Выбор между скрученным многожильным и одножильным кабелем
13 июня 2022 г. / Общие сведения, 101 обучение, установка и тестирование, передовые практики
You’re probably aware that both shielded and unshielded twisted-pair copper cable comes in either stranded or solid wire versions. There are plenty of considerations when it comes to choosing one or the other, including standards, environment, application, and cost. Let’s take a look at the differences and considerations, so you can decide which cable type is right for your specific installation.
The Differences Between Stranded and Solid Wire Cable
The first difference between stranded and solid wire cable is in the way they’re made: the terms stranded and solid refer to the actual construction of the copper conductors within the cable.
- • In a stranded cable, each of the eight copper conductors are made up of multiple “strands” of small-gauge wires that are wound together concentrically in a helix, much like a rope. Stranded wire is typically specified as two numbers, with the first number representing the quantity of strands and the second representing the gauge. For example, a 7X32 (sometimes written as 7/32) indicates that there are 7 strands of 32 AWG wire that make up the conductor.
- • In a solid cable, each of the eight conductors are made up of a single larger-gauge wire. Solid wire is specified by just one gauge number to indicate the size of the conductor, such as 24 AWG. Whether it’s a one-, two-, or four-pair cable, the gauge of the solid conductor is the specification for the cable.
This leads to the most noticeable difference: flexibility. Stranded cables are much more flexible and can withstand more bending; in terminations, however, those flexible strands can break or become loose over time. Solid cables are more rigid and can break if flexed too far or too frequently, but solid conductors will hold their shape over time and seat properly within insulation displacement connectors (IDCs) on jacks, patch panels, and connecting blocks.
A less-obvious difference between stranded and solid wire cable is performance. In general, solid cables are better electrical conductors and provide superior, stable electrical characteristics over a wider range of frequencies. They’re also considered more rugged and less likely to be affected by vibration or susceptible to corrosion, since they have less surface area than stranded conductors.
Solid wire can also carry more current than stranded wire. Higher-gauge (thinner) conductors have more insertion loss than lower-gauge (thicker) conductors; stranded cables exhibit 20 to 50 percent more attenuation than solid copper conductors (20 percent for 24 AWG and 50 percent for 26 AWG). Because the cross section of a stranded conductor is not all copper (there’s some air in there), they also have higher DC resistance than solid cables.
Now that you understand the differences between stranded and solid cable, let’s cover what you need to consider when making a choice.
Is Stranded or Solid Wire Better?
When it comes to 90-meter horizontal permanent links, there’s no choice: whether it's shielded or unshielded twisted pair, both TIA and ISO/IEC standards require solid cable. Stranded cable (24 and 26 AWG) is limited to patch cords and 10-meter lengths within a 100-meter channel.
Because stranded cables are more flexible and can withstand bending, they make excellent patch cords for equipment connections and cross-connects where cables are frequently bent and manipulated; at just 10 meters of the channel, the increased insertion loss and resistance aren’t a factor in the overall channel performance. However, smaller 28 AWG stranded patch cords that have even more insertion loss and resistance due to their smaller gauge do have some limitations — get the Skinny on 28 AWG Patch Cords to learn more.
Open office environments, however, are special situations; they’re faced with regular reconfigurations and may require a more flexible cabling system. In those installations, the standards allow stranded patch cords to take up more than 10 meters of the channel. However, if you’re using more than 10 meters of stranded cable in a channel, industry standards require de-rating the overall channel length to accommodate for the greater insertion loss and DC resistance.
When it comes to de-rating stranded cable per industry standards, the overall gauge is a factor: higher-gauge (thinner) cables have a higher de-rating factor. The de-rating for 26 AWG stranded cable is 0,5, while 24 AWG is only 0,2, and 22 AWG stranded cables require no de-rating at all.
Here are the calculations to determine overall channel length, where H=horizontal cable length, D=de-rating factor, C= total stranded cable length and T=total channel length.
For example, if you’re using 60 meters of horizontal solid category 6A cable and 40 meters of stranded 24 AWG category 6A patch cable with a 0,2 de-rating factor, the total length of the channel must be reduced to 97,5 meters. (If you prefer the actual math: total stranded cable length = [105-60]/[1 + 0,2], or 37,5, and total channel length = 60+37,5, or 97,5 meters.) If you’re using 26 AWG stranded cable with a 0,5 de-rating, the channel length needs to be reduced to 90 meters.
PoE Makes Greater Demands on Stranded Cable Patch Cords
While stranded cable is the norm for patch cords at patching areas in the telecommunications room (TR) and at the work area (perhaps longer than 10 meters in open offices), a primary application to consider in today’s LANs warrants the use of solid patch cords: Power over Ethernet (PoE). When PoE is delivered over twisted-pair copper cable, some of the power dissipates as heat. When power dissipates as heat, the cable temperature increases. With higher insertion loss and DC resistance, stranded patch cords are more likely to exhibit degraded transmission performance at elevated temperatures.
While not typically a concern in environmentally controlled spaces like the TR, stranded patch cords could become an issue once you start connecting devices in the ceiling — think wireless access points, security cameras, LED lights, and more. In best practice, if the environment isn’t temperature controlled and the cable doesn’t have to bend much, solid cable patch cords should be used. If you do use stranded patch cords in uncontrolled environments, it’s better to keep them short (about 5 meters or less). And when it comes to higher-temperature environments, industry standards require de-rating channel length for that, too; more cables in a bundle, each generating more heat, can require even more length de-rating (though there can be An Exception to Every Rule).
Balancing the Cost Difference Between Stranded and Solid Wire
While more strands in a conductor mean greater flexibility, a higher strand count also means a higher price. To keep costs down, stranded category 6 and category 6A cable are designed to strike a balance: enough strands to maintain proper flexibility, but not so many that it creates a dramatic price difference. You don’t have to compromise performance (or standards compliance) by selecting stranded wire cable instead of solid for environments and applications for which it’s not suited. Keep your stranded cables in environmentally controlled areas that require greater flexibility, and use solid wire cable where rugged performance (and little bending) is required.
Whether it’s patch cords, permanent links, or channels, you can configure a Fluke Networks Versiv™ tool to certify the cable's performance.