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Field guide · Cabling & low-voltage

Cat5e vs Cat6 vs Cat6a vs Cat8

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.

Diagram

Speed, distance, and power decide it — not the bigger number.

Speed you need
Power over the same cable

PoE+ — up to 30 W (802.3at)

70 m

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.

Specify

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.
Twisted-pair category comparison
CategoryBandwidthFull 100 m channelShortened channelØUnder PoE
Cat5e100 MHz1GBASE-T, and 2.5GBASE-T—≈ 5.0 mmCarries 802.3af and at without difficulty. Permitted for bt, but derate hard in large bundles — the thinner conductors run hotter.
Cat6— recommended for these inputs250 MHz1GBASE-T and 5GBASE-T10GBASE-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 mmComfortable to 802.3at. Fine for bt on shorter runs and modest bundles.
Cat6a500 MHz10GBASE-T over the full 100 m channel—≈ 7.1–8.0 mmThe sensible floor for 802.3bt at 90 W. Larger conductors mean lower resistance, less heat, and more margin in a full bundle.
Cat82000 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 mmCapable, 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.

Fig. 01Set the three things that actually decide it — speed, power, and channel length — and the recommendation follows. The table underneath marks the answer and shows what each category gives up.

Questions

Questions people actually ask

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.

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