Structured Cabling: Data and Coax Installation

Almost every building now needs data cable, and a bad termination can quietly turn a fast network into a slow one. A cable can pass a simple continuity test and still fail at speed, because data depends on how the pairs are twisted, how far they are bent and how they are pulled. This guide covers Category cable and coax from the pull to the test, plus the small panel that ties a home or office network together.

What You Will Be Able to Do

• Choose the right category of cable, and stay within length limits.
• Pull cable without changing its performance.
• Terminate a jack, a patch panel and a coax connector.
• Set up a structured wiring panel.
• Test a run, and understand why it failed.

How Twisted Pair Works

A Category cable holds four twisted pairs. Each pair carries one signal as opposite voltages on its two wires. Any noise from outside lands almost equally on both wires, because the twisting keeps moving each wire closer to and farther from the source. The receiver looks only at the difference between the two wires, so the equal noise cancels out. Two straight parallel wires would pick up noise unevenly.

That is why untwisting too much, or crushing the cable, hurts performance. Each pair has its own twist rate, and that rate is part of the design.

Choosing Cable

The TIA standards limit a horizontal channel to 100 meters, or about 328 feet. That breaks down into a permanent link of up to 90 meters, plus up to 10 meters of patch cords. Measure your route, not the straight line. A run that goes up a wall, along a ceiling and down again adds up quickly.

Most jobs use unshielded twisted pair (UTP). Shielded cable costs more, needs shielded connectors and has to be grounded as designed, so use it only when it is specified.

Fire ratings. Cable jackets carry listings such as CM, CMR and CMP, based on where they can run. Plenum spaces need CMP. Do not use a cheap copper-clad aluminum cable. It fails on heat and on Power over Ethernet (PoE), and it may not be listed.

Power over Ethernet. PoE sends power on the data cable to cameras, phones and access points. Large bundles of cable carrying PoE get warm, and the NEC limits the current allowed in large bundles. Follow the design, and keep bundles loose.

Pulling Cable

Cable performance can be lost during the pull, and no connector can bring it back.

Check the cable maker's data, because these are common values and yours may differ.

• Pull by hand or with a tension-limiting tool. Do not put a power winch on a data cable with no limit.
• Use pull boxes in long conduit runs, and cable-rated lubricant.
• Do not kink or step on the cable. A kink is permanent, even if it looks fine later, and it can fail certification.
• Use hook-and-loop straps, not tight zip ties, which crush the pairs.
• Keep data cable about 6 inches from parallel line-voltage runs, and cross power at 90 degrees.
• Stay away from fluorescent ballasts and motors.
• Do not lay cable on ceiling tiles. It must be supported by the structure.
• Restore any fire-rated wall you go through, using listed firestop.

Label each cable at both ends during the pull. A run that has no label costs far more time later.

Terminating Category Cable

Terminations follow one of two pin orders. T568B is common in US commercial work. T568A shows up in government and some residential specs. They work equally well, but both ends of every run must match.

The steps for a jack:

1. Strip only as much jacket as you need, usually 1 to 2 inches. Do not nick the conductors.
2. Keep each pair twisted right up to the termination. The most common cause of failure is untwisting more than about 1/2 inch.
3. Follow the jack's color label for the scheme you chose.
4. Seat each wire with a 110 punch-down tool, cutting side facing out.
5. Keep the jacket as close to the jack as you can.

Patch panels use the same method. Dress the cables neatly behind the panel, and keep the bend radius.

Field-made plugs fail more often than jacks. For permanent runs, terminate to a jack or panel, and use factory-made patch cords.

Coax

RG6 is the standard for cable TV, satellite and antennas. RG59 is older and thinner, and it should not be used for new work.

1. Use a two-step stripper to cut the jacket and dielectric to the right length.
2. Fold back the braid as the connector maker directs.
3. Push the compression connector on until the dielectric is flush.
4. Compress it with the tool.

Use compression connectors, not crimp or twist-on types. They seal better, and hold better.

Signal loss. Each two-way splitter costs about 3.5 dB on each leg. Use only the splitting you need, and cap unused ports with terminators. Bend coax gently, and do not staple through it.

Bonding at the entrance. Coax from outside the building must have its shield bonded to the building's grounding system, usually with a grounding block near the entry. Use the intersystem bonding termination at the service.

The Structured Wiring Panel

A structured wiring panel is a central enclosure that gathers cables from around a house or a small office. It may hold the fiber terminal (ONT), a router, a switch, a patch panel and coax splitters.

Get these things right:

1. Separation: keep 120-volt power apart from low-voltage cable. Use a separate power section or a divider inside the enclosure.
2. Ventilation: switches and routers make heat. A closed metal box can overheat them, so use a vented cover or leave room.
3. Service loops: leave 12 to 18 inches of slack on each cable so that it can be re-terminated.
4. Labels: mark each cable at both ends with the room and jack number.
5. Power: provide a dedicated receptacle in or near the enclosure.
6. Signal: metal enclosures can block Wi-Fi, so keep wireless access points outside them.

Plan for growth. Many installers pull two data cables and one coax to each location, so that a move does not mean opening a wall.

Testing

Test every run before you leave the job.

A cheap tester proves that the wires connect. It does not prove speed. A commercial job usually needs a certifier, which measures length, wire map and crosstalk, and gives a report for the customer.

Crosstalk is signal leaking from one pair to another. A failure called near-end crosstalk (NEXT) is usually caused by too much untwisting. Return loss failures often come from kinks or poor terminations.

If one end is T568A and the other T568B, you have made a crossover, and the wire map fails.

Fiber

Fiber is a specialty of its own, but you may run across it. It carries light on thin glass, so it has tight bend limits and needs special tools. Do not look into the end of a fiber, because the light may be invisible and can damage your eyes. Dispose of glass scraps in a marked container.

Common Mistakes

Check Your Understanding

1. A run measures 86 meters in the walls, and each end has a 3-meter patch cord. Does it fit within the channel?

Yes. The permanent link is under 90 meters, and the total of 92 meters is under 100 meters.

2. A Cat 6A cable has a diameter of 0.3 inch. What is the minimum bend radius?

4 x 0.3 = 1.2 inches.

3. A wire map passes, but the certifier shows a NEXT failure on one jack. What do you check?

Look at how far the pairs are untwisted at that jack, then re-terminate.

4. A home's switch keeps overheating inside a metal enclosure. What is the likely problem?

Poor ventilation. Use a vented cover or a larger enclosure, or move the equipment.

Key Takeaways

• Twisting cancels noise, so keep the twist right up to the termination.
• Keep a channel within 100 meters: 90 of permanent link and 10 of patch cords.
• Stay within the pull tension and the bend radius, because a kink is permanent.
• Use one pin order at both ends of every run.
• Use compression connectors on coax.
• Leave slack, separate power, ventilate the panel, label both ends, and test every run.