LAN Cable Length Limit: 100 m, Patches and Derates

October 24, 2026

Quick Answer / Summary: An Ethernet channel is limited to 100 m: 90 m of permanent link plus 10 m of patch cords at each end, for 1000BASE-T on Cat5e, Cat6 and Cat6a. 10GBASE-T runs 100 m on Cat6a but only 55 m on Cat6. Beyond 100 m, use fiber or a copper extender rather than a coupler. PoE devices lose voltage over distance, so check the voltage at the device on long runs.

The 100 m figure is a channel limit, not a cable limit. It includes every patch cord, coupler and outlet in the path, which is why a 95 m drum plus two 5 m leads fails even though no single cable looks too long.

Channel, Link and Patch Cords

Standards split the path into a permanent link, which is the cable in the wall, and a channel, which adds the patch cords at both ends. The permanent link is allowed 90 m; the channel is allowed 100 m overall.

Field testers report both. If a certification report shows a 92 m permanent link, the installation already exceeds the standard and no patch cord choice will fix it.

Why 100 m

The limit comes from round-trip delay and attenuation at the highest specified frequency. Beyond it, the receiver cannot reliably distinguish symbols, so the link either trains at a lower speed or drops.

Couplers and outlets

Every mated connection adds insertion loss and a reflection. Two couplers in a channel can consume the margin that the cable had left, which is why daisy-chained patch leads are a bad habit.

Speed Changes the Limit

1000BASE-T runs 100 m on Cat5e and above. 2.5GBASE-T and 5GBASE-T also reach 100 m on Cat5e and Cat6 respectively. 10GBASE-T is where cable category starts to shorten the reach.

Cat6 supports 10GBASE-T to 55 m in good conditions; Cat6a supports it to the full 100 m. Above 10G, copper stops being practical and fiber takes over.

Cat6 at 10G

The 55 m figure assumes limited alien crosstalk. In a dense bundle, real-world reach can be shorter. If 10G is the plan, specify Cat6a rather than hoping Cat6 passes.

Temperature and bundling

Hot ceilings and tight bundles raise attenuation. Cables run through a roof void in summer can behave worse than the datasheet suggests, so leave margin on long runs.

Power Over Long Runs

PoE delivers power over the same pairs, and the cable resistance converts some of it into heat. A 30 W load on a 100 m run of 24 AWG cable loses a measurable share of its voltage before it reaches the device.

Cameras that reboot at night or access points that drop under load are often a voltage problem, not a data problem. Measure at the device end and consider 23 AWG cable for long high-power runs.

PoE classes

Higher classes mean more current and more loss. A Class 4 device at 100 m is the worst case for a thin conductor, and the switch may refuse to deliver full power if it cannot detect the expected signature.

Heat in bundles

Bundled PoE cables warm up, which raises resistance and attenuation together. Keep large PoE bundles ventilated and avoid running them inside insulated voids.

Length Planning

Reach for each common combination.

Standard	Cable	Max channel	Note
1000BASE-T	Cat5e or better	100 m	90 m permanent plus 10 m cords
2.5GBASE-T	Cat5e or better	100 m	Check alien crosstalk in bundles
5GBASE-T	Cat6	100 m	Cat5e may be shorter
10GBASE-T	Cat6	55 m	Sensitive to bundle density
10GBASE-T	Cat6a	100 m	Design target for new builds

Real Installations

Two common over-length situations.

Warehouse run to a far corner

At 130 m, no copper standard applies. Run fiber to a small switch in the corner and distribute copper from there, or use a copper extender rated for the distance. A coupler in the middle does not extend reach.

Home office in a detached garage

A 60 m buried run is fine for 1G. Leave the permanent link at 55 m or less if 10G might follow, and terminate on a patch panel at each end so the channel stays measurable.

Frequently Asked Questions

Is the 100 m limit per cable or per link?

Per channel, end to end. It includes patch cords, outlets and couplers, with 90 m allowed for the permanent link.

What happens if I exceed 100 m?

The link may train at a lower speed, show CRC errors, or drop entirely. The behaviour is unpredictable, which is why the limit is treated as a design rule.