TL;DR: T568A and T568B are the two pin and pair assignments used to terminate an eight wire ethernet cable. They differ only in swapping the green and orange pairs, and neither performs better than the other. What matters is that one building uses one standard everywhere. Mixing them inside a single run creates a crossover cable and breaks the link. If you are planning a commercial install, our structured cabling team standardises every drop in the building before the first cable is pulled.
Nearly every ethernet fault we get called out to in a Fort Myers office has a boring cause, and a surprising number of them trace back to this one decision. A previous contractor terminated half the floor to T568A, someone else added drops later to T568B, and now a handful of jacks behave strangely under load. This post covers what the two standards actually are, how to tell which one a building already uses, and how to stop the problem recurring.
What is the difference between T568A and T568B?
T568A and T568B are two wiring schemes defined in the ANSI/TIA-568 cabling standard for terminating eight conductor twisted pair cable. The only difference between them is that the green pair and the orange pair swap positions on pins 1, 2, 3 and 6. Electrically the finished link is identical, so neither standard is faster, quieter or more reliable than the other.
The Fiber Optic Association, a non profit trade body, puts it bluntly in its guide to UTP cabling termination: “Either work equally well, as long as you don’t mix them!” That is the whole engineering argument in one sentence. The two schemes exist for historical reasons, because different parts of the industry started from different pinouts and both ended up written into the standard.
The blue and brown pairs sit on pins 4, 5, 7 and 8 in both schemes and never move. Only pairs 2 and 3 are reversed.
The ethernet cable color code for T568A and T568B
The ethernet cable color code assigns each of the eight conductors to a numbered pin on the RJ45 connector. In T568A, pin 1 is white and green. In T568B, pin 1 is white and orange. Everything else follows from that single swap, and the blue and brown pairs are identical in both.
| Pin | T568A | T568B |
|---|---|---|
| 1 | White / Green | White / Orange |
| 2 | Green | Orange |
| 3 | White / Orange | White / Green |
| 4 | Blue | Blue |
| 5 | White / Blue | White / Blue |
| 6 | Orange | Green |
| 7 | White / Brown | White / Brown |
| 8 | Brown | Brown |
One practical warning from the field. The color code above describes the RJ45 pinout, not the punch down sequence printed on the back of a jack. Cat5e and Cat6 jacks are built to keep each pair twisted as close to the contacts as possible, so the physical layout of the punch down slots often looks scrambled compared to the pin order. Always follow the labels moulded into the jack itself. Getting this wrong is one of the more common causes of the intermittent faults covered in our guide to data jack wiring.
What happens if you mix T568A and T568B?
If one end of a cable is terminated to T568A and the other end to T568B, you have built a crossover cable rather than a straight through patch cable. Modern switches and network cards with auto MDI-X will usually correct for it and the link will come up, which is exactly what makes this fault so difficult to find later.
The damage shows up further down the line. A link that only works because auto negotiation is quietly fixing it will fail the moment it meets equipment without that feature, which still includes plenty of cameras, access control panels, older paging hardware and industrial gear. It will also fail certification testing, so the cabling cannot be warranted.
The FOA guidance is to check what is already installed before adding anything: “Always check to see which scheme has been used and continue using that one.” In practice that means opening a jack in an existing area and reading the terminations before you cut a single new cable. On an expansion where drops are being added to a floor somebody else wired, that five minute check is the difference between a clean cutover and re-terminating a whole run of new outlets.
How do I know if my ethernet cable is T568A or T568B?
Look at the clip side of a terminated RJ45 plug with the contacts facing you and the cable running away from you, then read the color of pin 1. If the first wire is white with a green stripe, the cable is T568A. If it is white with an orange stripe, it is T568B.
For terminations you cannot see, such as a wall jack or a patch panel port, pull the faceplate or check the rear of the panel in the network rack. Well documented sites label this at the rack, which is one of the reasons we record the scheme in the as built documentation on every job we hand over.
If a building has no records and no consistency, a cable certifier will map the wire pairs for you and show exactly which pins each conductor lands on. That is the only reliable way to audit a floor that several contractors have touched over the years.
Do you use T568A or T568B for Cat6?
Both. The wiring scheme and the cable category are independent choices, so a Cat6 wiring diagram is the same as a Cat5e or Cat6A diagram for the purposes of pin assignment. Cat6 changes the cable construction and the tested bandwidth, not which color lands on which pin.
What does change with Cat6 and above is how much the termination workmanship matters. Higher category cable is less forgiving of untwisting the pairs at the connector, so the small habits that were survivable on Cat5e will cost you margin on a Cat6 link. If you are weighing the categories themselves rather than the pinout, our comparison of Cat6, Cat6A and Cat7 covers where each one earns its cost, and Cat6 vs Cat8 covers the top end.
Why most commercial buildings in Southwest Florida land on T568B
Neither standard is mandated for commercial work, so the decision is usually inherited rather than made. In the commercial buildings we work in across Fort Myers, Cape Coral, Naples and Bonita Springs, T568B is the more common scheme, largely because it is what most of the region’s low voltage contractors have historically used and what most pre terminated patch cords ship as.
That is a convention, not a rule, and it is worth saying plainly: if your building is already wired to T568A throughout, the correct decision is to keep using T568A. Converting a floor to gain nothing is wasted money and creates exactly the mixed environment you are trying to avoid.
The decision only becomes genuinely open on new construction. There we pick one scheme, write it into the project documentation, and hold every subcontractor to it, which is part of the wider discipline described in our post on future proof data wiring.
Getting it right on your next cabling project
Standardising a wiring scheme is cheap at design stage and expensive to retrofit. The work is not difficult, but it does require somebody to own the decision for the whole building and to document it where the next contractor will find it.
CommX Networks has spent nearly two decades designing and installing structured cabling, fiber, enterprise wireless and security systems for commercial clients across Southwest Florida. Every job we hand over includes labelled terminations and documentation that records the scheme in use, so the next person to open the rack is not guessing.
If you are planning a fit out, an expansion or a cleanup of cabling somebody else left behind, get in touch with our team or read more about our network cabling services and the areas we serve.
Frequently asked questions
Is T568B better than T568A?
No. The two schemes are electrically equivalent and neither offers a performance advantage. T568B is more widely used in US commercial installations, but that is convention rather than a technical benefit. The only wrong answer is using both in one building.
Can I plug a T568A cable into a T568B jack?
Yes, and it will usually work. Each end of a link is terminated independently, so a patch cord wired to one scheme will connect fine to a jack wired to the other as long as both ends of that patch cord match each other. Problems begin when the two ends of the same cable differ.
Does the wiring standard affect Power over Ethernet?
No. PoE uses the same conductors regardless of scheme, so both T568A and T568B carry power identically. Cable quality, run length and the power budget on the switch matter far more, as covered in our guide to Power over Ethernet.
Which standard should I specify for a new commercial building?
Pick one, write it into the specification, and apply it to every outlet, patch panel and patch cord in the building. For new commercial work in Southwest Florida we generally specify T568B, unless the client already has an estate wired to T568A.