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 error.

Non-Gripped U-BoltU-bolts, pipe clips, pipe clamps, hangers and of course our own 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 awry.

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 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 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 can vary hence the inside diameter is 3” nominal (around 3” up or down dependent upon the wall thickness)

There are various places on the internet to find the relationship between Nominal bore and pipe outside diameter, of course course you need the wall thickness figures also, but there are standards:

But really, look no further than our own pages. All of our brochures are available on line and contain full dimensional data including pipe outside diameter and nominal bore. The brochures can be viewed on line for example our U-bolt broucher is:

U-Bolt Brochure

And of course all of our products in the store have full dimensions also

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