Power over Ethernet does not add wires — it re-uses the eight contacts already inside every RJ45 jack. Knowing exactly which pins carry DC in each PoE mode is the difference between a socket that powers a camera reliably and one that overheats or never negotiates. This guide maps the RJ45 jack (female 8P8C socket / 母座) pinout for every PoE mode, backed by the IEEE 802.3 standard, and shows why the magnetics inside the jack decide whether Mode A and 4-pair PoE work at all.
Short answer: A PoE-capable RJ45 jack still has 8 pins in the standard 8P8C layout. In Mode A (Alternative A) power rides on the data pairs — pins 1, 2 and 3, 6. In Mode B (Alternative B) power rides on the spare pairs — pins 4, 5 and 7, 8. In 4-pair PoE (4PPoE / IEEE 802.3bt Type 3 and Type 4), all four pairs — every pin 1–8 — carry power at once. Power reaches the wire through the center taps of the jack's transformers, so the socket's magnetics, not its plastic housing, are what make PoE possible.
The 8-position/8-contact (8P8C) modular jack is defined mechanically by IEC 60603-7, and the pair-to-pin mapping by ANSI/TIA-568 (the T568A/T568B schemes). PoE — defined by IEEE 802.3 Clause 33 (Type 1/2) and Clause 145 (Type 3/4) — sits on top of that same pinout. The table below shows what each pin does for data and for power.
| Pin | Pair | 10/100BASE-T signal | 1000BASE-T signal | Mode A (2-pair) | Mode B (2-pair) | 4PPoE (Type 3/4) |
|---|---|---|---|---|---|---|
| 1 | 2 | TX+ | BI_DA+ | Power | data only | Power |
| 2 | 2 | TX− | BI_DA− | Power | data only | Power |
| 3 | 3 | RX+ | BI_DB+ | Power | data only | Power |
| 4 | 1 | — (spare) | BI_DC+ | unused | Power | Power |
| 5 | 1 | — (spare) | BI_DC− | unused | Power | Power |
| 6 | 3 | RX− | BI_DB− | Power | data only | Power |
| 7 | 4 | — (spare) | BI_DD+ | unused | Power | Power |
| 8 | 4 | — (spare) | BI_DD− | unused | Power | Power |
The IEEE standard lets a Power Sourcing Equipment (PSE) port be wired either as MDI or MDI-X, which flips the polarity on the powered pairs. In Mode A, an MDI-X port puts +V on pins 3&6 and −V on pins 1&2, while an MDI port reverses it. In Mode B, pins 4&5 are typically +V and pins 7&8 are −V. Because polarity is not guaranteed, every compliant Powered Device (PD) sits behind an input diode bridge that accepts either polarity. For 4-pair 802.3bt, the standard even fixes the reference: a Type 4 PSE presents Alt-A as MDI-X and Alt-B as MDI simultaneously.
In Mode A the same pair carries a differential data signal and a DC supply. This works because the DC is applied as common-mode "phantom" power to the center tap of the jack's isolation transformer. The two conductors of a pair sit at the same DC potential, so the transformer sees no DC across its winding and the differential data passes untouched. Take away the center-tapped magnetics and Mode A / 4PPoE simply cannot be delivered — which is why the jack you choose matters as much as the PSE.
This is the core of an integrated-magnetics RJ45 socket (magjack): the transformer, common-mode choke and center-tap connections live inside the connector housing. VOOHU's SYT-series integrated RJ45 jacks bring the center taps out for PoE, rated for 57 VDC / 350 mA PoE operation with 2250 VDC isolation and 350 µH OCL magnetics — the electrical foundation Mode A and 4PPoE rely on.
The pinout tells you where power flows; the PoE type tells you how much. Higher power classes drive more current per conductor, so the jack's contacts and magnetics must be rated for it — a critical, and often skipped, selection step.
| Standard | Common name | Type | Max PSE power | Max PD power | Pairs used | Max current / pair |
|---|---|---|---|---|---|---|
| 802.3af | PoE | Type 1 | 15.4 W | 12.95 W | 2 | 350 mA |
| 802.3at | PoE+ | Type 2 | 30 W | 25.5 W | 2 | 600 mA |
| 802.3bt | PoE++ / 4PPoE | Type 3 | 60 W | 51 W | 2 or 4 | 600 mA |
| 802.3bt | PoE++ / 4PPoE | Type 4 | 90 W | 71.3 W | 4 | 960 mA |
Typical PSE output is 44–57 VDC (rising to ~50–57 V for higher types); a PD must operate down to about 37 VDC at its input to tolerate cable drop. Full class negotiation and power budgets are covered in our PoE / PoE+ / PoE++ standards guide.
Match the socket to the highest PoE type it must ever pass, not just today's load:
| If your design is… | Pins carrying power | Jack requirement |
|---|---|---|
| 10/100 + Mode B midspan | 4,5,7,8 | Any jack with the spare pairs terminated; discrete or integrated magnetics. |
| 10/100/1000 + Mode A endspan | 1,2,3,6 (via center taps) | Integrated-magnetics magjack with accessible, current-rated center taps. |
| 802.3bt Type 3/4 (4PPoE) | All 8 | Magjack rated for full per-pair current (up to 960 mA) and 57 VDC isolation on every pair. |
VOOHU integrated RJ45 sockets cover 100M, 1000M, 2.5G, 5G and 10G/HDBaseT speeds in 1×1, 1×N and 2×N stacked bodies, with center-tapped magnetics sized for PoE through 802.3bt. See the THT vs SMT mounting guide when you fix the footprint.
In Mode A (Alternative A), power rides on the data pairs at pins 1, 2, 3 and 6. In Mode B (Alternative B), power uses the spare pairs at pins 4, 5, 7 and 8. Four-pair PoE (4PPoE, IEEE 802.3bt Type 3 and Type 4) energizes all eight pins at once.
Mode A delivers DC power over the same pairs used for data (pins 1-2 and 3-6) using the center taps of the jack magnetics, so data and power share the conductors. Mode B delivers power over the spare pairs (pins 4-5 and 7-8) that 10/100 Ethernet does not use for data. A compliant Powered Device accepts either mode via an input diode bridge.
Yes. 1000BASE-T uses all four pairs for bidirectional data (BI_DA to BI_DD on pins 1-2, 3-6, 4-5, 7-8). PoE still coexists because power is injected onto the transformer center taps as common-mode phantom power, so both 2-pair and 4-pair PoE work on a gigabit link.
For Mode A and 4PPoE you need an integrated-magnetics RJ45 jack (magjack) whose transformers have accessible center taps and adequate DC current and isolation ratings. VOOHU SYT-series integrated RJ45 sockets provide center-tapped magnetics, 2250 VDC isolation and 57 VDC / 350 mA PoE handling, sized for IEEE 802.3af through 802.3bt designs.