PoE Wiring Diagram: Power over Ethernet Standards and Pair Pinouts

PoE Wiring Diagram: Power over Ethernet Standards and Pair Pinouts — interactive diagram. Open it in the editor to customise components and wiring.

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A PoE wiring diagram shows how power is delivered alongside data over Ethernet cable, identifying which twisted pairs carry DC voltage under IEEE 802.3af, 802.3at, and 802.3bt standards and how they relate to the RJ45 connector pinout.

Power over Ethernet (PoE) is a technology defined by IEEE 802.3 that allows a single Ethernet cable to carry both network data and DC electrical power simultaneously to a powered device (PD) such as a VoIP phone, IP camera, wireless access point, or thin client. Power is delivered by a power sourcing equipment (PSE) — typically a PoE switch or midspan injector.

An Ethernet cable contains four twisted pairs (eight conductors total) terminated on an RJ45 connector. The standard 8P8C RJ45 pinout assigns pairs as follows: pair 1 (blue, blue/white — pins 4 and 5), pair 2 (orange, orange/white — pins 1 and 3), pair 3 (green, green/white — pins 2 and 6), and pair 4 (brown, brown/white — pins 7 and 8). Pair 2 and pair 3 carry data in 10BASE-T and 100BASE-TX (Fast Ethernet), using pins 1, 2, 3, and 6. Pair 1 and pair 4 are unused for data at these speeds.

IEEE 802.3af (PoE, 2003): Power is delivered on either the data pairs (Alternative A: pins 1 and 2 positive, 3 and 6 negative, or pins 3 and 6 positive, 1 and 2 negative depending on polarity) or the spare pairs (Alternative B: pins 4 and 5 positive, 7 and 8 negative). Maximum power at the PSE is 15.4 W; minimum guaranteed power at the PD is 12.95 W after cable losses. Voltage at the PSE is 44–57 V DC.

IEEE 802.3at (PoE+, 2009): Maintains compatibility with 802.3af but raises maximum PSE power to 30 W (minimum 25.5 W at PD). Still uses only two pairs (Alternative A or B). Voltage range remains 50–57 V DC. Both power pairs use the same two conductors.

IEEE 802.3bt (PoE++, 2018): Type 3 delivers up to 60 W at the PSE (51 W at PD); Type 4 delivers up to 100 W at the PSE (71.3 W at PD). 802.3bt requires all four pairs to carry power simultaneously — Alternative A powers data pairs and Alternative B powers the spare pairs at the same time. This requires a cable that supports all four pairs, meaning Cat 5e or higher (Cat 5e has adequate current capacity for 802.3bt at moderate cable lengths, but Cat 6 or Cat 6A is recommended for full-length runs at maximum power).

For PoE to work, the cable must be Ethernet-standard straight-through wiring. Crossover cables are not compatible with Alternative A. The cable length limit for PoE is the standard Ethernet maximum of 100 metres, though voltage drop increases with length and high-power 802.3bt may deliver less than rated power at 100 m. Use the lowest cable resistance possible (larger conductor gauge) for long runs at high power.

