A good RJ45 crimp needs all eight conductors in the correct order, every conductor pushed to the end of the plug, the cable jacket seated inside the strain relief, and the pairs untwisted by no more than 13 mm. Strip 3 cm of jacket, trim the conductors level so that 1.2 cm shows, insert, crimp once with a proper tool and test the cable straight away. The usual faults are a conductor that stopped short, a split pair caused by laying the wires in plain colour order, and a jacket left outside the strain relief.
Part of Networking
The RJ45 plug termination technique in depth: wire order, trim lengths, tool use, and the failure modes a tester will and will not catch.
The Pair Order Is Not The Colour Order
The eight pins of T-568B run: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown (Twisted Pair Wiring). Read the pairs off that list. Orange is on pins 1 and 2, blue on pins 4 and 5, brown on pins 7 and 8, and green is on pins 3 and 6, straddling the blue pair. The order looks odd, and a fitter who lays the wires out pair by pair (green on 3 and 4, blue on 5 and 6) makes a cable where every conductor reaches a pin, so a continuity test passes, but two pairs are split.
Twisted Pair Wiring states what follows: a split pair passes the continuity test and fails at gigabit speeds because the twisting that rejects noise is broken. So the order must be memorised or written on the bench, not reasoned out from “pairs go together”.
What You Need
- RJ45 plugs of the right type for the cable — solid cable and stranded cable use different contact styles in many plugs (general knowledge); check the packaging or test one
- A crimping tool made for 8P8C plugs — Twisted Pair Wiring is plain that hand pressure is insufficient, so do not substitute pliers or a screwdriver
- A cutter that leaves the conductor ends square and a jacket stripper
- A tester — Cable Testing
- Eye protection — trimmed copper ends fly
Method: Crimp A Plug
Step 1: Strip 3 cm of outer jacket The carrier Internet Infrastructure gives 3 cm. Do not score the conductor insulation; a nicked conductor breaks later at that point.
Step 2: Untwist, straighten and arrange the pairs Straighten each conductor by drawing it between your fingers, then lay all eight flat in T-568B order. Hold them together; a conductor that drifts out of position is the usual cause of a wrong crimp.
Step 3: Trim level Cut all eight to the same length so that 1.2 cm shows beyond the jacket (carrier figure). A conductor trimmed too long will bend inside the plug and may touch its neighbour; one trimmed too short will not reach the contact.
Step 4: Insert and check Push the conductors into the plug until the jacket sits inside the strain relief. Look through the transparent plug: every conductor must reach the end (Connectors says each must be visible at the end of the housing), and the jacket must be inside the plug body.
Step 5: Crimp once, firmly The tool pushes the eight contacts through the insulation and deforms the strain relief onto the jacket (Connectors). Test immediately; re-terminating is faster than repairing, so cut the plug off and start again if it fails.
Worked Example: Strip And Trim Lengths Against The Untwist Limit
Inputs: jacket stripped 3 cm = 30 mm; final exposed conductor length 1.2 cm = 12 mm; Cat5e untwist limit 13 mm (Twisted Pair Wiring).
Conductor trimmed off: 30 - 12 = 18 mm. The 12 mm that stays inside the plug is the only part of each pair that is untwisted in the finished cable, and 12 mm is below the 13 mm limit by 1 mm.
The sources in this section disagree: Physical Connections gives 25 mm as the maximum untwisted length for Cat5e, against 13 mm in Twisted Pair Wiring. The carrier’s 12 mm trim is consistent with the stricter figure, so use that one. The tolerance is only 1 mm, and a conductor left 3 mm long puts the cable over the limit.
Diagnosing A Bad Crimp
| Symptom | Likely cause |
|---|---|
| No link light | a conductor stopped short, or a pair in the wrong place |
| Link light, but errors or a slow speed | split pair, or too much untwist (Twisted Pair Wiring) |
| Works, then drops when the cable moves | jacket outside the strain relief, or one contact not seated |
Common Mistakes
- Crimping before checking the wire order — a plug cannot be uncrimped; cut it off and re-strip, which shortens the cable by the length you remove
- Pulling a cable by its plug — the plug is crimped on after the cable is pulled, not before
- Skipping the test because the cable “looks right” — Connectors says to test at once, because incomplete insertion cannot be seen once the cable is routed
Safety
Blades and flying copper
Strip away from your hand, keep your fingers out of the crimper’s jaws, and wear eye protection when cutting conductors. Never crimp or strip a cable that might still be connected to a powered device or that you pulled from a wall without proving the wall wiring dead (Electrical Safety).
What’s Next
- Learn next: Cable Testing — proving the crimp, and finding the faults a simple tester misses
- Combine with: Twisted Pair Cabling — the whole run, not just the plug