Terminating and Testing an Ethernet Cable

Why this matters

A bad crimp is the cheapest part of a network run and the most expensive to find later. The cable can be the right category, pulled cleanly, and still fail because one conductor stopped a millimetre short of its contact or the pairs were untwisted an inch back from the plug. Most of the time the result is not a dead link, it is a link that comes up, works for a while, and drops. This is the hands-on procedure for putting a plug on the end of a cable and proving it carries a signal before you leave. Choosing the cable, the jacket and the wiring pattern is covered in the sibling card. See related: Ethernet Cable Categories and Terminations.

Tools: a cable cutter, a jacket stripper set for round network cable, an RJ45 crimp tool that matches the plugs you use, plugs rated for the cable's conductor (solid or stranded), and a tester with a remote unit for the far end. A tester that reports split pairs is worth the extra.

Step 1: Identify the run and take it off power

Find both ends of the run and label them before you cut anything. If you are re-terminating an existing cable, unplug it from the switch or injector at the far end first. Many runs to cameras and access points carry power over Ethernet (PoE), which is low-voltage DC on the same conductors; it is not a shock hazard in the way household current is, but crimping a live run can arc across the contacts and damage the switch port or the device.

Why this step: a run you cannot match to its other end cannot be tested, and a cable cut at the wrong end of the wrong run is a second cable to fix. What breaks if skipped: you cut and terminate a perfectly good run to the doorbell, and the camera run you were sent for is still bad.

If the work puts you in an attic, step only on joists or a walking board, never on the ceiling drywall between them. Fiberglass insulation irritates skin and lungs, so wear long sleeves and a disposable particulate respirator if you will be moving it. If the insulation is loose, gray-brown and pebbly rather than fibrous, it may be vermiculite, which can contain asbestos: do not disturb it, and stop and tell the customer it needs testing before anyone works in it.

Step 2: Cut square and leave yourself a service loop

Cut the cable straight across with a proper cutter, not side cutters that pinch it flat. Leave enough slack at each end that the cable can be re-terminated at least twice: 300 mm (about a foot) coiled loosely at a jack or in a ceiling space is a sensible starting point.

Why this step: every failed crimp costs you about 15 to 25 mm of cable when you cut it off and start again. What breaks if skipped: a run cut to exact length cannot survive a single re-do, and a run that came up one plug short has to be pulled again.

Step 3: Strip the jacket without nicking the conductors

Strip about 25 mm (an inch) of outer jacket. You will trim the conductors back later; the extra length is only so your fingers can arrange them. Use a stripper set to score the jacket rather than cut through it, then flex the jacket at the score and pull it off. Cut away any separator spline (the plastic cross in the middle of many Cat6 cables) and the ripcord flush with the jacket.

Before going on, look at every conductor's insulation where the jacket ended. Acceptance: no nicks or exposed copper. A nicked conductor can test fine on the day and break inside its insulation months later.

What breaks if skipped: a scored conductor inside the plug where you cannot see it, failing intermittently once the cable has been flexed a few times.

Step 4: Untwist only as much as you need, and set the order

Separate the four pairs and fan them out, then untwist each pair only as far back as you need to straighten it for the plug. The standard limit is no more than 13 mm (half an inch) of untwist at a termination, and the twist should run right up to where the jacket ends.

Arrange the conductors in T568B order, the most common pattern in North American residential work. (If your shop uses T568A, swap the orange and green pairs; whichever you use, use the same at both ends.)

PLUG FACE: clip DOWN, gold contacts toward you,
tip away from you, cable toward you. Pin 1 is LEFT.

 pin  1     2     3     4     5     6     7     8
     W-OR  OR    W-GR  BL    W-BL  GR    W-BR  BR

 orange pair    pins 1 + 2   side by side
 blue pair      pins 4 + 5   side by side, center
 green pair     pins 3 + 6   split, straddles blue
 brown pair     pins 7 + 8   side by side

The green pair is the one that catches people. It is the only pair that does not sit side by side; its white-green conductor goes on pin 3 and its solid green on pin 6, with the blue pair between them. Wire it in simple color order instead and the two ends will match each other, pass a basic continuity test, and leave a split pair that fails at speed. See related: Ethernet Cable Categories and Terminations.

What breaks if skipped: a long untwisted tail is where crosstalk gets in, and a Cat6 run terminated with an inch of untwist performs like a lower grade.

Step 5: Flatten, trim, and size to the plug

Hold the eight conductors flat and side by side in order, pinched between thumb and finger. Hold the plug beside them, clip down, and trim the conductors straight across so that, when inserted, they reach the front of the plug and the jacket sits inside the plug's rear, under the strain-relief tab. For most plugs that leaves about 13 mm of conductor beyond the jacket.

