Cable categories
What actually separates the twisted-pair categories, and how to choose between them. The answer is set by three things at once: the speed you need, how long the run is, and how much power the cable has to carry — a category that is right at 30 metres can be wrong at 90. Includes a selector that takes those three and tells you which cable to specify.
Speed, distance, and power decide it — not the bigger number.
PoE+ — up to 30 W (802.3at)
The 100 m limit is the whole channel — 90 m of solid horizontal cable plus up to 10 m of cords. Past 100 m nothing in this table applies.
Cat6
Gigabit runs on anything from Cat5e upward. Cat6 is the sensible new-install floor — barely more expensive than Cat5e, and it leaves 5 gigabit available later.
- If the cable is already in the wall and it is Cat5e, it will do this job. Certify it rather than replacing it on principle.
| Category | Bandwidth | Full 100 m channel | Shortened channel | Ø | Under PoE |
|---|---|---|---|---|---|
| Cat5e | 100 MHz | 1GBASE-T, and 2.5GBASE-T | — | ≈ 5.0 mm | Carries 802.3af and at without difficulty. Permitted for bt, but derate hard in large bundles — the thinner conductors run hotter. |
| Cat6— recommended for these inputs | 250 MHz | 1GBASE-T and 5GBASE-T | 10GBASE-T — 55 mReach depends on alien crosstalk from neighbouring cables, so a tightly bundled run may fall short of 55 m and a loosely dressed one may exceed it. TIA does not warrant 10G on Cat6 at any length. | ≈ 5.7 mm | Comfortable to 802.3at. Fine for bt on shorter runs and modest bundles. |
| Cat6a | 500 MHz | 10GBASE-T over the full 100 m channel | — | ≈ 7.1–8.0 mm | The sensible floor for 802.3bt at 90 W. Larger conductors mean lower resistance, less heat, and more margin in a full bundle. |
| Cat8 | 2000 MHz | — (not a horizontal cable) | 25GBASE-T and 40GBASE-T — 30 mSpecified only to 30 m, and always shielded. This is a data-centre patching cable for top-of-rack to server, not a building cable. | ≈ 9.0 mm | Capable, but irrelevant — nothing at this distance is powered over the same cable in practice. |
Cat5eStill perfectly good cable, and still what most existing buildings are full of. It is not a mistake to find it — it is a mistake to install it on a run you expect to carry 10 gigabit later.
Cat6The 10-gigabit figure is the trap. It is a conditional maximum, not a rating — if 10G matters, the cable to install is Cat6a.
Cat6aNoticeably fatter and stiffer. It eats pathway fill, needs a larger bend radius, and a conduit sized for Cat6 will not take the same count of Cat6a.
Cat8Specifying Cat8 for office drops is the classic over-buy: it cannot reach the far end of the floor, requires shielded terminations and proper bonding, and buys nothing a workstation can use.
- Is Cat6 enough, or should I install Cat6a?
- For gigabit, Cat6 is comfortably enough. For 10 gigabit over a full-length run, it is not — Cat6 carries 10GBASE-T only over a shortened channel, and that reach depends on crosstalk from neighbouring cables rather than being a rating. If 10 gigabit matters now or later, install Cat6a.
- How far will Cat6 carry 10 gigabit?
- Commonly quoted as 55 metres, but it is a practical limit rather than a specified one. Alien crosstalk from adjacent cables sets the real figure, so a tightly bundled run can fall short and a loosely dressed one can exceed it. TIA does not warrant 10GBASE-T on Cat6 at any length.
- Do I need Cat6a for PoE?
- Not for 802.3af or at, which Cat5e handles without difficulty. For 802.3bt at 90 W, Cat6a is the sensible floor — its larger conductors mean lower resistance and less heat, which is what actually limits a big PoE bundle in a warm ceiling.
- Should I just install Cat8 to future-proof?
- No. Cat8 is specified only to 30 metres, so it cannot reach the far end of most floors, and it is always shielded, which means every termination needs correct bonding. It is a data-centre patching cable for top-of-rack to server, not a building cable.
- What is the maximum length of an Ethernet cable?
- 100 metres for the channel, in every category — 90 metres of solid horizontal cable plus up to 10 metres of patch cords. Beyond that the answer is fibre with media conversion, or an intermediate telecom room.
The rest of the section
- 01RJ45 Wiring: T568A and T568BBoth colour codes, pin by pin, side by side.
- 02RJ11 Wiring and Phone PinoutsSix positions, three lines, two colour systems.
- 03Keystone Jack TerminationStrip, seat, punch, snap — without failing the certification.
- 0466 Block WiringFifty rows, four columns, and where the bridging clips go.
- 05110 Block WiringLay the pairs in, press the connecting block on.
- 06Fibre Types, Connectors and PolishRead the jacket, read the connector, don't mate blue to green.
- 07Subnetting, ExplainedWhere the numbers come from, and the meaning behind them.
- 08PoE Standards and Why It Won't PowerThe switch reserves the class, not the draw.
- 09Camera Lens, Field of View and Pixel DensityCovering the room is not the same as identifying a face in it.
- 10Patch Panels and Labelling to TIA-606A label that survives is worth more than a tidy rack.
- 11Rack Planning: Units, Depth, Weight and HeatIt fits vertically. That was never the hard part.
- 12Writing a Switch ConfigurationFrom factory default to a switch you would hand over.
- 13Networking and Cabling GlossaryThe terms, and what they mean on a job.
We do this work
as well as document it
Anyone specifying cable for a buildout, or working out whether what is already in the wall is good enough.