T568A and T568B colour codes for ethernet cable wiring

Ethernet cables are wired to one of two colour standards: T568A or T568B. Both use the same four twisted pairs - orange, green, blue and brown - and differ only in one detail: the orange and green pairs swap positions. Electrically the two are identical. What matters is consistency: wire both ends of a cable to the same standard and it works; the problems start when terminations get mixed by accident.

This guide gives the full pin-by-pin colour codes for both standards, explains which one to use, what each wire pair actually does and how to terminate networking cable so a gigabit link stays a gigabit link.

T568A and T568B colour codes

Hold an RJ45 plug with the clip facing away from you and the contacts facing up: pin 1 is on the left. The wiring order for each standard runs as follows.

Notice that the blue and brown pairs sit in the same positions in both standards - only orange and green trade places. The blue pair's position on the centre pins is a legacy of telephone wiring, where the first phone line occupied pins 4 and 5; the ethernet standards were designed around it.

Which standard should you use?

T568B is the de facto choice for commercial installations in the UK and most existing structured cabling, so if you're adding to a building that's already wired, match what's there - a cable tester or a look inside an existing socket will tell you. T568A is specified in some government and residential standards and is the formally preferred scheme in the TIA standard itself, but in practice the installed base wins the argument.

The genuinely important rule is the boring one: pick one standard for a site and use it everywhere - sockets, patch panels and field-terminated plugs alike. Mixed standards don't fail obviously; they produce accidental crossover cables and intermittent confusion that costs far more time than the original choice ever saved.

What's the difference between straight-through and crossover cables?

A straight-through cable has both ends wired to the same standard - either both T568A or both T568B - and is what almost every modern link uses. A crossover cable has one end wired A and the other B, which swaps the transmit and receive pairs. Historically that was needed to connect two computers directly, or switch to switch, without a hub in between.

Today crossover cables are essentially obsolete: auto MDI-X, which detects and corrects swapped pairs automatically, has been mandatory since gigabit ethernet and near-universal on 10/100 hardware for two decades. If you find a crossover cable in a drawer, label it or bin it - unlabelled, it's a fault waiting to be discovered the hard way.

What do the four pairs actually do?

Which wires carry data depends on the link speed and the difference explains one of the most common network faults.

10/100 Mbps ethernet only uses two pairs, but gigabit uses all four, transmitting in both directions on each. The practical consequence: a cable with a broken wire in the blue or brown pair will happily run at 100 Mbps and refuse to link at gigabit -a fault that looks like a configuration problem but is actually a termination problem.

Power over Ethernet rides on the same wiring. Mode A injects power over the data pairs (pins 1–2 and 3–6), Mode B uses pins 4–5 and 7–8, and the equipment negotiates before applying power — another reason all eight conductors should be terminated properly even on a link that only needs two pairs today.

Cat5e, Cat6 or Cat6a?

The colour codes are identical across categories; what changes is the cable's bandwidth and how far it can carry higher speeds.

The 100 m figure is the total channel limit - conventionally 90 m of fixed cabling plus 10 m of patch leads. Construction matters as much as category: solid-core cable is for fixed runs and punch-down terminations, while stranded cable is for flexible patch leads and each needs its matching type of RJ45 plug, because the contacts pierce solid and stranded conductors differently.

How do you terminate an RJ45 plug?

Strip around 25 mm of jacket, separate and untwist the pairs, arrange the conductors in the chosen colour order, trim them square and push them fully home in the plug before crimping - every conductor should be visible seated against the end of the connector. Plugs with a load bar make the ordering step considerably less fiddly and a proper crimping tool is non-negotiable; pliers improvisations produce the intermittent joints that haunt networks for years.

The detail that separates a working crimp from a good one is twist retention: the pairs are twisted at precise rates to cancel interference and every millimetre you untwist undoes that. Keep untwisting to the absolute minimum - no more than about 13 mm at the termination - and don't yank the twists out of the cable further back to make the conductors easier to arrange. Finally, test: a basic cable tester from the networking diagnostics range confirms continuity and pair order in seconds, which is a great deal cheaper than diagnosing the same fault through a patch panel later. For everything at the other end of the link - from switches to industrial ethernet hardware for panel and factory-floor use - wiring discipline pays off in links that negotiate at full speed first time.

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Frequently asked questions

What is the T568B wiring order?

From pin 1 to pin 8: white/orange, orange, white/green, blue, white/blue, green, white/brown, brown — with the plug clip facing away and contacts up, pin 1 on the left. It's the most common standard in UK commercial installations.

Can I mix T568A and T568B?

Not on the same cable — one end of each standard creates a crossover cable. Across a site it technically works if every individual cable is internally consistent, but it invites termination mistakes, so pick one standard and use it everywhere.

Does T568A or T568B perform better?

Neither. The same four pairs connect to the same pins pair-for-pair; only the colour assignment of the orange and green pairs differs. Performance depends on cable category, termination quality and run length, not the colour standard.

Why does my link run at 100 Mbps instead of gigabit?

The classic cause is a fault on the blue or brown pair — pins 4, 5, 7 or 8. Fast ethernet only uses two pairs, so it survives the fault; gigabit needs all four and falls back. Re-terminate and test all eight conductors.

Do I need Cat6a for gigabit?

No — Cat5e carries gigabit to the full 100 m channel limit. Cat6a earns its premium where 10 gigabit over full-length runs is a genuine requirement, or as future-proofing in new permanent installations where recabling later would be disruptive.

Should patch leads be solid or stranded core?

Stranded — it tolerates repeated flexing that fractures solid conductors. Solid core belongs in fixed runs terminated onto punch-down sockets and patch panels. Match the RJ45 plug type to the conductor type, as the contact design differs between them.

Wire it once, test it, forget it

From boxed cable and patch leads to crimping tools and testers, everything for a termination that negotiates at full speed first time.