推荐回答（3个）
公称直径为250到400毫米
一般来说，管子的直径可分为外径、内径、公称直径。管材为无缝钢管的管子的外径用字母D来表示，其后附加外直径的尺寸和壁厚，例如外径为108的无缝钢管，壁厚为5MM，用D108*5表示，塑料管也用外径表示，如De63,其他如钢筋混凝土管、铸铁管、镀锌钢管等采用DN表示，在设计图纸中一般采用公称直径来表示，公称直径是为了设计制造和维修的方便人为地规定的一种标准，也较公称通径，是管子（或者管件）的规格名称。管子的公称直径和其内径、外径都不相等，例如：公称直径为100MM的无缝钢管有102*5、108*5等好几种，108为管子的外径，5表示管子的壁厚，因此，该钢管的内径为（108-5-5）＝98MM，但是它不完全等于钢管外径减两倍壁厚之差，也可以说，公称直径是接近于内径，但是又不等于内径的一种管子直径的规格名称，在设计图纸中所以要用公称直径，目的是为了根据公称直径可以确定管子、管件、阀门、法兰、垫片等结构尺寸与连接尺寸，公称直径采用符号DN表示，如果在设计图纸中采用外径表示，也应该作出管道规格对照表，表明某种管道的公称直径，壁厚。
碳钢管外径壁厚对照表
公 称 直 径 国标无缝管 国标焊管(GB/T3091-2001) 美 标(ASTM A53) 英 标(BS) 日 标(JIS)
外径 (mm) 壁厚 (mm) 外径 (mm) 壁厚(mm) 外径 (mm) ANSI壁厚(mm) 外径 (mm) 壁厚(mm) 外径 (mm) 壁厚(mm)
国标管 加厚管 STD SCH20 SCH40 SCH80 A级 B级 M级 SGP(一般 配管用) SCH40 SCH80
200A 219.00 6.0 219.10 5.00 219.10 8.20 6.30 8.20 12.70 216.30 3.60 5.70 216.30 5.80 8.20 12.70
250A 273.00 7.0 273.10 273.10 9.30 6.30 9.30 15.10 267.40 5.50 267.40 6.60 9.30 15.10
300A 325.00 8.0 325.00 323.90 9.50 6.30 10.30 17.40 318.50 6.00 318.50 6.90 10.30 17.40
350A 377.00 7/8/9 377.00 355.60 9.50 8.00 11.10 19.10 355.60 6.00 355.60 7.90 11.10 19.10
400A 426.00 7/8/9 426.00 406.40 9.50 8.00 12.70 21.40 406.40 6.00 406.40 7.90 12.70 21.40
450A 478.00 7/8/9 478.00 457.20 9.50 8.00 14.30 23.80 457.20 7.90 14.30 23.80
500A 529.00 7/8/9 529.00 508.00 9.50 10.00 15.10 26.20 508.00 7.90 15.10 26.20
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