Cable is made up of resins and metals that are vital parts of making a world-class power cord or cord set. If cable materials are damaged, the result could be loss of electrical continuity or shortened lifespan. Making sure you know the recommended minimum bend radius for cable is critical to the performance and longevity of the cable.

Calculating Bend Radius

Calculating how far a cable can bend without causing material damage depends on the jacket material and the wires and insulators inside—and the diameter of the cable. The outer diameter of the cable times the cable multiplier equals the minimum bend radius. Cable multipliers vary according to the type of cable and per industry standards.

What is the recommended minimum bend radius for cable? The National Electrical Code Handbook (NEC) and HD 516 lists minimum bend radius requirements.

“It depends on the size of cable and the material,” Dan Ford said, Technical Support Specialist at Interpower. “Some types of cable bend easier than others with their jacket materials offering varying degrees of flexibility. Fillers or shielding inside the cable will decrease its ability to bend.” Ford added that finer strands of wire allow for more flexibility than larger strands.

If cable repeatedly exceeds the minimum bend radius, issues may include cracked insulation, broken conductors, or splits in the jacket or conductor wires. For example, if the jacket material is a harder material, it may not survive an “8” radius, but possibly a “12.” The harder the material, the more prone it is to damage due to rigidity.

“It is important to avoid excessive bending of the cable which can shorten its life, potentially leading to shock or fire,” Ford said. “The end use of the cable plays a big role in what the actual bend radius will be—how it will be used? For example, if the cord is plugged into the wall and then jerked side to side for removal, that will violate the bend radius. If the cord is repeatedly twisted, that will affect it as well.”

Ford compared the bend radius of a cable to that of a garden hose that is bent to the point where it kinks and blocks the flow of water. In the same way a cable can be bent to where it can no longer transmit the flow of electrons.

“Bend radius failures usually occur over time and eventually the cable will no longer transmit power,” Ford said.

Indicators of Bend Radius Failures:

• Discoloration of the cable
• Wrinkling, pinching, or kinking of the cable
• Stress tears on the cable jacket
• Increased resistance of the cable with the meter (a high resistance rating can indicate damage)

Strain Reliefs

The main purpose of a strain relief or cable gland is to provide protection to a power cord. They help extend cord life and add stability to the equipment. In generic terms, it is used to secure a power supply cord to a piece of electrical equipment. The term strain relief is commonly used in North America, while cable gland is the preferred term in Europe and other continents.

Thread Types

Different types of mounting threads are available: National Pipe Thread (NPT) is the American standard and is most commonly used in the United States; Stahlpanzerrohr-Gewinde, also called Panzergewinde (PG), is the German military thread used in Germany and Europe; the metric type which originated in Europe is used worldwide.

Location is a major factor in determining type. If located in North America, NPT or metric threads should be chosen. In Europe, metric is often used for new equipment while PG is often used for replacement parts. While all thread types secure fittings to equipment, minor differences include the thread spacing and tapering in each type. Using the wrong type for your application may result in an improper fitting or connection resulting in cross-threading.

Cable and Wire Size

Cable diameter determines the size of the strain relief. The outer diameter of the cable must be known in order to select the appropriate strain relief which will be listed in specification guidelines.

Body Styles

Interpower offers strain reliefs in two general configurations: dome and flex.

Dome	Flex
Named for its dome shape, this configuration helps secure the wire connection to the equipment. This extends cord life by preventing friction across rough surfaces and sharp edges on the equipment panel.	This flexible style safely bends the cord entering the equipment. It is designed to eliminate kinking at the point where the cord enters the equipment. The spiral-shaped spring wraps around the cord and adds strength where the cord is most vulnerable.

Methods of Attachment

Threaded strain reliefs offered by Interpower are assembled in two different ways. Mounting threads can be threaded directly into the panel of the equipment. Or it can be eased through a clearance hole in the panel of the equipment and fitted with a locking nut. Each option requires a different size clearance hole in the equipment panel. Specifications are found by part number on the Interpower website to assist in determining the correct hole sizing.

Direct Thread into Panel	Clearance Hole with Nut