Mandrel size should be how much smaller than the duct diameter?

Study for the BICSI OSP Design Test. Prepare using multiple choice questions, with hints and explanations to enhance understanding. Succeed in your exam journey!

When pulling a cable through a duct, a mandrel is used to keep the duct round and protect the walls from damage or snagging during the pull. The mandrel should be sized just a bit smaller than the duct’s inner diameter so it can slide through easily while still providing clearance to prevent deformation or abrasion on the duct walls.

A practical guideline is about a quarter to a half inch smaller than the duct diameter. This range lets lubricant and the cable pass with minimal resistance while maintaining the protective spacing. For example, in a 2-inch inner-diameter duct, a mandrel around 1.75 to 1.5 inches works well. If the mandrel is only a tiny bit smaller (like 1/8 inch), friction can be higher; if it’s much smaller or larger, it either offers insufficient protection or won’t fit properly. Hence, 1/4 to 1/2 inch smaller is the recommended rule of thumb.