Not all copper wire products are the same – in fact, even the way the wires are stranded can have a massive impact on their application.

Cable strand bundling is an art and a skill. A bundled wire is more flexible than solid copper wiring, has reduced metal fatigue, and allows heat to dissipate more quickly. Its increased flexibility makes it easier to pull through conduit, around bends, and install in tight spaces, though you lose some current capacity.

How your cable is stranded may also dictate other things, including its class. But what does stranding look like and how does it work?

What Types of Cable Stranding Are There?

As with most industries, if you want to become more familiar with the stranding process, there is some manufacturing jargon you’ll need to know.

Bunched – Like a handful of dry spaghetti, a bunched cable features copper strands compiled in no particular geometric arrangement. Bunched cable is less expensive to produce because the process to make it isn’t as intricate and doesn’t require many additional steps. This wire is more flexible than a solid copper conductor of the same gauge, but not as flexible as other options mentioned later.

Concentric – Concentric copper wire stranding involves one or more layers of strands applied around a central wire called the core. Layers are added in sixes, with the first layer being six strands, the second 12, the third 18, and so on. The direction reverses with each layer added.

Each layer added also increases its lay length. The lay refers to how long something is when it’s laid flat on the ground. In the case of a concentrically bundled wire, the middle strand is the longest, with each layer being a little shorter.

Though it isn’t as flexible as bunch-stranded wire, concentric cables are strong, durable, and crush resistant, making them great for power distribution. It’s also worth noting that concentrically laid cables can be bundled in several ways, decreasing their overall size.

  • Round – This is your standard concentric stranding layout. If you cut the wire and look at a cross-section of it, you’d see air pockets between the individual strands. Concentric strands in a round pattern have a larger diameter than a solid conductor of the same gauge.
  • Compressed – For a compressed concentric layout, the conductor is assembled and pulled through a die to bring the strands closer together. There are still air pockets, but the diameter drops to about 97% of the round configuration.
  • Compact – Compact concentric layouts require additional pre-forming for the strands before they’re bundled. Strands are shaped into trapezoids and then placed together to remove as much empty space as possible. Although this is the smallest version of a concentrically bundled cable, it’s still not as small in diameter as a solid conductor.

Unilay – Unilay bundled cables are very similar to concentric, but all the layers go in the same direction. The resulting wire has a smaller diameter and can be in round, compressed or compact layouts to decrease the diameter even further.

Rope Lay – Rope lay is the most intricate form of cable bundling. For a rope lay, smaller groups of strands are twisted together. The resulting bundles are then twisted together to create a larger one. Rope lay cables are the most flexible stranding type and are typically found in large gauge cables.

What Types of Cable Classes Are There?

Like bundling options, cables can fall into several classes, each with its own applications.

Concentric-Lay Conductors

  • Class B: General power cables, like THHN and XHHW, among other standard 600v control cables.
  • Class C: These are just more flexible versions of Class B cables.
  • Class D: Power cables with extra flexible stranding.

Rope-Lay and Bunch Stranded Conductors

  • Class G: These are portable flex power cables designed for portable use.
  • Class H: These cables are extremely flexible, for example, take-up reels.
  • Class I: This is a rope-lay style made using bunched stranded conductors. This class works for apparatus cable and motor leads.
  • Class K: Class K stationary service cords and cables are rope-lay made using bunched stranded conductors comprised of #30 AWE copper. One example of this class is SIS, which is used in switchboards and panelboards.
  • Class M: This rope-lay cable is made from bunched stranded conductors using #34 AWG copper and is considered a constant service cord. Primarily used in portable cords, an example would be DLO (Diesel Locomotive), though cables like this are found on cranes, in shipyards, mining operations, and other heavy-duty locations.

Why Does Stranding and Class Matter?

In the same way your flexibility may impact your ability to run an obstacle course, stranding and class directly relate to how well a wire will react during and after installation.

Stranding type impacts how flexible the wire is overall, its diameter, and even how much heat it can dissipate. Even the type of metal used in the strands can make a difference, with the options of either copper or tin-coated copper available.

The class type also matters, depending on the application and the flexibility you’ll need to effectively work with the cable. Class B, C, and D are durable electrical cables that are usually more cost-effective than other options. For example, electrical utilities use these wires for primary and secondary voltage applications.

Other classes, G through M, are designed to be easy to bend, strong, and malleable. Popular applications for these cables include machinery and machine tools, power cords, extension cables, and tray cables.

Want to Learn More?

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Kris-Tech’s experienced team can guide you through the wire ordering process, ensuring the right products are shipped at the right time.

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