The client asked for "the best cable, spare no expense" for a two-floor office, and the shop salesman sold him pricey rolls of "shielded Cat7". The installer shows up, opens the box, and finds a thick, stiff S/FTP with a drain wire per pair that nobody knows where to ground. He ends up fighting every connector, the rack fills with loose braids hanging in the air, and to top it off the link certifies exactly like a Cat6A U/UTP at half the price. The field lesson is an old one: the "best" cable doesn't exist; the right cable for this job does.
Choosing copper well means understanding five decisions that travel together: the category (how much bandwidth), the shielding (how much noise protection), solid or stranded (stiffness versus flexibility), the AWG gauge (how thick the copper is) and the jacket (what the building code demands). None of them is a matter of taste or of what the box says: each has a measurable why. Let's take them one at a time.
Categories: what each one actually delivers
The category sets the cable's guaranteed bandwidth, in megahertz, and from there comes the speed and distance it can run. The trick is that the box sells you the big number and drops the distance. Here is what each category truly delivers, always over the full 100 m channel:
| Category | Bandwidth | Max speed | At that speed | Typical use |
|---|---|---|---|---|
| Cat5e | 100 MHz | 2.5 Gb/s | 100 m | Basic office, 1G, tight budget |
| Cat6 | 250 MHz | 10 Gb/s | 37–55 m | Sensible office default today |
| Cat6A | 500 MHz | 10 Gb/s | 100 m | 10G, WiFi 6/7, 4K cameras, backbones |
| Cat7/7A | 600/1000 MHz | 10 Gb/s | 100 m | Almost never: TIA doesn't recognize it |
| Cat8 | 2000 MHz | 25/40 Gb/s | ~30 m | Datacenter only (switch to server) |
Cat5e looked retired, but 802.3bz gave it a second life: it runs 2.5GBASE-T at a full 100 m, exactly what a modern WiFi 6 AP asks for. For plain 1 Gb/s it's still perfect and cheap. Cat6 is today's sensible office floor: 1G at 100 m, no drama. The number marketing hides is its 10G: Cat6 does run 10GBASE-T, but only up to 55 m in a good environment, and in a tight bundle with many cables running side by side the recommendation drops to 37 m because of the crosstalk between neighboring cables (alien crosstalk). Cat6A is the one that runs 10G over the full 100 m, which is why it's the answer whenever the project talks about ten gigabit across a floor.
Shielding: when to, and when not to
Shielding protects against external electromagnetic interference (EMI), and it comes in three flavors worth naming properly. U/UTP is unshielded: four twisted pairs and the jacket, that's it. F/UTP adds an overall foil wrapping all four pairs together. S/FTP is the full package: a braided screen around everything, plus an individual foil on each pair. The nomenclature reads "overall-shield / per-pair-shield-UTP".
When should you shield? In industrial plants full of variable-frequency drives, big motors and welding; near radio transmitters; in trays that share space with power; and in very dense 10G installs, where the foil cuts the alien crosstalk between tightly packed neighbors. Outside those cases —which is to say, the vast majority of offices, schools and clinics— a well-installed U/UTP certifies just the same and is far easier to terminate. Shielding adds stiffness, raises the cost of every connector and, above all, demands a flawless grounding chain.
Solid or stranded: each to its post
Copper comes in two builds. Solid is a single wire per conductor: lower attenuation, punches down cleanly into jacks and patch panels, and it's what goes in the permanent link —the fixed run through wall, trunking or tray. Stranded twists several fine wires per conductor: it bends a thousand times without breaking, which is why it's what patch cords are made of. The price of that flexibility is more attenuation, on the order of 20 % more per meter.
That's why the 100 m channel splits into 90 m of solid plus 10 m of stranded: the standard did its math with exactly that mix. Adding extra patch cords, or —worse— punching solid cable into a patch-cord plug (which expects stranded wires), steps outside the model and collects intermittent faults. Rule of thumb: solid for what doesn't move, stranded for what gets plugged and unplugged.
The AWG gauge: thickness matters (especially with PoE)
AWG measures conductor thickness, and it runs backwards from intuition: smaller number, thicker copper. A 23 AWG Cat6 is thicker than a 24 AWG one and has about 21 % less resistance (66.6 vs 84.0 Ω/km per conductor). Less resistance means less voltage drop, less heat and more usable distance, especially when the cable carries power. That's why 23 AWG is the favorite for high PoE and long runs.
At the other end are the "slim" 28 AWG patch cords, those skinny ones that look tidy in the rack. They're handy and take little space, but their thin copper has considerably more resistance: you must shorten the channel distance when you use them (makers publish reduction tables) and you shouldn't run them on links with high PoE. Lovely for 30 cm between panel and switch; bad as a replacement for a 5 m patch that also powers a camera.
