Cat5 Wiring Diagram: T568A vs T568B Pinout and Pair Assignment

Cat5 Wiring Diagram: T568A vs T568B Pinout and Pair Assignment — interactive diagram. Open it in the editor to customise components and wiring.

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A Cat5 wiring diagram shows the T568A and T568B colour-coded pair assignments for the 8P8C (RJ-45) connector, used in Ethernet and structured cabling installations.

Category 5 (Cat5) and its successor Category 5e (Cat5e) are twisted-pair copper cable standards widely used for Ethernet networks and voice applications. The cable contains four twisted pairs of 24 AWG (0.51 mm) copper conductors, totalling eight conductors. Each conductor is insulated in one of eight colour-coded insulations, with four being solid-colour and four being white with a coloured stripe.

The eight conductors are terminated into an 8-position 8-contact (8P8C) modular connector — commonly but imprecisely called an RJ-45 — following one of two wiring standards defined in TIA/EIA-568: T568A or T568B. Both standards define the assignment of each of the eight conductors to the eight connector positions (pins 1 through 8, counted left to right with the locking tab facing away from you).

T568A pin assignment: 1 – White/Green, 2 – Green, 3 – White/Orange, 4 – Blue, 5 – White/Blue, 6 – Orange, 7 – White/Brown, 8 – Brown

T568B pin assignment: 1 – White/Orange, 2 – Orange, 3 – White/Green, 4 – Blue, 5 – White/Blue, 6 – Green, 7 – White/Brown, 8 – Brown

The only difference between T568A and T568B is the swap of the orange pair (pins 1–2 in T568B) and the green pair (pins 3–6 in T568B). Pairs 1 (blue, pins 4–5) and 4 (brown, pins 7–8) are identical in both standards.

For 100BASE-TX and 1000BASE-T Ethernet, the pairs assigned to pins 1–2, 3–6, 4–5, and 7–8 carry the data signals. It is critical that the pairs are not split — each twisted pair must terminate on its designated pair of pins to preserve the differential signalling and noise immunity provided by the twisting.

Straight-through patch cables use the same standard (T568A or T568B) at both ends. Crossover cables use T568A at one end and T568B at the other, swapping the transmit and receive pairs. Modern network equipment uses auto-MDI/MDIX crossover detection, making crossover cables largely unnecessary in new installations.

T568B is more widely installed in North America for commercial structured cabling. T568A is specified by the US government (TIA-568-C) and is common in residential installations. Either is electrically equivalent — consistency within an installation is what matters.

How to wire cat5 diagram

Cut the cable to length and strip the outer jacket Use a cable stripper or utility knife to score and remove approximately 25–30 mm of the outer jacket. Take care not to nick the conductor insulation. Remove any spline filler (ripcord) and any foil shielding if present. Do not untwist the pairs more than necessary — excessive untwisting degrades crosstalk performance.
Separate and arrange the pairs in the correct pin order Untwist each of the four pairs just enough to separate the conductors. Arrange all eight conductors in the pin order for your chosen standard: T568A (W/Grn, Grn, W/Org, Blu, W/Blu, Org, W/Brn, Brn) or T568B (W/Org, Org, W/Grn, Blu, W/Blu, Grn, W/Brn, Brn), reading left to right with the connector tab down. Flatten and straighten the conductors side by side.
Trim the conductors to the correct insertion length Hold all eight conductors tightly in line and trim them square with a sharp cutter so they are exactly 12–14 mm long from the jacket edge. This length allows the conductors to reach the connector pins while leaving enough jacket inside the connector to be gripped by the strain relief boot.
Insert conductors into the 8P8C connector Hold the connector with the tab facing down and the open end towards you. Push all eight conductors simultaneously into the connector channels, ensuring each conductor slides fully to the front of its channel and is visible through the transparent connector body. Verify the colour order against the standard before crimping.
Crimp the connector Insert the loaded connector fully into the crimping tool die. Apply firm, steady pressure until the tool ratchet releases, confirming a complete crimp cycle. The crimp action simultaneously drives the IDC (insulation displacement contact) pins into each conductor and locks the strain relief bar onto the outer jacket.
Test with a cable tester Connect both ends of the completed cable to a cable continuity tester. Verify that all eight conductors map correctly (1-to-1 for a straight-through cable or the expected crossover pattern). A quality tester will also detect split pairs — a common installation error that passes a simple continuity test but fails at higher data rates.

Specifications

Cable standard	TIA/EIA-568-B.2 (Cat5e), ISO/IEC 11801 (Class D minimum)
Conductor size	24 AWG (0.51 mm) solid copper for horizontal cable; 24–26 AWG stranded for patch cords
Number of pairs / conductors	4 pairs / 8 conductors
Connector type	8P8C (8-position 8-contact) modular plug — IEC 60603-7
Wiring standards	T568A or T568B (TIA/EIA-568); must be consistent throughout installation
Maximum channel length (horizontal)	100 m total (90 m fixed horizontal + 10 m combined patch cords)
Bandwidth rating	Cat5e: 100 MHz; supports 1000BASE-T (Gigabit Ethernet) and 100BASE-TX
Applicable standards	TIA/EIA-568-C.2, ISO/IEC 11801, EN 50173

