The 2inch PVC pipe that joins a 45degree elbow to the long b

The 2-inch PVC pipe that joins a 45-degree elbow to the long branch of a 45-degree wye is 24 inches long. How far are the two fittings offset? A. 19 7 / 8 inches C. 20 5 / 8 inches B. 20 7 / 16 inches D. 20 7 / 8 inches

Solution

Ans-

If the duct is of 22 gage (0.85 mm), the second column shows that it may be unreinforced.

If the duct is of 24 gage (0.70 mm), the 12” (305 mm) side may be unreinforced, but grade B joints are required at 10 feet (3 m) maximum spacing on the 18” (457 mm) sides. However, Table 1-18 allows the T-1 drive slip (alone) to be used on the 18” (457 mm) sides. Any joint used on the 18” side must meet grade B regardless of joint spacing.

If the duct is of 26 gage (0.55 mm), the 12” (305 mm) side may be unreinforced, but on the 18” (457 mm) sides, the maximum reinforcement spacing is 8 feet (2.4 m) and the minimum size is grade B. By Table 1-18, the T-1 drive slip is acceptable as grade A (up to 20” (508 mm) width and 8 feet (2.4 m) spacing). Substitution of C or D for B would still not permit a reinforcement spacing greater than 8 feet (2.4 m) because the minimum gage associated with 10 feet (3 m) spacing is 24 (0.70 mm).

Example 2, 30” × 18” (762 × 457 mm) duct:

The choices for the 30” (762 mm) side are: 16 ga (1.61 mm) for unreinforced; E on 22 ga (0.85 mm) at 10 feet (3 m); D on 24 ga (0.70 mm) at 8 feet (2.4 m), or D on 26 ga (0.55 mm) at 6 feet (1.8 m) max. or C at 4 feet (1.2 m) max. For the 18” (457 mm) side, the choices are the same as outlined in Example 1 for 18” (457 mm) width.

Example 3, 54” × 30” (1372 × 762 mm) duct, with 5 feet (1.5 m) joint spacing preselected:

For 54” (1372 mm) width, F code is required if 22 ga (0.85 mm) is selected. 24 ga (0.70) duct may be used with 5 feet (1.5 m) joint spacing but an E rated intermediate (between E rated joints) would be added (effectively changing the reinforcement spacing to 2-1/2 feet (0.76 m)).

For the 30” (762 mm) side: Grade D is required (for 5 feet (1.5 m) maximum spacing) on any duct gage less than 16 (1.61 mm).

If you did not have the 5 feet (1.5 m) joint limitation and it was economical to do so, 16 ga (1.61 mm) duct unreinforced on the 30” (762 mm) sides and G joints at 10 feet (3 m) spacing on 54” (1372 mm) sides is acceptable.

Example 4, 72” × 72” (100 × 188 mm), with 5’ (1.5 m) joint spacing and no intermediate reinforcing use 18 ga (1.31 mm) duct with H rated joints or G rated joints with tie rods added (Gt) or use 24 ga (0.70 mm) with 21/2 feet (0.75 m) spacing and F ratings.

This introduction does not review all aspects of construction. It is an overview. Certain other limitations will appear later in the text and tables. For example, certain joints have been assigned maximum pressure classes. However, other limits may be acceptable if they can be shown to result in equivalent construction. See the appendix for other useful tables and commentary.

Consult NFPA Standard 90B and other codes and specifications applicable for use of materials thinner than 26 gage (0.55 mm) in residential work.