Editor’s Note: This article is based on one that first ran in the November/December 2012 issue of IEC Insights, a publication of the Independent Electrical Contractors organization.
Flexible-conductor cable, or flex cable, has become increasingly popular for its ease in maneuvering in tight spaces, particularly where larger conductors are needed, as well as where cable movement may be an issue (as in the wind turbine application shown here, for example). The increased use of flex cable has led to the need for guidance in selecting the proper compression connectors and terminals appropriate for these applications, as well as the proper method for installing these connectors for secure, trouble-free termination. In April 2012, the National Electrical Manufacturers Association (NEMA) issued Bulletin No. 105, which advises that mechanical set-screw connector lugs and terminals are not intended for use with fine-stranded conductors.
Even though mechanical set-screw connectors are commonly used with solid, B- or C-Code cables, they are not recommended for fine-stranded flexible conductor cable because the fine strands break too easily from the stress of a mechanical connection. Broken strands can cause overheating and wire pullout. NEMA recommends compression connectors for termination of fine-stranded flexible conductor cable.
Table I lists flex cable classifications and strands. Some connectors are dual-rated for code and flex cable, while others require a different connector for each application: the connector should identify the proper application clearly, whether B- or C-Code or flex cable (Fig. 1). According to the UL standard, 486 A–B:
“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.”
Fig. 1. As magnified here, per UL standard, 486 A–B connectors must show conductor classes and number of strands. Other information that’s important to the user is also carried on these components, including, among other things, application.
Compression connectors come in a variety of configurations—including one- and two-hole lugs, butt splices, H-taps and C-taps, among others—and are available for copper, aluminum or copper-clad aluminum conductors. Among the advantages a compression connector offers over a mechanical connector is the permanence of its connection when properly installed with the correct tooling: it cannot be loosened accidentally. Other advantages include its irreversibility, which may be required for grounding applications; its low-profile crimp, which is easy to insulate; and some compression connectors 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.
There are two well-known methods of crimping a connector termination: the indent-style crimp, made by die-less compression tools, and the hex-style crimp, made by compression tools equipped with interchangeable hex dies.
Correct execution with a proper tool that corresponds to the size of cable and connector enables the indent-style crimp to ensure reliable electrical performance and excellent pullout resistance. An indent-style crimp results in rounded edges and no flash on the connector. It also forms strands tightly together inside the connector, which eliminates air gaps from the conductor. The indent-style crimp, however, does not provide the ability to inspect for a proper crimp.
The hex-style crimp has been the industry’s preference for crimping compression connectors onto B- and C-Code copper, and aluminum/copper cables up to 1000 kcmil. The hex-style crimp provides superior electrical performance and excellent pullout strength, and hex dies emboss the die code onto the connector for easy inspection and verification of a proper crimp after installation.
Hex-Flex® Die System…
Recently, a third method of attaching connectors to flexible conductors was introduced, which combines the best of the indent- and hex-style crimps: superior pullout ratings and the ability to inspect for a proper crimp. The Hex-Flex Die System consists of standard hex die halves and an indenter. The hex portion provides color-coding and die embossing for easy identification and inspection. The indenter is round and smooth and produces the higher pullout values across all types of flex cables. Due to the higher pullout values created by the Hex-Flex® system, it also reduces the number of crimps required on most connectors.
The right way to install connectors
Steps in installing a compression connector properly are quick and easy with the right tools:
Step 1. Preparing the cable properly…
Without damaging the conductors, strip the insulation to a length that allows the conductors to be fully inserted into the connector barrel with no bare wire exposed (see Figs. 5 and 6).
Fig. 5. Wire stripped
Fig. 6. Wire length
Step. 2. Picking the proper connector…
As stated previously, compression connectors—along with their packaging materials and enclosed literature—clearly point out several important pieces of information. Pay close attention to them. Look for marks that indicate:
IMPORTANT: Connectors marked with just the cable size or “CU” should be used on copper conductors only. Connectors marked “AL( )” with a cable size should be used on aluminum conductors only. Connectors marked “AL( )CU” with the cable size may be used on aluminum or copper conductors.
Step 3. Choosing the proper tool and die…
A wide range of gear, from manual tools to battery-operated hydraulic crimping units, is available to make terminating compression connectors easier. Connectors with colored bands or dots that correspond to the colored markings on the dies facilitate proper matching of connectors with dies. Connectors and dies also have a die code number marked or stamped on them. Dies have a code number engraved on the crimping surface.
Fig. 7 Dies and connector markings
Step. 4. Crimping properly…
Locate the markings on the connector and die. Keeping your fingers away from the crimping mechanism, insert the connector into the tool and align the die with the connector.
With multiple crimps, the first should be nearest to the tongue and subsequent crimps should move toward the barrel end. Connectors are marked with colored stripes to indicate the number and location of each crimp and the die code number at each compression location.
A proper crimp will emboss the die code number on the connector for easy inspection to determine if the correct die and connector combination were used. MT
Chad Smith is Director, Product Management, for Thomas & Betts.