2020 年 1 月 9 日 / General
You’ve probably heard of balanced twisted-pair copper cabling referred to as either stranded or solid, and if you’re on the fence as to which you should use when and where, you’ve come to the right place. There are plenty of considerations when it comes to choosing, including standards, environment, application and price. Let’s take a look at the differences and delve into these considerations so you know which cable type is right for your specific situation.
When it comes to twisted-pair copper cable, the terms stranded and solid refer to the actual construction of the copper conductors within the cable, and the names themselves make it obvious to tell them apart. In a stranded cable, each of the eight copper conductors are made up of multiple “strands” of small gauge wires that are concentrically wound together in a helix, much like a rope. Stranded cable is typically specified as two numbers with the first number representing the number 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 just one solid larger gauge wire and are specified by just one gauge number to indicate the size of the conductor, such as 24 AWG.
With both stranded and solid cables within the same category (i.e., category 5e, category 6 or category 6A) specified by one gauge size, this can seem a bit confusing. Just remember that whether the conductor is made up of multiple strands or one solid conductor, the final overall size of the conductor is the same. In other words, a 24 AWG cable is still a 24 AWG cable.
The most significant difference between stranded and solid cable is performance. Because higher gauge conductors (thinner) have more insertion loss than lower gauge (thicker) conductors, stranded cables exhibit 20 to 50% more attenuation than solid copper conductors (20% for 24 AWG and 50% for 26 AWG). And because the cross section of a stranded conductor is not all copper (there’s some air in there), they also have a higher DC resistance than solid cables. In general, solid cables are better electrical conductors and provide superior, stable electrical characteristics over a wider range of frequencies. They are also considered more rugged and less likely affected by vibration or susceptible to corrosion since they have less surface area than stranded conductors.
Another difference is flexibility. Stranded cables are much more flexible and can withstand more bending compared to rigid solid conductors that can break if flexed too many times. However, when it comes to terminating stranded cable, the individual strands of the conductors can break or become loose over time. Solid conductors will hold their shape and seat properly within IDCs on jacks, patch panels and connecting blocks.
Now that you understand the differences between stranded and solid cable, let’s cover what you need to consider when making a choice.
Industry Standards and Environment
When it comes to 90-meter horizontal permanent links, there really is no choice as 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, and at just 10 meters of the channel, the increased insertion loss and resistance are not 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. Check out our blog for the Skinny on 28 AWG Patch Cords.
There are special situations in open office environments where standards allow stranded patch cords to take up more than 10 meters of the 100-meter channel as they recognize that offices are faced with regular reconfigurations and may require a more flexible cabling system. However, if 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 derating 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. The calculations to determine overall all channel length are as shown, where H=horizontal cable length, D=de-rating factor, C= total stranded cable length and T=total channel length.
For example, if 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 using 26 AWG stranded cable with a 0.5 de-rating, the channel length then needs to be reduced to 90 meters.
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 office areas), there is a primary application to consider in today’s LANs that warrants the use of solid patch cords – power over Ethernet. When PoE is delivered over twisted-pair copper cabling, some of the power dissipates as heat. When power dissipates as heat, the temperature within the cable can increase. 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, once you start connecting devices in the ceiling (think wireless access points, security cameras and LED lights), stranded patch cords could be an issue. A good rule of thumb is that if the environment is not temperature controlled and there’s not a lot of manipulation going on (i.e., bending), patch cords should be constructed using solid cable. And 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, and more cables in a bundle generating more heat can require even more length de-rating (yes, we did a blog on that too).
What’s the Price Difference?
While more strands in a conductor means greater flexibility, strand count has an impact on price – the more strands that make up a cable, the greater the cost. To keep costs down, stranded category 6 and category 6A cable are designed with enough strands to maintain proper flexibility but not so many that it creates a dramatic price difference. In other words, the delta is really not enough to compromise performance (or standards compliance) by selecting stranded cable over solid for environments and applications for which they’re not suited. Keep your stranded cables to environmentally-controlled areas that require greater flexibility.