Why this step: the strain-relief tab clamps the jacket, not the conductors. If the jacket stops short of it, the tab clamps bare conductors and every tug on the cable pulls on the contacts. What breaks if skipped: a termination that works on the bench and fails when the cable is pulled taut into a wall plate.

Pass-through plugs, where the conductors poke out of the front and get trimmed flush after crimping, make this step easier to check; use them only with a crimp tool that has the matching flush-cut blade.

Step 6: Insert and inspect before you crimp

Push the conductors into the plug firmly, keeping them flat and in order, until each one hits the front wall. Then look through the plug before crimping:

• From the top, read the colors left to right against the diagram. All eight must be in sequence.
• From the front, every conductor's copper end must be visible right up against the front of the plug. One that stops short will not reach its contact blade.
• The jacket must sit past the strain-relief tab.

Acceptance: all three hold. If any one fails, pull the conductors out, re-flatten, and re-insert; you have not crimped yet, so this costs nothing.

What breaks if skipped: the crimp is permanent. Once crimped, a short conductor means cutting the plug off.

Step 7: Crimp fully

Seat the plug in the crimp tool's die and squeeze the handle to its full stroke, until the ratchet releases if the tool has one. The tool pushes eight contact blades down through the insulation into the conductors, and drops the strain-relief tab onto the jacket.

Acceptance: all eight gold contacts are level with each other and pushed down below the plug's top surface, and a firm pull on the cable does not move the plug.

What breaks if skipped: a half-stroke crimp leaves one or two blades high, which can make contact on the tester and lose it when the cable heats up or moves.

Step 8: Test both ends, then test the link

Plug the tester's main unit into one end and the remote into the other, and run the wiremap. Acceptance: all eight pins pass, with no open, short, reversed pair or crossed pair, and no split pair if the tester detects them.

A wiremap pass proves the conductors land on the right pins. It does not prove the run carries data at speed. Plug the device in and check the link speed on the switch, the router's device list or the device itself. Acceptance: the link comes up at the speed the device supports, typically 1 Gb/s for an access point or a gigabit camera. A run that passes the wiremap and links at 100 Mb/s is telling you something is marginal: untwist, a split pair, a damaged section of cable, or a run beyond the 100 m channel limit.

A run from start to finish

A tech is wiring an access point in a hallway ceiling from a switch in the basement. The run is 22 m of solid-core Cat6, and both ends get plugs rated for solid conductors: one into the switch, one into the access point.

First crimp, basement end. Steps 3 to 7 go normally. The tech checks the color order through the top of the plug: W-OR, OR, W-GR, BL, W-BL, GR, W-BR, BR. Front view shows all eight copper ends at the front wall. Crimped.

Second crimp, ceiling end. Working overhead from a ladder, the tech trims and inserts, glances at the color order, and crimps. The front-view check in Step 6 is skipped because the light is poor.

Test. The wiremap shows pins 1 through 7 passing and pin 8 open.

Finding it. An open on one pin with the others good is almost always a conductor that did not reach its contact. Looking at the ceiling-end plug from the front, the brown conductor stops about 2 mm short of the front wall; it slid back when the conductors were pushed in at an angle. The basement end looks perfect, so it stays.

Fix. The tech cuts the ceiling-end plug off, losing about 20 mm of cable from the 300 mm service loop, and repeats Steps 3 to 7, this time with a work light for the Step 6 inspection. The wiremap now shows all eight pins pass. The access point powers up from the PoE switch and the switch reports the port at 1 Gb/s.

What the case shows. The step that failed was the inspection, not the crimp. A crimp tool cannot fix a conductor that is not in position, and the inspection is the only moment that position is visible. Had the service loop been skipped, the 20 mm lost on the re-do would have left the cable too short to reach the access point's mount, and the run would have needed a splice or a new pull.

What would have changed the diagnosis. If the wiremap had passed on all eight pins and the link had come up at 100 Mb/s, the open-pin logic above does not apply. The suspects become a split pair, too much untwist at either end, or a damaged section along the run, and the first check is the color order at both plugs.

References

• TIA-568 structured cabling standard (T568A/T568B pin assignments, untwist limit, channel length)
• Manufacturer documentation for your crimp tool and plugs (solid versus stranded compatibility)
• See related: Ethernet Cable Categories and Terminations
• See related: Power over Ethernet for Cameras and Access Points
• See related: Tracing a Low-Voltage Run in a Finished House