.0mm
300mm
12"
323.9mm
350mm
14"
355.6mm
400mm
16"
406.4mm
450mm
18"
457.0mm
500mm
20"
508.0mm
550mm
22"
559.0mm
600mm
24"
610.0mm
650mm
26"
660.0mm
700mm
28"
711.0mm
750mm
30"
762.0mm
800mm
32"
813.0mm
850mm
34"
864.0mm
900mm
36"
914.0mm
1000mm
40"
1016.0mm