What is the maximum size of a mandrel for a 4-in duct?

What is the maximum size of a mandrel for a 4-in duct?

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

Answer: (D)

When you pull cables through a duct, a mandrel is used to check that there’s enough clearance inside and to simulate the size of the cable bundle you’ll pull. The mandrel must fit inside the duct and be able to negotiate bends and fittings, so it needs to be slightly smaller than the duct’s actual inside diameter. For a four-inch duct, the standard maximum mandrel size is 3¾ inches. That leaves about 1/8 inch of radial clearance, providing a little room for rough edges, dirt, and tolerances while allowing the mandrel to pass through bends and connectors without getting stuck. Using a larger mandrel could jam the duct or damage the installation, which is why 3¾ inches is the correct maximum.

When you pull cables through a duct, a mandrel is used to check that there’s enough clearance inside and to simulate the size of the cable bundle you’ll pull. The mandrel must fit inside the duct and be able to negotiate bends and fittings, so it needs to be slightly smaller than the duct’s actual inside diameter. For a four-inch duct, the standard maximum mandrel size is 3¾ inches. That leaves about 1/8 inch of radial clearance, providing a little room for rough edges, dirt, and tolerances while allowing the mandrel to pass through bends and connectors without getting stuck. Using a larger mandrel could jam the duct or damage the installation, which is why 3¾ inches is the correct maximum.