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I would like to make a run of cat 6 ethernet cable between 2 dwellings, underground. It appears I may need just a little over the maximum length for 1 Gbps (100 meters), maybe 110 meters or so. Putting a switch in between is impractical. And actually for the application, 200 to 300 Mbps should be ample. POE is not required.

Is there a formula or graph available which describes the degradation over length, eg maximum speed vs length?

Cat 6 is rated 50 meters for 10 Gbps and 100 meters for 1 Gbps. Perhaps it is something like, for every doubling of cable length, speed is reduced by a factor of 10?

As I already have the cable for the job I'd rather not add the expense of going with fiber.

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    OSP (Outside Plant) cabling is a specialty that must be left to professionals. For example, you need proper outside cabling, bury to a depth of 24" or below the frost line (whichever is deeper, use proper grounding, bonding, and lightning protection, not exceed the inside length of outside cable, etc. You could be both civilly and criminally liable if an electrical transient from lightning starts a fire in your equipment. See this answer for the required category test suite that requires an expensive tester.
    – Ron Maupin ♦
    Commented Jan 8, 2023 at 19:41
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    Remember that the horizontal cable length is a maximum of 90 meters of solid-core cable, and you are allowed 10 meters of stranded cabling divided between both ends to achieve the maximum of 100 meters. Your company needs to hire a professional install with OSP experience to do something like this.
    – Ron Maupin ♦
    Commented Jan 8, 2023 at 19:43
  • Other than flexibility, what is the purpose of the stranded cable at the ends?
    – KevinHJ
    Commented Jan 9, 2023 at 9:13
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    Flexibility is the reason for stranded patch cables. The horizontal cable uses solid-core because it performs better. but it is more fragile than stranded cable, and it does not get moved around like patch cables, which use stranded with less performance, but is much more flexible than solid core cable.
    – Ron Maupin ♦
    Commented Jan 9, 2023 at 13:24
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    The stranded cable is limited to 10 meters, divided between the two ends, not two meters. That is the standard. You need to buy and read ANSI/TIA 568-D, Commercial Building Telecommunications Cabling Standard. You seem to have a limited understanding of what modern network cabling entails; it is not simple electrical connectivity the way analog voice cabling is. Your company really needs to hire a cabling professional, preferably one that is BICSI certified.
    – Ron Maupin ♦
    Commented Jan 9, 2023 at 15:50

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Generally, the maximum length is 100 m for a twisted-pair cable with the correct specs (up to 90 m stiff, solid-core and up to 10 m flexible, stranded patch cable). In practice you can exceed maximum lengths somewhat but ignoring the standards isn't on topic here.

Ethernet doesn't probe the cable (with the notable exception of 'smartrate' 2.5/5/10 Gbit/s ports), so if it's below specs you'll get increasing errors and ultimately a link failure.

Perhaps it is something like, for every doubling of cable length, speed is reduced by a factor of 10?

No, it's not that simple. Different twisted-pair speeds use vastly different line codes and cable length isn't only about attenuation but also about (near-end and far-end) crosstalk which isn't linear.

Effectively, each cable behaves differently in fringe cases. If you want a reliable link I'd seriously recommend using fiber. It isn't really much more expensive if you use a pre-terminated cable in an empty conduit. Make sure the cable is suited for mixed indoor and outdoor use as per your local regulations.

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  • Other than flexibility, what is the purpose of the stranded cable at the ends?
    – KevinHJ
    Commented Jan 9, 2023 at 9:14
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    It's the flexibility that's required - you can't move solid-core cable around and plug it into a NIC, a switch or whatever.
    – Zac67 ♦
    Commented Jan 9, 2023 at 10:40
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Try it. If it works, great. If not, it is too long and it will either get a lot of packet loss or it will negotiate a lower speed, maybe 100 Megabit. If that happens and you need 1 Gigabit link speed, then switch to fiber optic.

The electromagnetic issues involved with copper cabling mean that max length specifications are designed for common use scenarios. A single cable in a conduit buried under ground is not a common use case (though it does happen fairly often) compared to the normal use of many cables in a wiring closet or office environment. You may get away with a slight bit of extra length without issue.

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