Data-center facilities of all sizes rely on a steady, reliable, and well-maintained electrical power system. Even small electrical failures or faults can have devastating consequences, including extensive operational downtime and the expense of locating and repairing damaged equipment. Data center applications put unique demands on cables that transmit the generated power to the power conversion equipment, and through to the step-up and step-down transformer, coupled with grid interconnection switchgear.

Space within the data center floor build-out is at a premium; the area must accommodate conversion and termination equipment, as well as main and auxiliary power resources, such as transformers and an immense variety of control circuits. Cables there must be capable of tight radius bends and sweeps, and carrying the ampacity (ampere capacity) of the prime power resource. For that reason, designers prefer flexible diesel locomotive (DLO) cable in the high-power, tight-fit cabinets from where power cables run within floor and overhead raceways and restricted spaces. It is capable of carrying voltages from 120 Vac through 220 Vac to 480 Vac, and ampacities up to 2,000 A.

Due to the tight-fitting overhead and under-floor raceways, as well as cabinet design, proper cable terminations are among critical considerations for data center projects. Understanding ampacity loads will inform the choice of the specific type of metallurgy and the crimping methodology to use for high- or low-voltage terminations.

Within a data center, critical environmental conditions like dewpoint, moisture, and temperature controls are always a concern when applying cable terminations. For example, there is a data center that is responsible for switching the main data stream of South America, Central America, and the Caribbean's Layer 1, Layer 2, and Layer 3 traffic, bound to more than 148 countries in the world, making this data center the unrivaled gateway to the Americas. These types of data centers are critical paths for business, government, and media. In data centers such as this one, there is no room for error in the power design of conductor and wire terminations.

There are different Underwriters Laboratories (UL)-approved methods of crimping connectors to conductors for these applications, but identifying the proper installation method is easily done with the right tools. However, some confusion has arisen over the proper connectors and lugs to use with flexible cable. The National Electrical Manufacturers Association (NEMA)’s Bulletin No. 105 advises the industry 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 in applications where solid, B- or C-Code cables are used, they are not recommended for use with DLO cable. The mechanical connection of solid B- and C-Code cables can cause breakage of the fine strands in DLO cable, resulting in overheating and wire pullout. NEMA calls for compression-type connectors when terminating DLO (also known as fine-stranded flexible conductor) cable.

This article guides the proper selection of compression connectors and terminals (e.g., standard length, long barrel, copper splice, and reducer adapter) for these applications, as well as the proper method for installing these connectors to ensure a secure, trouble-free termination.


Flexible-conductor cable, like DLO, also commonly known as flex cable, has become increasingly popular in the past few years for conducting solar-powered electricity. Flex cable is easier to maneuver in tight spaces, above or below grade, particularly with sizes beginning at 250 million cubic metres (MCM), as well as when cable changes position frequently, as in underground applications.

There are many different flex cable classifications and strands, so it is important to match the termination to the proper conductor size or specification.

Regardless of whether connectors are manufactured for specific applications or are dual rated for both B- or C-Code and flex cable, they must be clearly identified for the proper application. This complies with UL standard 486 A–B, NFPA 70 (as with NEC and Standard for Electrical Installation NOM-001-SEDE, as well), which 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 [single input wire] or Class C stranding shall also be marked with the conductor class or classes and the number of strands.”

There is a wide variety of compression connectors that includes, for example, one- and two-hole lugs, butt splices, and H-taps and C-taps. There also are connectors available for copper, aluminum, or copper-clad aluminum conductors. Compression connectors offer several advantages over mechanical connectors:

  • When properly installed with the correct tooling, connections are permanent and cannot be loosened accidentally.
  • Connections are irreversible, which is sometimes required for grounding applications.
  • Low-profile crimps are easy to insulate.
  • Some 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 properly calibrated connection.


Traditionally, there were two crimping methods for installing compression connectors: the indent-style crimp, made by compression tools without dies, or the hex-style crimp, made by compression tools equipped with interchangeable hex dies.

Indent-style crimp. The indent-style crimp offers reliable electrical performance and approved, calibrated pullout resistance, as long as it is correctly done with a proper tool that corresponds to the size of cable and connector. An indent-style crimp leaves the connector with rounded edges and the ability to produce an arc flash. The strands are formed tightly together inside the connector, which eliminates virtually all air gaps from the conductor.

One disadvantage of an indent-style crimp is that it does not allow for the inspection of a proper crimp.

Hex-style crimp. The hex-style crimp has been the industry standard for crimping compression connectors onto B- and C-Code copper, and aluminum/copper cables up to 1,000 MCM. The hex-style crimp results in superior electrical performance and calibrated pullout strength, while hex dies emboss the die code onto the connector for easy inspection and verification of a proper crimp after installation.

Hex-Flex®die system. A third method of attaching connectors to flexible conductors was recently introduced. It combines the best of the indent- and hex-style crimp — superior calibrated pullout ratings and the ability to inspect for a proper QA/QC measured and documented crimp. The higher pullout values created also reduce the number of crimps required on most connectors.


With the correct tools, the proper installation of compression connectors is quick and easy:

• Prepare the cable in accordance with the appropriate quality procedures. Strip the insulation carefully and according to proper procedures to avoid damaging the conductors. Strip the insulation to the proper length, so that the conductors can be fully inserted into the connector barrel without exposing bare wire or un-insulated cable.

• Select the proper connector. Consult the wire schedule for the correct design criteria for the cable’s ampacity and voltage. Compression connectors include markings for the proper application, which are clearly visible on the connector, as well as on product packaging and enclosed literature. The connector must match the application.

Connectors are marked with several important pieces of information that ensure safe installation and comply with all applicable codes and regulations associated with this procedure:

  • Manufacturer
  • Application by color and die code
  • Wire size
  • Crimp indicator bands
  • UL and/or CSA listings

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.

Select the tool and proper die. A wide range of tools, from manual tools to battery-operated hydraulic crimping tools, makes installing compression connectors easier.

The Color-Keyed® method simplifies the selection of the proper installing die: colored bands or colored dots on the connector correspond to the colored markings on the dies. Connectors and dies also have a die code number marked or stamped on them. Dies also have a code number engraved on the crimped surface.

• Making the crimp. Be familiar with all applicable procedures, codes, and regulations. Locate the markings on the connector and die. Keep fingers away from the crimping mechanism. Insert the connector into the tool and align the die with the connector.

For multiple crimps, make the first one nearest to the tongue and work toward the barrel end. Connectors are banded with color stripes to indicate the number and location of each crimp, and marked with the die code number at each compression location.

When properly crimped, the die code number will be embossed on the connector to facilitate inspection to confirm that the correct die and connector combination was used.

With the increasing use of fine stranded, flexible conductor cable in solar power applications, the selection of the proper connector and connection method is critical. Compression connectors are preferable to mechanical connectors, but the installation must be properly executed. By following the steps outlined above, electrical contractors will be following NEMA- and UL-approved methods.