Ethernet PoE — pinout and power
Definition
IEEE 802.3 Power over Ethernet (PoE) carries DC power and Ethernet data over the same four-pair cable. Power Sourcing Equipment (PSE) supplies energy, while the Powered Device (PD) receives it. A compliant PSE checks for a valid PD before applying operating power.
T568A and T568B conductor identification
The illustrations show the contacts visible, the cable pointing down, and pins 1 through 8 from left to right. They identify conductor colours; the PoE alternative is always determined by pin number.
Do not confuse them: T568A/T568B are two colour orders. Alternative A/Alternative B, also called Mode A/Mode B, are two pairsets available for PoE power.
PoE pairset pinout
| Pin | T568A | T568B | 10/100BASE-T signal on MDI | 1000BASE-T and faster signal | PoE alternative | Common-mode power role |
|---|---|---|---|---|---|---|
| 1 | White/green | White/orange | TX+ | BI_DA+ | Alternative A | Label A1 — same DC pole as pin 2 |
| 2 | Green | Orange | TX− | BI_DA− | Alternative A | Label A1 — same DC pole as pin 1 |
| 3 | White/orange | White/green | RX+ | BI_DB+ | Alternative A | Label A2 — same DC pole as pin 6 |
| 4 | Blue | Blue | Not used | BI_DC+ | Alternative B | Label B1 — same DC pole as pin 5 |
| 5 | White/blue | White/blue | Not used | BI_DC− | Alternative B | Label B1 — same DC pole as pin 4 |
| 6 | Orange | Green | RX− | BI_DB− | Alternative A | Label A2 — same DC pole as pin 3 |
| 7 | White/brown | White/brown | Not used | BI_DD+ | Alternative B | Label B2 — same DC pole as pin 8 |
| 8 | Brown | Brown | Not used | BI_DD− | Alternative B | Label B2 — same DC pole as pin 7 |
- Alternative A: power travels in common mode on pairs 1–2 and 3–6, which also carry 10/100BASE-T data. Injection through transformer centre taps is sometimes called phantom power.
- Alternative B: power uses pairs 4–5 and 7–8. They are “spare” only on 10/100BASE-T; they also carry data on 1000BASE-T and faster links.
- Four-pair powering (4PPoE): Alternatives A and B are powered together. This enables the highest Type 3 and Type 4 power levels and reduces cable loss compared with two-pair powering.
In the table, A1/A2 and B1/B2 are polarity-neutral labels, not electrical signs. Within each twisted pair, both conductors carry the same DC pole in common mode; another pair provides the return path. The Ethernet signal remains differential. A compliant PD is designed to accept the polarity configurations permitted by IEEE 802.3, so no universal “+” and “−” should be assigned to pins or cable colours.
PoE types and available power
| IEEE type | Original amendment | Common name | Classes | Powered pairs | Maximum PSE power | Maximum power available at PD* |
|---|---|---|---|---|---|---|
| Type 1 | IEEE 802.3af | PoE | 0–3 | 2 | 15.4 W | 13.0 W |
| Type 2 | IEEE 802.3at | PoE+ | 4 | 2 | 30 W | 25.5 W |
| Type 3 | IEEE 802.3bt | Often “PoE++” | 1–6 | 2 or 4; four for maximum power | 60 W | 51 W |
| Type 4 | IEEE 802.3bt | Often “PoE++” | 7–8 | 4 | 90 W | 71.3 W |
The marketing term PoE++ is ambiguous: it may refer to different Type 3 or Type 4 levels. 4PPoE describes simultaneous power on all four pairs, not one specific power level. To avoid a sizing error, check the Type, class, PSE output power, and power guaranteed at the PD.
Detection, classification, and power-up
- Detection: the PSE probes the link at low voltage and looks for a PD’s nominal 25 kΩ resistive signature. Without a valid signature, an IEEE PSE must not apply normal operating power.
- Classification: the PSE and PD establish the power class through physical-layer signalling. LLDP can supplement this exchange to refine requested and available power.
- Start-up: the PSE limits inrush while the PD input capacitance charges, then supplies the negotiated budget.
- Maintain power: the PD presents a maintain-power signature. The PSE may shut down the port when the load is unplugged or no longer meets the required conditions.
IEEE 802.3bt additionally provides four-pair capability checks, single- and dual-signature PD operation, and optional Autoclass. Use a compliant PSE or PD controller instead of an improvised discrete injector.
Passive PoE warning
A so-called passive PoE injector may apply voltage immediately, without IEEE detection, using vendor-specific voltage, polarity, and pins. It can damage ordinary Ethernet equipment or a PD designed for a different input.
- Never treat “24 V passive”, “48 V passive”, or a proprietary pinout as equivalent to IEEE 802.3af/at/bt.
- Explicitly verify voltage, polarity, pins, and compatibility at both ends before connection.
- A simple continuity tester cannot establish that a port supplies standards-compliant PoE.
Cable and installation integrity
- Use a compliant four-pair channel that meets the category required by the data rate and is suitable for the PoE current.
- For high power and cable bundles, check temperature rise, conductor gauge, DC resistance, and resistance unbalance between conductors.
- The PD figures in the table account for loss in a standards-compliant channel; thin, long, or poorly terminated cable increases voltage drop and heating.
- Connector shielding or shell does not replace the power conductors and must not be used as the PoE return path.
Related documentation
- Power over Ethernet: operation, power, and design
- Ethernet RJ45 pinout — T568A and T568B
- Straight-through and crossover Ethernet cables
- Ethernet cable categories from Cat 5 to Cat 8
- Ethernet BASE-T rates from 10 Mbit/s to 40 Gbit/s
Official and technical sources
- IEEE 802.3-2022 — Standard for Ethernet and power via the MDI
- IEEE 802.3bt-2018 — DTE Power via MDI over 4-Pair
- Ethernet Alliance — Overview of IEEE 802.3bt Power over Ethernet
- Microchip — PoE power interfaces, Alternatives A/B, and four-pair powering
- Microchip — PoE detection and the 25 kΩ signature
- ANSI/TIA-568.2-E — balanced twisted-pair cabling and components