PoE and heat: the bundle heats up
Today that same twisted pair carries watts: 802.3af delivers 15.4 W, 802.3at raises it to 30 W, and 802.3bt (PoE++, Type 3 and 4) reaches up to 90 W from the switch. That current heats the copper, and when you bundle 24, 48 or more PoE cables tightly, each one's heat adds up and the core of the bundle climbs in temperature. The problem is twofold: copper resistance rises with temperature (more drop, fewer watts to the device) and attenuation gets worse too. That's why the standards apply a derating: the fuller and tighter the bundle, the lower the current capacity per cable.
The recipe against heat has three parts. First, thick gauge: for PoE Type 3/4, 23 AWG (and, frankly, Cat6A) dissipates and drops less. Second, don't saturate the pathway: leaving air between cables helps the heat escape. Third, don't coil the slack: that tight loop behind the camera was harmless with data, but with PoE it's a miniature space heater where every turn warms its neighbor. Leave a loose "S" or cut to length. You can size the total switch load with the PoE budget tool and check the effect of distance with the voltage drop calculator.
The jacket: it's a code matter, not a preference
The outer jacket doesn't change data performance, but it defines where you're allowed to run the cable under the building and fire-safety code. You don't pick it because it looks nice: you pick it by where it goes.
| Rating | Name | Where it goes | Why |
|---|---|---|---|
| CM | General | Within a single floor, horizontal | Common use, no special fire requirement |
| CMR | Riser | Vertical shafts between floors | Resists fire climbing the shaft |
| CMP | Plenum | Above a drop ceiling used as air return | Low flame and little toxic smoke in the breathed air |
| LSZH | Low emission | Tunnels, hospitals, enclosed spaces | Little smoke, no halogens: safer evacuation |
The costliest confusion is plenum: if the drop ceiling doubles as air-conditioning return (very common in offices and malls), the code requires CMP in that space, because a fire there sends smoke straight into the ducts that ventilate the building. Running CM where CMP was required goes unnoticed by everyone… until the inspection or the fire. And in Chile and much of the world, LSZH is increasingly the default for public spaces, because of the smoke. Always check what the project spec demands before buying the roll.
The decision, no detours
Let's boil it all down to field rules. Cat5e when the project is plain 1G, no heavy PoE, tight budget: it's still honest and cheap, and it even gives you 2.5G for an AP. Cat6 as the sensible floor of a new office when 10G isn't on the horizon or the runs are short. Cat6A when you want 10G at real distance, comfortable PoE++ and a future-proof install: it's today's sweet spot. "Cat7" in an office is almost always marketing, and "Cat8" is datacenter. On shielding: U/UTP by default, and F/UTP or S/FTP only when the environment (industrial EMI or very dense 10G) justifies it and you can ground it properly.
A real case to tie it together: you have to pick cable for a WiFi 6 access point with PoE++ (802.3bt Type 4), 90 m from the switch, and the client wants it "10G-ready" for the next upgrade. The reasoning, step by step:
- Distance: 90 m of permanent link + patch cords fit inside the 100 m channel. Any category from Cat5e up meets the distance for 1G, so distance alone doesn't decide. Confirm it with the TIA-568 channel.
- 10G-ready requirement: at 90 m, Cat6 doesn't run 10GBASE-T (its ceiling is 55 m, and less in a bundle). Only Cat6A guarantees 10G at 90–100 m. That already rules out Cat5e and Cat6.
- High PoE: with PoE++ (802.3bt) the switch delivers up to 60 W per port on Type 3 and up to 90 W on Type 4 (PSE); the load receives about 51 W or 71 W (PD) depending on the type. That current heats the copper and drops with distance, so the thicker conductor is preferable: choose 23 AWG, standard on good-brand Cat6A.
- Shielding: it's an office, not an industrial plant. A U/UTP Cat6A certifies 10G with no trouble and is easier to terminate; there's no EMI to justify shielding or the hassle of grounding braids.
- Bundle and heat: since it carries PoE++, don't saturate the trunking and don't coil the slack; leave air. Check the fill with the conduit fill calculator.
- Conclusion: Cat6A U/UTP, 23 AWG, jacket per where it runs (CMP if it goes above a ceiling with air return, LSZH if the project asks for it). It meets the distance, guarantees the future 10G, and the thick gauge appreciates the high PoE.
With that you have the whole map. Validate distances against the 100 m channel, check the load with the PoE budget, watch the conduit fill, and keep the color code handy when you terminate. The "right" cable isn't the most expensive or the most shielded: it's the one physics and code approve for your job.