The 100-Metre Rule and Where It Comes From
Every Ethernet installation lives under one quiet rule: a single cable run must not exceed 100 metres (328 feet). This limit comes from the TIA-568 structured-cabling standard, which divides the run into 90 metres of solid-core "horizontal" cable inside walls, plus up to 10 metres of flexible patch cords at each end. It is not a suggestion — it is the distance the protocol's electrical budget was engineered around, and crossing it makes the network behave unpredictably.
Why exactly 100 metres? The answer is half electrical and half historical. Electrically, copper attenuates the signal the further it travels, and beyond about 100 metres the loss budget of a standard Ethernet transceiver runs out. Historically, Ethernet's original collision-detection scheme assumed a round-trip propagation delay consistent with roughly this distance. Modern full-duplex links rarely use collision detection, but the length limit survived because the physics demands it.
Attenuation: What a Long Cable Actually Does to the Signal
Attenuation is the gradual weakening of the electrical signal as it pushes through copper. Every metre of cable eats a small, measurable slice of the signal's strength, and its appetite grows with frequency — the higher the data rate, the faster the signal decays. A cable that handles 1 Gbps comfortably can be marginal for 10 Gbps, because 10GBASE-T transmits at roughly 400 MHz compared with 125 MHz for 1000BASE-T, and copper punishes high-frequency signals far more aggressively.
A rough rule of thumb for decent twisted-pair cable is about 0.2 dB of insertion loss per metre at 100 MHz. Over 100 metres that becomes roughly 20 dB of total loss — and a 1-gigabit Ethernet receiver is designed to recover a signal degraded by about that much. Stretch the same cable to 130 metres and the arithmetic stops working: the receiver can no longer tell ones from zeros reliably.
Formula to remember: Total loss (dB) ≈ attenuation per metre × cable length. Compare the result against the loss budget of your link speed (roughly 20–24 dB for 1 Gbps at 100 MHz). If the run's loss exceeds that budget, expect corrupted frames, a link that renegotiates down to a slower speed, or no link at all.
The estimator linked below does this math for you, but it helps to understand why the number matters.
Why Speed Does Not Fade Gradually
Wi-Fi fades gracefully — walk away from the router and your speed decays in steps. A cable does not behave that way. Because Ethernet is a digital link, it works at full speed right up to the edge of its budget, then collapses. A 95-metre run and a 30-metre run deliver the identical full gigabit; a 120-metre run may deliver nothing at all. There is no gentle slope — there is a cliff.
What happens past the limit is packet loss. Weak, distorted bits arrive corrupted, frames fail their checksums, and TCP retransmits them. Throughput does not halve neatly; it stutters and craters while latency spikes. Many network cards then "renegotiate" the link to a lower speed such as 100 Mbps, which looks like a mysterious slowdown on a run that worked fine for years — the classic symptom of a cable that is slightly too long, or of one that has degraded.
Cable Category Changes the Distance Math
Not all twisted-pair cable is equal. The category printed on the jacket decides how far each speed can travel, because better cable loses less signal per metre:
| Cable type | 1 Gbps | 10 Gbps | Notes |
|---|---|---|---|
| Cat5e | 100 m | — | The minimum for gigabit; fine for most homes |
| Cat6 | 100 m | 55 m | Tighter twists and separators; best value for new installs |
| Cat6a | 100 m | 100 m | Thicker, often shielded; full 10-gigabit range |
| Cat7 | 100 m | 100 m | Heavily shielded; mostly a data-centre choice |
| Cat8 | 100 m | 30 m (40G) | Short high-speed runs between switches |
Two runs of the same length can behave very differently. A certified Cat6a run holds 10 Gbps across the full 100 metres, while a Cat6 run at the same distance is only rated for about 55 metres at 10 Gbps. And anything older than Cat5e should be replaced, not measured.
The Hidden Killers: Cheap Cable and Bad Terminations
Even a run comfortably under 100 metres can fail if the cable itself is substandard. Copper-clad aluminium (CCA) cable — common in the cheapest reels — has roughly 55% higher DC resistance than pure copper and degrades badly past 60–80 metres; it is not compliant with TIA standards at all, yet it floods online marketplaces. Bad terminations are the other classic: a single poorly punched keystone jack or a nicked conductor can introduce more loss than twenty metres of good cable.
Other culprits worth checking: flat "designer" cables that cheat on the twisted-pair geometry, cable run parallel to mains power lines (electromagnetic interference couples straight into the pairs), and patch cords that push the total "channel" beyond the length you measured on the plan.
How to Plan a Long Run Before You Pull It
- Measure the route, not the straight line. Add slack, vertical drops and service loops, then add 10% contingency.
- Keep the solid-core permanent link to 90 metres and the patch cords to 10 metres total — that is the 100-metre channel budget.
- Buy pure-copper, properly certified cable. Avoid CCA and unmarked reels no matter how cheap.
- Keep data cable at least 20 cm from mains power; where they must cross, do it at right angles.
- Test every run before closing the walls — a cable certifier is ideal, a wire-map tester is the minimum.
- Label the installed length on the patch panel. Future-you will be grateful.
Estimate Your Loss Before You Run the Cable
Rather than guessing whether your planned run is safe, plug the numbers into the site's estimator:
https://veltrixnews.online/tools/network-cable-length-loss-calculatorEnter the cable category, length and target speed, and it estimates the signal loss and tells you whether the run sits inside the safe budget — much cheaper than discovering the answer after the cable is in the wall.
Four Ways to Fix a Run That Is Too Long
- Insert a switch at the midpoint. Each Ethernet segment resets the distance budget, so a switch at 90 metres buys you another full 100.
- Convert to fibre. A media-converter pair with a fibre run goes kilometres without breaking a sweat — the real answer for outbuildings and garden offices.
- Use a PoE-powered Ethernet extender where there is no power available at the midpoint.
- As a fallback, use powerline adapters or a point-to-point wireless bridge. Either beats a dead 130-metre copper run.
Frequently Asked Questions
What is the maximum length of an Ethernet cable?
100 metres (328 feet) per the TIA-568 standard: 90 metres of solid-core cable plus up to 10 metres of patch cords. Beyond that, signal loss causes dropped packets, speed renegotiation, or a dead link.
Does a longer Ethernet cable slow down my internet?
Not gradually. Ethernet is digital: it runs at full speed until the signal degrades past its budget, then throughput collapses through packet loss. A 95-metre cable of the same category performs identically to a 30-metre one.
How can I extend Ethernet beyond 100 metres?
Add a network switch at the midpoint to reset the distance budget, convert the run to fibre with media converters (good for kilometres), use a PoE-powered extender where there is no midpoint power, or bridge with powerline adapters or a wireless point-to-point link.