How to wire poe wiring diagram

Determine the power requirement of the powered device (PD) Check the PD's specification for its PoE class (0 through 8) or watt requirement. Class 0 is up to 15.4 W (unclassified); Class 1 is up to 4 W; Class 2 is up to 7 W; Class 3 is up to 15.4 W (802.3af); Class 4 is up to 30 W (802.3at); Classes 5 and 6 are up to 45 and 60 W (802.3bt Type 3); Classes 7 and 8 are up to 71.3 W (802.3bt Type 4). The PSE must support the required class.
Verify cable specification Confirm the installed cable is Cat 5e or better and is terminated as a straight-through (T568A or T568B, consistently applied at both ends). Ensure the cable run does not exceed 100 m. For 802.3bt Type 4 at full power, measure the loop resistance of all four pairs; it must not exceed the 802.3bt specification to ensure adequate voltage at the PD.
Confirm PSE PoE standard support Verify the PoE switch or midspan injector supports the required PoE standard (802.3af, 802.3at, or 802.3bt). Check the per-port and total PoE power budget of the switch: a 24-port switch with 370 W PoE budget cannot simultaneously supply 30 W to all 24 ports. Calculate the expected total PoE load and verify it is within the switch's rated budget.
Make the physical cable connections Terminate both ends of the Ethernet cable to RJ45 plugs or keystone jacks using the same wiring standard (T568A or T568B) at both ends. T568B is the more common choice in commercial installations: orange/white on pin 1, orange on pin 2, green/white on pin 3, blue on pin 4, blue/white on pin 5, green on pin 6, brown/white on pin 7, brown on pin 8. Use a cable tester to verify correct pinout and continuity before connecting PoE equipment.
Connect the PD and PSE Plug the cable into the PD (camera, access point, phone) at one end and the PoE switch port or midspan injector at the other. The PSE will automatically detect the PD signature and apply power. The PD will power on within the startup time specified in its datasheet. No additional configuration is usually required on the cable; PoE power allocation may need configuration on managed switches.
Verify operation and measure delivered power if needed Verify the PD has powered on and is communicating on the network. On a managed PoE switch, the per-port power consumption is usually displayed in the management interface. For 802.3bt applications at high power, verify the delivered power meets the PD's minimum requirement, particularly on longer cable runs. A PoE analyser can measure actual voltage and current delivered to the PD.

Specifications

IEEE 802.3af (PoE) PSE output power	Maximum 15.4 W per port; minimum 12.95 W guaranteed at PD
IEEE 802.3at (PoE+) PSE output power	Maximum 30 W per port; minimum 25.5 W guaranteed at PD
IEEE 802.3bt Type 3 (PoE++) PSE output power	Maximum 60 W per port; minimum 51 W at PD; uses all 4 pairs
IEEE 802.3bt Type 4 (PoE++) PSE output power	Maximum 100 W per port; minimum 71.3 W at PD; uses all 4 pairs
PSE output voltage range	44–57 V DC (802.3af/at); 50–57 V DC (802.3bt)
PoE Alternative A pairs (pins)	Pins 1/2 and 3/6 (data pairs in 10/100 Mbit/s; same pairs used by 1G and 10G)
PoE Alternative B pairs (pins)	Pins 4/5 (positive) and 7/8 (negative) — spare pairs in 10/100 Mbit/s
Maximum cable length	100 m (328 ft) per Ethernet standard; minimum Cat 5e for PoE, Cat 6A recommended for 802.3bt

Safety warnings

PoE voltages (typically 48–57 V DC) are below the 50 V AC / 75 V DC SELV threshold in IEC 62368-1, and are therefore classified as Safety Extra Low Voltage (SELV) in most jurisdictions. However, current at 48 V over short circuits is sufficient to damage equipment and create fire risk from sustained arcing. Handle PoE cabling with normal electrical care.
A PoE injector or switch draws significant mains power. Installation of the PSE in a building's electrical infrastructure must comply with applicable wiring standards (NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000). The mains input must be on a circuit protected by an appropriate circuit breaker or fuse. This work must be done by a licensed electrician.
Outdoor PoE cable runs exposed to lightning risk must be protected by an appropriate surge protection device (SPD) rated for data lines (typically IEC 61643-21) installed at the point of entry to the building. A direct or nearby lightning strike to an outdoor Ethernet cable can destroy connected equipment and cause fire.
Do not use non-standard or unrated cable (flat cable, telephone cable, solid-core cable not rated for Ethernet) for PoE installations. Non-compliant cable has higher resistance, causing excessive voltage drop and heat generation, and may not meet safety or performance specifications.

Tools needed

Ethernet cable tester (wire map, continuity, split pair detection)
RJ45 crimping tool (for field-terminated plugs)
Punch-down tool (for keystone jacks or patch panels)
Cable stripper (for removing cable outer sheath)
Multimeter (DC voltage range, for verifying PoE voltage at PD connector)
PoE network analyser (for verifying delivered power class and wattage)
Cable fish tape or pull string (for routing cable through walls or conduit)

Common mistakes

Using a crossover cable instead of a straight-through cable: Alternative A PoE (the most common mode) requires straight-through wiring. A crossover cable reverses the data pairs and prevents correct data operation; it may also cause incorrect polarity on the power pairs.
Exceeding the 100 m cable length limit: signal quality and PoE power delivery both degrade beyond this length. For longer distances, use PoE extenders or add intermediate network equipment.
Underestimating the total PoE power budget of a switch: a 24-port PoE switch with a 185 W budget cannot power 24 devices that each draw 15 W — it can only supply 185 W total. Plan the power budget before deployment.
Using Cat 5 (not Cat 5e) for PoE++ installations: original Cat 5 has higher resistance per pair than Cat 5e; at full 802.3bt power levels this causes excess heat in the cable and may cause the PSE to reduce power or shut off the port.
Mixing T568A and T568B termination standards on opposite ends of the same cable: this creates a crossover cable, not a straight-through. Always use the same standard at both ends.

