I'm looking at IID Si vs IID USC versions. Table 3, page 418, line 29. It's indeed strange, as for other materials the conversion factors are different. For example, 3" relates to 75 mm. (line 26), 1.5" converts to 38 mm (line 27), 2.5" to 65 mm (line 30), etc.
I think this may be a typo. I suggest you forward this to the applicable Code committee for consideration, this is likely an error that sneaked into the Code books.
If you do, you may also point them that 2.5" sometimes converts to 64 mm. (same page, line 32), sometimes to 65 mm. (same page, line 30). This seems inconsistent.
This issue was initially observed in the 2019 Code edition. The SUMMARY OF CHANGES section addresses this matter as follows: "Size/Thickness added or revised for lines containing pound sign in margin (17-583, 17-2305)" and "For dual-unit specifications, revised the intermediate values in the Size/Thickness columns of Tables 1A, 2A, 3, 4, 5A, U, and Y-1 to conform to the intermediate size/thickness limits in the material specifications in Section II, Part A." In my professional opinion, it appears to be a typographical error in the metric units. However, given that this has not been rectified in the 2021 and 2023 editions, it cannot be definitively classified as a mere typographical error. Is there anyone with a clear understanding of this matter?
