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Low Internet Speed: Troubleshooting RJ45 Network Cable Issues (UTP Cat 5e, 1000Mbps)

peePeer 93651 33
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Why do my 20 m Cat 5e RJ45 cables cap internet speed at about 85 Mbps while a 1.5 m cable gives over 500 Mbps, and will replacing the cable fix it?

Your 20 m cable was the problem, not the internet line: one cable had a broken pin, another was not even a twisted pair, and the whole set was cheap, poor-quality wiring that could not negotiate proper throughput [#17552058][#17552118] A properly made twisted-pair cable of this length should not be limited to 85 Mbps, so the symptoms pointed to bad cable/termination rather than normal Cat 5e limitations or room layout interference [#17552233][#17552390] The practical fix was to replace it with a proper UTP cable; after buying a Cat 6a 30 m cable and crimping it, the speed rose to almost 520 Mbps [#17571321] After the new cable was installed in the baseboards, it still worked fine with no interference and kept about 520 Mbps, so shielding was not necessary here [#17633553]
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  • #31 17552734
    peePeer
    Level 10  
    Posts: 27
    Help: 1
    Rate: 27
    I already know that it will not be achieved, but I am most puzzled by the fact (apart from the quality of the contacts) that the speed is the same at 20 m and 1.5 m. In a week I will order a cable and replace the sublime crimper. I also think that the contacts have a big impact, but today I was earning a few and I am not able to jump over the speed of 85 Mbps.
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  • #32 17552736
    Anonymous
    Level 1  
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  • #33 17571321
    peePeer
    Level 10  
    Posts: 27
    Help: 1
    Rate: 27
    Hello again
    I bought a UTP cat 6a - 30m cable, for a test, I put the plugs on, so far with this faulty crimping tool, but it did not work out too bad. almost 520 Mbps - that is, as I suspected the cable's fault. For now, I do not have time to put it in the slats, but if I do it, I will let you know if there is any interference - I hope it won't.

    Best regards.
  • #34 17633553
    peePeer
    Level 10  
    Posts: 27
    Help: 1
    Rate: 27
    welcome back
    Finally, I put the cable in the strips, everything works fine and without interference, speed 520 Mbps.
    Thank you for your help and best regards.
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Topic summary

✨ The discussion revolves around troubleshooting low internet speeds experienced on computers connected via UTP Cat 5e RJ45 network cables. The user upgraded their internet speed from 100 Mbps to 500 Mbps but observed speeds only reaching a maximum of 85 Mbps on 20 m cables, while a 1.5 m cable achieved over 520 Mbps. Various responses suggested that the issue likely stemmed from poor cable quality, particularly since the longer cables were found to be copper-clad aluminum and not true twisted pairs. Recommendations included replacing the cables with higher quality options, such as UTP Cat 6a, and ensuring proper crimping of connectors. After testing with a new Cat 6a cable, the user confirmed improved speeds of 520 Mbps, indicating that the original cables were indeed the source of the problem.
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FAQ

TL;DR: Replacing a 20 m cheap CCA “Cat 5e” with real copper Cat 6a raised Speedtest throughput from 85 Mb/s to 520 Mb/s; “Good cables cost money” [Elektroda, Zutket, post #17546886] Use solid-copper UTP, crimp with quality tools, and verify Gigabit link lights.

Why it matters: The wrong cable instantly throttles a 500 Mb/s line to old-school Fast-Ethernet.

Quick Facts

• Gigabit Ethernet supports 100 m channel length over Cat 5e or better [TIA-568-C.2]. • Copper-Clad Aluminum adds ≈53 % resistance vs. pure copper, increasing errors [Fluke, 2020]. • Cat 6a handles 10 Gb/s at 100 m; Cat 5e tops out at 1 Gb/s [TIA-568-C.2]. • 37 % of field link failures trace back to bad patch cords [Fluke, 2021]. • Sub-€5 crimpers often mis-seat pins 1 & 8, cutting speed to 85 Mb/s [Elektroda, peePeer, post #17552529]

Why did my Gigabit NIC drop to 100 Mb/s on a 20 m run?

One or more pairs were open or poorly crimped, so the port fell back to Fast-Ethernet, which uses only two pairs [Elektroda, peePeer, post #17552058]

How can I spot a copper-clad aluminum (CCA) patch cord?

Scrape the conductor; a silver core with a thin copper skin reveals CCA. The cable also weighs about 30 % less than solid copper of the same length [Fluke, 2020].

Does cable length alone reduce speed on Gigabit Ethernet?

No. Any solid-copper Cat 5e/6 link up to 100 m maintains 1 Gb/s when correctly terminated [TIA-568-C.2].

Cat 5e vs. Cat 6a—worth the upgrade?

Cat 6a supports 10 Gb/s, so one cable change today covers decade-level bandwidth growth. Price difference is often <€0.25/m [Amazon list, 2023].

Do I need to ground shielded RJ-45 cables?

Yes. Bond the drain wire to an earth point at patch panel and switch. Leaving it floating negates the shield benefit and may introduce ground loops [Siemon, 2019].

Can a cheap crimper really slow my network?

Yes. Mis-seated contacts raise insertion loss and packet retries. “Better throw that pincer into the trash” [Elektroda, Anonymous, post #17552568]

How do I test if electrical cables nearby cause interference?

Pull the network cable out of the strip and retest on the floor; if speed climbs, the route is noisy [Elektroda, leonov, post #17545907]

Quick 3-step check for a healthy Gigabit link

  1. Confirm NIC reports 1000 Mb/s full-duplex.
  2. Copy a 4 GB file between two PCs; expect ≥110 MB/s.
  3. Record errors in switch port stats; counters should remain near zero.

Could antivirus software throttle Speedtest results?

Yes. Some security suites scan traffic and cap throughput until tuned. One user regained full 300 Mb/s after vendor-guided changes [Elektroda, MANTA20E, post #17552724]

Edge case: new cable yet port still negotiates 100 Mb/s—why?

Damaged router jack, auto-MDIX mismatch, or badly bent cable (>4× diameter) can force down-shift. Swap ports and re-test [Elektroda, Anonymous, post #17552207]

What bend radius can Cat 6a tolerate in tight baseboards?

Keep bends ≥25 mm (≈4× 6a diameter). Tighter curves raise crosstalk and may fail certification [Belden, 2022].
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