How this calculation works
The only difference between A and B
Green and orange swap. That is it. Blue stays on 4 and 5, brown stays on 7 and 8, and the electrical performance is identical because the pair assignments are the same either way.
T568B is the de facto standard in US commercial work. T568A shows up in federal and residential specs and in older installations. If you terminate one end A and the other end B you have built a crossover cable, which modern auto-MDIX equipment will silently work around — and which the next tech to troubleshoot the run will not find for an hour.
Pair assignments
The four pairs are 1/2 (orange on B), 3/6 (green on B), 4/5 (blue) and 7/8 (brown). Notice that pair 3/6 is split around the blue pair — that is a legacy of telephone wiring and it is why the colour order looks scrambled.
10/100 Ethernet uses only pairs 1/2 and 3/6. Gigabit and above use all four. That is why an old cable with two pairs punched down works fine at 100 megabit and dies the moment somebody upgrades the switch.
What actually breaks Cat 6
Untwisting. The twist is what cancels crosstalk. Untwist more than half an inch at the termination and you have created an antenna. Cat 6 is far less forgiving about this than Cat 5e because it is certified to 250 MHz.
Bend radius. Four times the cable diameter, roughly one inch for typical Cat 6. Tight bends deform the geometry between conductors and change the impedance.
Overtightened ties. Same problem. If the jacket is dimpled, the pair spacing has changed.
Length. 100 metres total, which is 90 metres of horizontal run plus 10 metres of patch cords on both ends combined.
Running it alongside power
Article 800 governs communications cable. Keep separation from power conductors, use plenum-rated cable in plenum spaces, and remove abandoned cable — that last one is a real requirement and it gets cited.
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Frequently asked questions
T568A or T568B?
Either, as long as you use the same one on both ends of every run in the building. B is the commercial default in the US; A appears in federal specs and some residential work. They are electrically identical — the difference is only which pair lands on which pins.
What happens if I mix A and B?
You get a crossover cable. Modern equipment with auto-MDIX will negotiate around it and the link will come up, which is worse than failing outright — the mistake stays hidden until someone connects gear that cannot compensate, and then it takes an hour to find.
How much can I untwist at the termination?
Half an inch, maximum. The twist rate is what cancels crosstalk between pairs, and Cat 6 is certified to 250 MHz where that cancellation matters much more than it did on Cat 5e. Untwisting an inch is a common cause of a run that fails certification but still passes traffic.
Why do only two pairs get used sometimes?
10 and 100 megabit Ethernet only use pairs 1/2 and 3/6. Gigabit and faster use all four. Older installations were often punched down with two pairs only, which works until someone upgrades the switch and the link drops to 100 megabit or fails entirely.
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