Making a Secure Connection with Flexible Cable
With the increasing use of fine-stranded, flexible conductors, especially for alternative energy applications, there has arisen some confusion over the proper connectors and lugs to use. The National Electrical Manufacturers Association (NEMA) recently issued a bulletin (No. 105, dated April, 2012) advising the industry that mechanical set-screw connector lugs and terminals are not intended for use with fine-stranded conductors.
Although mechanical set-screw connectors are commonly used in applications where solid, B or C-Code cables are used, they are not recommended for use with fine-stranded flexible conductor cable, because the mechanical connection can cause breakage of the fine strands, resulting in overheating and wire pullout. NEMA calls for compression-type connectors when terminating fine-stranded flexible conductor cable.
Here are guidelines for selecting the proper com-pression connectors and terminals for these appli-cations, as well as the proper method for installing these connectors to ensure a secure, trouble-free termination.
Matching the connector to the cable
Flexible-conductor cable, commonly known as flex cable, has become increasingly popular in the past few years. Flex cable is easier to maneuver in tight spaces, particularly with larger sizes, as well as when cable movement is prevalent, such as in wind turbine applications.
There are many different flex cable classifications and strands, so care should be taken to match the connector to the proper cable type (See Figure A).
Some connectors are dual rated for both code and flex cable, while others require a different con-nector for each application. The connectors should clearly identify the proper application, whether B or C-Code, or flex-type cables. This is in compliance with UL standard 486 A–B that states, “A connector, a unit container, or an information sheet packed in the unit container for a connector tested with conductors other than Class B, SIW, or Class C stranding shall also be marked with the conductor class or classes and the number of strands.”
Compression connectors come in almost limitless variety that includes one and two-hole lugs, butt splices, H-taps and C-taps, to name only a few. They also are available for copper, aluminum, or copper-clad aluminum conductors. Compression connectors offer a number of advantages over mechanical con-nectors: the connection is permanent when properly installed with the correct tooling and it cannot be loosened accidentally. The connection is irreversible, which is sometimes required for grounding applica-tions. The low-profile crimp is easy to insulate. Some connectors also are available with an oxide inhibitor. The disadvantages are that each conductor size requires its own connector, and crimp tooling is needed to make a proper connection.
An overview of crimping styles
When it comes to installing compression con-nectors, electrical contractors traditionally had two choices in crimping methods: the indent-style crimp, made by die-less compression tools; or the hex-style crimp, made by compression tools equipped with interchangeable hex dies.
When correctly performed with a proper tool that corresponds to the size of cable and connector, the indent-style crimp offers reliable electrical per-formance and excellent pullout resistance. With an indent-style crimp, the connector is left with rounded edges and no flash. Also, virtually all air gaps are removed from the conductor as the strands are formed tightly together inside the connector. The indent-style crimp, however, does not provide the ability to inspect for a proper crimp.
The hex-style crimp has long been preferred as the industry standard for crimping compression connectors onto B and C-Code copper, and aluminum/ copper cables up to 1,000 kcmil. The hex-style crimp provides superior electrical performance and excellent pullout strength. Plus, hex dies emboss the die code onto the connector for easy inspection and verification of a proper crimp after installation.
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