Which Is Better – Compression or Mechanical Connectors?

Determining the appropriate connector may not be that simple. Each style has its advantages and disadvantages. Let’s explore why one style is used over the other, and the benefits and challenges each has to offer.

Initial Cost

The cost of a mechanical connector is usually higher because mechanical connectors are range taking so they can accommodate a broad range of conductor sizes. Each electrical component manufacturer has developed these ranges for their connector series. The ranges developed must meet or exceed agency performance requirements per UL486A/B. An example of a range could be #4 AWG through 600 kcmil which includes 13 different size conductors #4, #2, #1, 1/0, 2/0, 3/0, 4/0, 250, 300, 350, 400, 500, and 600 kcmil. That is a broad range and arguably a stretch for a #2, but still meeting UL requirements, if used.

Another reason for a higher cost of mechanical connectors is the manufacturing processes. Mechanical connectors are made by extrusion when material is pushed through a die profile, much like the playdough toy you used as a kid. These extrusions come in ten-foot lengths, called “sticks,” to facilitate handling. These sticks must then be cut to length for each connector. The cutoff piece is referred to as the body of the connector. The body requires some machining, such as a basic drill and tap operation for the impingement screw and bolt hole(s). Lastly, the machined bodies are tin plated and assembled with the impingement screw. Specialized machinery assists in every step of this process.

Compression connectors are manufactured out of tubing. The tubing also comes in ten-foot sections. The tubing is cut to length depending on one- or two-hole requirement, chamfered, formed, and tin plated. A two-hole lug requires additional tongue length to accommodate the second hole. The greatest difference between mechanical and compression lugs is that compression lugs are designed for only one conductor size and there are no assembly requirements during manufacturing.

Cost Advantage: Compression

Installation Costs

Mechanical lugs are fairly easy to install. Follow the manufacturer’s instruction sheet as to what torque value is required to make an agency listed connection. The use of a torque wrench is recommended when making an assembly. Compression connectors require manufacturer’s recommended application tooling. Installation of a compression connector may specify the hydraulic head, specific die set, and number of crimps along the barrel beginning at the connector end farthest from the conductor and working towards the conductor. This technique will allow the barrel to extrude towards the conductor and the last crimp to have full contact with the conductor. Depending on the manufacturer’s connector being used, the purchase of additional dies may be required to meet the agency listing.

Installation Advantage: Mechanical

Conductor Types and Classifications

Mechanical connectors are better suited for a more rigid conductor, such as code classification B or C. In connector applications, resistance of the strands is applied to the connector walls when the impingement screw is properly torqued. When mechanical connectors are used on less rigid conductors with higher strand counts, like weld or battery conductors, the strands could separate, or be displaced, while the impingement screw is being tightened. This separation creates the potential for an inferior connection.

The use of mechanical connectors on less rigid conductors may not always deliver repeatable results. Repeatability is key when making electrical connections. Compression connectors are the superior choice for high strand count conductors. All of the strands are contained inside the barrel before the compression operation. The die profile is specifically designed to help ensure that assembly receives the correct compression ratio. Compression connectors demonstrate time and time again reliable and repeatable operations.

Conductor and Classification Advantage: Either, depending on which conductor classification is being installed

Inspectability

Mechanical connectors are not easy to inspect. Once the installer has released the pressure of the torque wrench, verification that torqueing was applied at the correct value cannot be done. The only way an inspector could conduct a check would be to overcome friction to move the screw. The value derived would be higher than the actual torque that was applied during assembly.

Alternatively, the crimping process for compression connectors generates a die embossment. Any electrical or QC inspector can identify the die by the embossment. By referring to the manufacturer’s instruction sheet, the correct number of crimps for the connector can be verified.

Inspectability Advantage: Compression

Reusability

Compression connectors are for one-time use, while mechanical connectors allow reassembly and reuse. The flexibility to reassemble is a benefit when equipment needs to be moved or connection needs to be reentered at a future time.

Reusability Advantage: Mechanical

Oxide Inhibitor

Oxide inhibitors are primarily used on aluminum connectors. The inhibitors help break through any aluminum oxides that may have formed prior to installation. Aluminum oxides are not conductive and could cause connector failure if not properly removed. Both mechanical and compression connectors are tin plated to help reduce any galvanic corrosion that might occur due to dissimilar metals. Because many mechanical connectors do not come with oxide inhibitor, a good practice is to apply oxide inhibitor during installation. Compression aluminum connectors are supplied with factory installed inhibitor which will break through the aluminum oxides during the crimping process. Excess inhibitor must be wiped away after crimping.

Oxide Inhibitor Advantage: Either, since both are agency listed

So, which is better?

Mechanical and compression connectors each have their pros and cons. Selection of the right connector for the job truly depends on the application. Consider a couple of applications that might help identify which connector would be the best fit.

Breakers most commonly use mechanical connectors for terminating power. The breaker manufacturer can limit the amperage being applied to a given breaker by offering a mechanical connector with a range that won’t accept conductors that exceed the ampere rating.

Rail Transit, like trains and subways, by nature have a lot of vibration due to expansion and contraction of rail joints. When a train passes over these joints, the train is jolted. Over time, the repetitive jolting could loosen a mechanical connector. In this application, compression would be the preferred method to securely join the electrical conductor.

Both mechanical and compression connectors have valuable roles in the electrical market. Select the method that has been proven over the years for target application and always follow the manufacturer’s recommended method of assembly.