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Nominal Size

– the history and origin and how is it used in industry

Definition and History of Nominal Size

To define Nominal Size, requires an explanation of size systems as this term is not referring to the dimensions of a product. In the case of measuring a steel beam, to calculate the load capacity, for example, there are tables to translate the nominal size of a beam into usable true dimensions.

The terminology of Nominal size began long ago to describe certain sized parts, pipes and lumber. At the beginning, there were less variables in size, thickness or borings of parts. As fabrication machines became more and more precise and could handle manufacture of more increments in measurements, the Nominal size term was less about actual measure of a part and more descriptive.

How to Use this Term of Nominal Size

nominal size image from beyondrigging, size:390*222.Here is an easy example of how history has changed standard measurement identifications. The differences between nominal size and actual measurement of certain cuts of lumber vary in terms of thickness times width. The nominal size of lumber is the terminology we use commonly when ordering product, like the term 2X4. Determining the actual grading of lumber size is related to a product standard used by all manufacturers, established by the United States Dept. of Commerce. The actual size of a “2 X 4″ wood plank is approximately 1½” thick and 3½” wide. This is because in the past, lumber was measured when still green, then the dimensions would change as wood dried. The true dimensions are calculated for measurements like in floor plans and wall sizes. The reference of “2 X 4” would not correctly calculate the actual measurements. The difference in thickness and width of wood changes the dimensions of a structure, wrong calculations affect window placement, doorways, right angles of walls, and many other standards. That is why actual measurement must be used in calculations and not nominal size.

The same changes have occurred in piping over time because inner wall thickness and outer wall thickness of pipes have become more precise and thinner piping can be manufactured as strong as thicker materials were at one time. These considerations are critical to engineering and construction fabrication. Dimensions, locations, pressure and weight will all factor in correct piping alignment and placement in various systems. This is the same for other fittings.

This term “nominal size” is an identifying number and it is not related to the specific measurements of parts or products, for example: Using specific sizes of wood screws is simplified by identifying the specific screw size with a number. These numbers do not identify exact sizes of the screws, but do organize the increments from high numbers like #8 being smaller than #5 screws. The opposite is true with wire gauges. If you want to know the actual measurement of a screw you need to use size tables to find the true length and diameter in inches.

Other cases involving diameter, pressure, weight bearing loads, speed, and volume, engineers use calculation tables to find exact sizes of the parts they use in blueprints and diagrams for true size structure measurements and construction.

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