Safety warnings

Cat5e and Cat6 structured cabling is low-voltage data wiring and does not carry mains electricity under normal use. However, Power over Ethernet (PoE) equipment injects DC voltage (typically 48 V) onto the cable pairs. When disconnecting PoE-powered devices, power down the PoE switch port first to avoid shock from the PoE voltage, which — while not typically lethal — can cause a startling and potentially injurious involuntary reaction.
When fishing cable through walls, ceilings, and floors, be aware of proximity to mains electrical wiring. Maintain the minimum separation distance from unshielded power wiring required by TIA-568 (minimum 50 mm from unshielded mains wiring, 150 mm when parallel to wiring above 480 V). Accidental penetration of a mains cable with a fish tape can be fatal.
Confirm the fire rating of any cable being installed in plenum (air handling) spaces. Standard PVC-jacketed cable must not be installed in plenum spaces without conduit; plenum-rated (CMP) cable is required in those locations. Use CMP-rated cable where required by local building code.
When installing structured cabling in commercial premises, work may need to be performed by a licensed cabling contractor or registered communications worker, depending on local telecommunications and building regulations.

Tools needed

RJ-45 / 8P8C crimping tool (matched to connector type)
Cable stripper (Cat5e-specific, to avoid nicking conductors)
Flush cable cutters or electrician's scissors
110 / Krone punch-down tool (for keystone jacks and patch panels)
Cable continuity and wire-map tester (detects split pairs, open circuits, shorts, miswiring)
Cable length meter / TDR (for long runs to verify under 100 m channel length)
Cable labels and permanent marker

Common mistakes

Split pairs — placing both conductors of a pair on adjacent pins that do not belong to the same pair (e.g. white/green on pin 1 and white/orange on pin 2 instead of green on pin 2). This passes a simple continuity test but fails for Ethernet because it destroys the differential pair signal and crosstalk rejection. A proper wire-map tester detects split pairs.
Mixing T568A at one end and T568B at the other on a straight-through patch cable, accidentally creating a crossover cable. Verify the colour order matches at both ends before crimping.
Untwisting pairs more than 13 mm (0.5 inch) at the termination point. Excessive untwisting degrades near-end crosstalk (NEXT) performance, particularly on Cat5e and Cat6.
Using a connector designed for solid-core conductors on a stranded patch cord, or vice versa. Solid-conductor IDC contacts are designed to pierce solid wire; stranded contacts are designed to grip multiple strands. Using the wrong type results in intermittent or failed connections.
Stapling or sharply bending the cable beyond its minimum bend radius (typically 25 mm / 1 inch for Cat5e). Excessive bending distorts the pair geometry and degrades performance.
Not testing completed runs with a wire-map tester before closing walls or hiding cables. Discovering a miswire after the cable is concealed requires costly rework.

Troubleshooting

Network link does not establish (no link light on switch or NIC): Cause: Open circuit on one or more conductors, or connectors not fully crimped Fix: Test the cable with a wire-map tester. Re-examine the connector at the failing end — ensure all eight conductors are fully seated to the front of the connector and visible through the clear body. Re-crimp or replace the connector.
Cable tests pass on continuity tester but network is slow or drops frequently: Cause: Split pairs, excessive untwisting at termination, or cable run exceeding 100 m channel length Fix: Re-test with a tester capable of detecting split pairs (not just simple continuity). Verify total channel length including patch cords is within 100 m. Inspect both ends for excessive untwisting and re-terminate if the untwisted length exceeds 13 mm.
Wire-map tester shows a short between two conductors: Cause: A stray conductor strand bridging adjacent contacts inside the connector, or conductors nicked during stripping Fix: Cut off and replace the faulty connector. Inspect the conductor insulation for nicks from the cable stripper. Ensure no stray strands are present when loading conductors into the connector.

Frequently asked questions

What is the difference between T568A and T568B wiring?

T568A and T568B differ only in the positions of the orange and green pairs. In T568A, the green pair occupies pins 1–2 and the orange pair occupies pins 3–6. In T568B these are swapped. Both are electrically equivalent for Ethernet. The choice matters only for consistency within a single installation — mixing them within a run will create a crossover effect.

What does 8P8C mean, and is it the same as RJ-45?

8P8C (8-Position 8-Contact) is the correct name for the modular connector used in network cabling. RJ-45 is a registered jack designation that originally applied to a specific telephone wiring configuration, but it has become colloquially used to refer to the 8P8C connector. Electrically and physically the connector is the same, but technically they are distinct designations.

Can I use Cat5 cable for Gigabit Ethernet?

Original Cat5 cable is not rated for Gigabit Ethernet (1000BASE-T) because 1000BASE-T uses all four pairs simultaneously and has tighter attenuation and crosstalk requirements. Cat5e (Category 5 enhanced) is the minimum standard for Gigabit Ethernet. If existing Cat5 cable tests to the Cat5e performance parameters, it may work in practice, but it is not standards-compliant for new installations.

What is a crossover cable and when do I need one?

A crossover cable uses T568A at one end and T568B at the other, which swaps the transmit and receive pairs. It was previously required to connect two computers directly or two switches without an uplink port. Modern equipment uses auto-MDI/MDIX, which automatically detects and corrects the pair orientation, making crossover cables unnecessary in most current installations.

How far can Cat5e cable run?

The maximum horizontal cable run for Cat5e in a structured cabling system is 90 m (295 ft) from the patch panel to the wall outlet, with a further 10 m (33 ft) allowance for patch cords at each end — totalling 100 m (328 ft) channel length. Running cable beyond this limit will cause signal attenuation that exceeds the specification, resulting in unreliable Ethernet operation.