For a 4-inch duct, the mandrel maximum should be which?

For a 4-inch duct, the mandrel maximum should be which?

• A. 4 in
• B. 3 in
• C. 2¾ in
• D. 3¾ in

Answer: (D)

The idea is to use a mandrel that represents the largest smooth object you can pull through the duct without snagging, while still leaving enough clearance for bends, fittings, and minor roughness inside the duct. For a 4-inch duct, you want a mandrel that is slightly smaller than the duct itself to ensure it can pass through corners and connections smoothly. In practice, that means about a quarter inch less than the nominal size, so roughly 3¾ inches. Using a mandrel any larger could snag on bends or joints, while a much smaller mandrel would not accurately reflect the actual pull conditions.

The idea is to use a mandrel that represents the largest smooth object you can pull through the duct without snagging, while still leaving enough clearance for bends, fittings, and minor roughness inside the duct. For a 4-inch duct, you want a mandrel that is slightly smaller than the duct itself to ensure it can pass through corners and connections smoothly. In practice, that means about a quarter inch less than the nominal size, so roughly 3¾ inches. Using a mandrel any larger could snag on bends or joints, while a much smaller mandrel would not accurately reflect the actual pull conditions.