Sheet Metal (C-43) · Lesson 2 of 8

Product-Conveying Duct Gauge: CMC Tables 506.2(1) and 506.2(2)

"Product-Conveying Duct Gauge: CMC Tables 506.2(1) and 506.2(2)" is lesson 2 of 8 in the Sheet Metal (C-43) Learning Hub. It opens with "Classify the Duct First" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Classify the Duct First
  2. Read Round Duct Gauge in Table 506.2(1)
  3. Read Rectangular Duct Gauge in Table 506.2(2)
  4. Know When the Table Does Not Apply, and the Add-Ons
  5. Hoods Have Their Own Thickness

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Decide whether the tables apply" through "If the product were sawdust".

What you'll be able to do

  • Explain the main idea of "Product-Conveying Duct Gauge: CMC Tables 506.2(1) and 506.2(2)" in plain language you can use on the exam.
  • Apply this step with confidence: "Read Rectangular Duct Gauge in Table 506.2(2)".
  • Apply this step with confidence: "Know When the Table Does Not Apply, and the Add-Ons".
  • Apply this step with confidence: "Hoods Have Their Own Thickness".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Duct Gauge: Table 506.2 by Class and BandsWelding-fume exhaust, Class 1, 6 in. w.c.18 in round48 in between bands30 x 12 rectangular24 in between bandsBands = reinforcement spacing. Not to scale.Class 1, to 7 in. w.c.Round 18 in, Table 506.2(1):no bands: 0.044 in (18 ga)bands at 96 in: 0.027 in (22 ga)bands at 48 in: 0.021 in (24 ga)Rectangular 30 in side, Table 506.2(2):bands at 48 in: 0.055 in (16 ga)bands at 24 in: 0.033 in (20 ga)bands at 12 in: 0.021 in (24 ga)Hood on a Class 1 system (Sec. 505.11):steel not less than 0.027 in (No. 22)Dash in a table cell = not permitted.Round 0.021 in (24 ga); rectangular 0.033 in (20 ga)At 6 in. w.c. the 4 in. w.c. SMACNA exception for Class 1 ducts does not apply.2025 CMC Table 506.2(1), Table 506.2(2), Sec. 505.8, 505.11, 506.2. Gauge in table = No. shown after the thickness.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Classify the Duct First

The 2025 California Mechanical Code (CMC) Section 505.8 sorts product-conveying ducts into five classes. Class 1 conveys nonabrasives such as smoke, spray, mists, fogs, noncorrosive fumes and gases, and light fine dusts. Class 2 conveys moderately abrasive particulate in light concentrations, such as sawdust, grain dust, and buffing and polishing dust. Class 3 is Class 2 material in high concentrations or highly abrasive material in low concentrations (manganese, steel chips, coke), Class 4 is the heaviest abrasives (alumina, bauxite, sand, slag), and Class 5 conveys corrosives such as acid vapors. The class decides the gauge, so you cannot pick a thickness until you know it.

Step 2: Read Round Duct Gauge in Table 506.2(1)

Table 506.2(1) is Minimum Sheet Metal Thickness for Round Ducts. Down the side are the negative pressure ranges (To 7, 8 to 11, 12 to 15, and 16 to 20 inches of water column) and the reinforcement spacing (0, 24, 48, or 96 inches). Across the top are the diameters, from Up to 7 inches to 60 inches, in separate blocks for Class 1 through Class 4. Each cell prints the thickness and the gauge, for example 0.044 (18 ga.). Class 1, To 7 in. w.c., 16 to 19 inch diameter: no reinforcement 0.044 (18 ga.), every 96 inches 0.027 (22 ga.), every 48 or 24 inches 0.021 (24 ga.).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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