T568A vs T568B: Which Wiring Standard

Before the first jack is punched down, someone has to decide on a pin order. The T568A vs T568B choice decides which colors land on which pins at every jack, patch panel and plug on the job. Electrically the two perform the same; the risk is a building where both are in use and nobody wrote down which.

T568A vs T568B pin by pin

Hold a plug with the clip facing away and the contacts toward you, and pin 1 is on the left. The table lists both color orders. The striped conductor of each pair is shown as white with its pair color.

Pins 4, 5, 7 and 8 never change. The blue pair always sits in the center on pins 4 and 5, which is why both schemes work with a two-line phone jack wired to the inner pins.

What actually changes between the two

The TIA-568 standard numbers the pairs: pair 1 blue, pair 2 orange, pair 3 green and pair 4 brown. T568A puts pair 3 (green) on pins 1 and 2 and pair 2 (orange) on pins 3 and 6. T568B reverses those two pairs, and nothing else moves.

Because each pair still lands on a matched set of pins, a link wired T568A at both ends and a link wired T568B at both ends are electrically identical. Speed, category rating and distance are the same. The cable’s twist and the quality of the termination decide performance, not the letter.

What about PoE?

Power over Ethernet is defined by pin positions, not colors. Alternative A uses pins 1, 2, 3 and 6, Alternative B uses pins 4, 5, 7 and 8, and four-pair PoE uses all eight. Either wiring scheme delivers power the same way, so the choice has no effect on a PoE power budget for IP cameras.

Which ethernet wiring standard to use on a job

The TIA standard lists T568A first and allows T568B where it is needed to match existing eight-pin systems. In practice T568B is widespread in US commercial work, while T568A appears in government and many institutional specifications. Neither is wrong.

Use this order to decide:

1. Follow the specification. Many owners write the pinout into their cabling standard. Montana State University’s telecommunications wiring and installation guidelines, for example, state that all outlets are terminated and tested to Category 6A standards with pinout T568A.
2. Match the building. On an addition or retrofit, open a jack and a panel port and copy what is there.
3. Otherwise pick one and record it. Choose one ethernet wiring standard for the company, use it on every job, and note it on the as-built drawings and the patch panel label.

Straight through vs crossover cable

A straight through cable has the same scheme on both ends, A to A or B to B. It is the normal patch cord between a device and a jack, or between a patch panel and a switch.

A crossover cable has T568A on one end and T568B on the other. That swaps the pairs on pins 1, 2, 3 and 6, which older 10BASE-T and 100BASE-TX ports needed to connect two like devices, such as switch to switch. A full gigabit crossover also swaps the blue and brown pairs: pin 1 to 3, 2 to 6, 4 to 7 and 5 to 8.

Most current switches, routers and network adapters detect the pair arrangement and correct it automatically, a feature called Auto MDI-X. On modern gear, straight through cables work almost everywhere, and a crossover is mainly kept for legacy equipment.

Mixing schemes on a permanent link

The real failure is a horizontal run with one scheme at the jack and the other at the patch panel. That run becomes a crossover in the wall. A modern switch may still link through it, but a certification or wire-map tester will fail it with crossed pairs, and the next technician loses time tracing it.

The usual cause is simple: most jacks and panels print both A and B color codes, and the installer reads the wrong row. Pick the row once, mark it on the tool bag if needed, and test every run before closing the ceiling.

Terminating to either scheme: field tips

• Keep the pair twists as close to the termination as possible. The TIA limit is about half an inch (13 mm) of untwisted conductor for Category 5e and higher.
• Do not strip more jacket than the jack or plug needs, and do not kink the cable at the back of the jack.
• Use plugs rated for the cable: solid-conductor plugs for bulk cable, stranded plugs for patch cable, and the correct category.
• Test every run for wire map, length and continuity, and label both ends.

Cable choice matters more than the pinout for performance. Our guides on Cat5e vs Cat6 for security camera runs and CMR vs CMP cable ratings cover the jacket and category side.

Patch cords and supplies for the job

Factory patch cords are wired straight through, almost always to T568B, and they work in a T568A building because both ends match. That is why a finished cord rarely needs checking against the building’s scheme.

Bursma supplies finished cables for dealers and integrators within its wire, cable and connector product division, including the 7 foot white Cat 6 patch cable, plus Cat5e and Cat6 bulk cable for low-voltage installs and the wholesale connectors category. Integrators in central Ohio can reach the Columbus, Ohio low-voltage distributor branch, and new installers can apply for a Bursma dealer account.

Frequently asked questions