Conductor Stranding: Solid vs. Stranded Conductors, Types of Stranding, and Stranding Classes
Solid vs. Stranded
Stranding is an important aspect of cable construction to consider. Some cables are available in solid and stranded forms, but smaller cables are usually solid, and larger cables (usually larger than 6 AWG) tend to be stranded. When you have the option to choose it's important to understand the construction of your cable and how the differences between solid and stranded conductors could affect your application.
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Solid: Solid conductors consist of just one strand of metal. They are easier to terminate than stranded conductors and are mechanically tough and inexpensive. The major disadvantage of solid conductors is their lack of flexibility. |
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Stranded: Stranded conductors are composed of multiple metal strands bunched together in any number of configurations (discussed below in “Types of Stranding”). They are much more flexible than solid conductors, and the higher the strand count, the more flexible they are. While stranding can be more expensive, it is essential for any wire or cable that will need to withstand any kind of movement or flexing. |
Types of Stranding
If a cable with a stranded conductor seems to be the appropriate choice for your application, you then need to consider the type of stranding. Although most constructions are specified by regulatory agencies, it is helpful to know some basic information about each type.
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Bunched: Strands are gathered together with no particular design or arrangement. This is the least expensive type of stranding as it involves the least amount of time and labor. Bunched stranding provides great flexibility. |
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Concentric: Strands are arranged in a circular pattern. Each layer alternates direction and has an increasing lay length (the center strand is longest). Concentric stranding is mostly chosen for its mechanical strength and crush resistance. |
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Unilay: Strands are arranged in a circular pattern, but all layers are twisted in the same direction and share the same lay length. A unilay arrangement is lightweight and allows for a small diameter. |
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Rope lay: Strands are arranged into cabled groups. Each group usually consists of 7, 13, 19, or 27 strands since those counts can be gathered into a circular configuration easily. Rope lay is the most flexible type of stranding and is generally found in cables size 10 AWG and larger. |
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Equilay: Strands are similar to those in concentric stranding, but lay length is the same for each layer. |
Bunched, unilay, and concentric are the most common types of stranding. Be sure to check spec sheets and standards for stranding regulations.
Stranding Classes
Stranding classes are designed for different applications. See the list below for common uses of each stranding class.
Concentric Lay
- Class B: Power cables
- Class C: Power cables; more flexible than Class B
- Class D: Power cables; extra flexible stranding
Rope Lay and Bunched
- Class G: All cables for portable use (concentric lay)
- Class H: All cables which require extreme flexibility (concentric lay)
- Class I: Apparatus cable and motor leads (bunched)
- Class K: 30 AWG copper wires; used for stationary service (bunched)
- Class M: 34 AWG copper wires; used for constant service (bunched)
To learn more about conductors, watch our "Cable Basics 101: Conductors" video.