Troubleshooting

PD does not power on when connected to PoE switch port: Cause: PoE switch port does not support the PD's power class; PoE standard mismatch (e.g., 802.3bt PD on 802.3af-only switch); cable wiring fault preventing PoE detection Fix: Verify the switch port PoE standard against the PD's requirement. Use a cable tester to confirm correct straight-through wiring on all four pairs. Check the switch's management interface for the port's PoE status and any error messages. Test with a different, shorter cable to rule out cable resistance issues.
PD powers on but performs erratically or reboots periodically: Cause: Insufficient power delivered to PD due to long cable run, high cable resistance, or undersized PSE power budget Fix: Measure the DC voltage at the PD's RJ45 port with a multimeter or PoE analyser. PD input voltage should be within 37–57 V for 802.3af/at or the 802.3bt specified range. If voltage is below minimum, shorten the cable run, upgrade to lower-resistance cable, or reduce the distance between PSE and PD.
Ethernet link works but PoE power is not delivered: Cause: Non-compliant detection signature on PD; passive (non-standard) PoE switch expected; or PSE PoE power budget exhausted by other ports Fix: Check whether the PSE is using passive (non-IEEE) PoE — if so, it outputs power unconditionally without detection and may not recognise a standard compliant PD. Check the PSE's total PoE power budget and reduce load on other ports if the budget is exhausted. Verify the PD is a genuine PoE-compliant device with the correct detection resistor.

Frequently asked questions

What is Alternative A and Alternative B in PoE wiring?

Alternative A delivers PoE power over the data pairs (pins 1/2 and 3/6 — the same pairs carrying 10/100 Mbit/s data). Power is superimposed as DC on top of the AC data signal using centre-tapped transformer isolation at each end. Alternative B delivers power over the unused spare pairs (pins 4/5 positive and 7/8 negative) in 10/100 Mbit/s installations. Both are valid under 802.3af and 802.3at.

Does PoE work with Cat 5, Cat 5e, or Cat 6 cable?

IEEE 802.3af and 802.3at PoE work reliably with Cat 5e or better; older Cat 5 (not Cat 5e) may work but is not recommended due to its higher conductor resistance causing greater voltage drop over long runs. IEEE 802.3bt (PoE++) at full power requires Cat 5e minimum, with Cat 6 or Cat 6A recommended to keep voltage drop within specification over 100 m runs.

Can a PoE switch damage a non-PoE device connected to it?

A compliant 802.3af/at/bt PSE first performs a detection phase — it applies a small probe voltage and checks for the characteristic signature resistor (typically 25 kΩ) that identifies a compliant PoE device. Power is only applied if the signature is detected. A standard Ethernet device presents no PoE signature, so a compliant PSE will not apply power. Non-compliant or faulty PSEs could theoretically deliver power without detection; always use standards-compliant equipment.

What is the maximum cable length for PoE?

The maximum cable length for PoE is 100 metres, the same as standard Ethernet. At this length, conductor resistance causes a voltage drop that reduces power delivered to the PD. For 802.3bt Type 4 (100 W) at 100 m, the power delivered at the PD is limited to 71.3 W. At shorter distances, more power is available. For long runs, use cable with larger conductor cross-section (lower AWG number) to reduce resistance.

What is a PoE midspan injector and when is it used?

A midspan injector (also called a PoE injector or power injector) is a device that adds PoE power to an existing Ethernet link. It sits between a non-PoE switch and a PoE-powered device, inserting DC power onto the cable pairs. It is used when upgrading an installation that has an existing non-PoE network switch, avoiding the need to replace the switch. Midspan injectors are available for single ports or as multi-port midspan panels.