You have a cable, a crimper and a plug, and one awkward fact: every RJ45 termination forces you to unwind pairs that are twisted for a reason. So how much untwisting is too much? As little as the plug allows, and never more than the commonly cited limit of about 0.5 inch (13 mm) for Cat5e. This guide explains why and shows how to keep the untwisted section short.
Why the untwist matters
An Engadget explainer on twisted pairs lays out the mechanism. Current in a wire creates a magnetic field, and two wires running parallel can disturb each other's signal. Twisting a pair creates opposing fields that cancel that interference. Interference between wires in the same bundle is called crosstalk.
Each of the four pairs is also twisted at a different rate. That keeps the pairs from disturbing one another the way untwisted parallel wires would. According to the article, gigabit Ethernet uses all four pairs, so none of them can be sloppy.
A plug is the one place where the cable loses this protection. The wires inside the connector run flat and side by side, and the longer that straight stretch gets, the more room crosstalk has to creep in. That conclusion is an inference from the mechanism the article describes. The article does not discuss terminations.
What the limit actually is
The Engadget article gives no measurements, so the number has to come from elsewhere. The figure repeated for Cat5e is a maximum of about 0.5 inch (13 mm) of untwisted pair at the termination. It is widely attributed to the TIA-568 cabling standard.
I have not verified that figure against the current text of TIA-568, which is sold behind a paywall. Treat 0.5 inch as a ceiling, not a target. The reliable source is the installation instructions or datasheet from the maker of your cable, plug or jack. If those give a tighter number, use theirs.
For Cat6 and higher, or for shielded cable, this guide gives no figures. Check the manufacturer's sheet before you start.
Step-by-step: terminating an RJ45 plug
This walkthrough uses the common T568B color order and a standard crimp plug. Each step notes where to keep the pairs twisted.
- Cut the cable end square. Snip off any damaged old plug and a few millimeters of cable behind it. Fresh copper and an even jacket make every later step easier.
- Strip only the jacket. Remove enough outer jacket to work with, without nicking the conductors. Leave the pairs twisted at this point. Cut off any pull string or separator, and fold back foil if the cable has it.
- Untwist one pair at a time, only as needed. Unwind just enough of each pair to straighten the two wires. Do not unravel all four pairs to the jacket edge out of habit. This step is where the common mistake happens (see below).
- Arrange the order. For T568B, from left to right with the plug tab facing away from you: white/orange, orange, white/green, blue, white/blue, green, white/brown, brown. The green pair splits around the blue pair, which is why some untwisting is unavoidable.
- Flatten and pinch the wires. Hold the wires parallel and flat between thumb and finger. Keep the twists intact right up to the point where you pinch.
- Trim to length. Cut the wires with a clean, even edge so the straight, untwisted section stays short. Follow the plug's instructions for the exact trim length. Make sure the jacket will sit inside the plug body so the strain relief grips it.
- Insert and check. Slide the wires into the plug until each one reaches the end. Look through the clear plastic from the front. All eight copper ends should touch the end wall, and the jacket should sit past the strain relief.
- Crimp. Squeeze the crimper fully. For a straight-through cable, repeat on the other end with the same color order.
The common mistake
The usual error is step 3 done too eagerly. Many people unwind every pair down to the jacket, straighten the lot and push it all into the plug. That leaves the full length untwisted. Because it makes the wires easier to line up, it feels like the careful method, but it is the opposite. Untwist a little, arrange, then trim so the plug does the rest.
A related snag is conductors that do not reach the contacts, or a jacket that stops short of the strain relief. Either one can cause dropouts when the cable moves.
Punching down a jack
The same principle applies to a wall jack, with a slightly different workflow. Strip the jacket, then untwist each pair only as far as its slot on the jack. Most jacks have color-coded slots, and the punch-down tool trims the excess. Keep the twist right up to the terminal block, and follow the maker's instructions for the maximum untwist.
Signs of a bad termination
A bad termination does not always mean no connection. Watch for these:
- The link negotiates at 100 Mbps on a gigabit-capable port.
- The connection drops when you touch or move the cable.
- Throughput is erratic, or error counters climb.
The 100 Mbps symptom has a plausible explanation. Gigabit uses all four pairs, per the Engadget explainer. Fast Ethernet (100 Mbps) generally uses only two pairs, the orange and green on pins 1, 2, 3 and 6. If the blue or brown pair makes poor contact, the link may fall back to 100 Mbps and still appear to work. This is an inference from how those speeds use the pairs, not something the article states.
To check, look at the negotiated speed in your operating system's network settings for the Ethernet adapter, or in your router or switch's port status. A cable tester helps too. A basic wiremap tester finds opens, shorts and swapped wires. It often misses a split pair, which is wired pin-to-pin correctly but pairs the wrong conductors. It also cannot measure crosstalk from excess untwist. Only a certification-grade tester measures that, and those are generally priced for professionals, not home users.
DIY or premade: a decision table
| Situation | Better choice | Who benefits | Limitation | | --- | --- | --- | --- | | Long wall run through studs or conduit | DIY with solid cable and punch-down jacks | Anyone who needs a custom length or a tidy in-wall install | Termination cannot fix a poor-quality cable or an overly long run | | Short patch cable (a few meters or less) | Premade | Most home users, since factory-molded plugs are consistent and convenient | Fixed lengths, so you may end up with excess slack | | Custom-length patch cable | DIY if you have the tools, premade if you can find the length | Hobbyists who enjoy it or need an odd length | Stranded cable needs plugs rated for stranded wire, and a poorly crimped plug is an easy mistake | | Repair of a broken plug | DIY re-termination | Anyone with a good cable and a crimper | If the cable is kinked or damaged beyond the plug, replace it |
The rule of thumb: if you need the length, go DIY. If you only need a cable, buy one. A crimp tool and a bag of plugs can cost more than a single premade patch cable, so DIY pays off mainly when you make several cables or need lengths that do not exist off the shelf.
Next-action checklist
- After crimping, plug in both ends and check the negotiated link speed in your network settings or on the router port.
- Run a cable tester across the cable if you have one, and note any open, short or miswire.
- If the link negotiates below gigabit on a gigabit-capable connection, cut off the plug and re-terminate. Check the color order, keep the untwist as short as the instructions allow, and confirm all eight conductors reach the contacts.
- If it still fails, swap in a known-good premade cable to rule out the port or the cable run.
- Keep the manufacturer's datasheet on hand so you can check the exact untwist limit for your cable category.
What to expect next
A clean termination is boring, which is the point: the link comes up at full speed and stays there. If you re-terminate once and the speed is still wrong, suspect the cable or an in-wall run before blaming your crimping.
The Engadget article notes that you do not need to hunt for twisted pair when buying cable, because it is standard. What you control is how much of that twist you give back at the plug.
