PoE Magnetics Are Chosen by Center-Tap Current: MagJacks for 802.3af, at and bt

Power over Ethernet pushes DC through the same magnetics that carry the data, so choosing a PoE MagJack turns on current, pairs and bias:

Key Takeaways

• Choose PoE magnetics by center-tap current rating for the PoE type, not by speed grade — a 1000BASE-T MagJack is not automatically rated to carry PoE.
• IEEE 802.3af (Type 1) delivers 15.4 W at the PSE at 350 mA; IEEE 802.3at (Type 2, PoE+) delivers 30 W at 600 mA per pair set.
• IEEE 802.3bt Types 3 and 4 power all four pairs — 60 W and 90 W at the PSE across classes 5 to 8 — so every center tap in the jack carries current.
• PoE current cancels in an ideal core, but pair resistance unbalance leaves net DC that lowers OCL, so a part meeting its OCL figure at 8 mA can fall short under PoE bias.
• A footprint-compatible non-PoE MagJack is not a PoE replacement: its cable-side center taps are often tied into a termination network instead of brought out to pins.

Engineering rule of thumb: size the center-tap rating from the pair-set current your highest supported class draws, then read OCL at a PoE-level bias — the headline OCL figure describes a non-PoE operating point.

Where PoE Current Flows Through the Magnetics

PoE never touches the differential signal. The power sourcing equipment (PSE) applies DC between two pair sets: current enters the cable-side center tap of one pair's 1CT:1CT transformer, splits equally across that pair's two conductors, and returns through the center tap of another pair. Because the current is common-mode on each pair, the 100 Ω differential data rides through unaffected.

IEEE 802.3af and 802.3at define two ways to do this. Alternative A powers the pairs on pins 1/2 and 3/6; Alternative B powers the pairs on pins 4/5 and 7/8. A powered device (PD) must accept power on either, which is why PD inputs sit behind a bridge rectifier. IEEE 802.3bt adds four-pair powering, using both sets at once.

The practical consequence for jack selection: every pair that can carry power needs its cable-side center tap available — brought out to a pin on a PD, or driven by the PSE controller on a switch. On RJ45 MagJacks, that center-tap topology is fixed inside the part, so it has to be read from the internal schematic before layout, not discovered at bring-up.

PSE Side vs. PD Side: What Changes for the Jack

The same speed grade of MagJack is specified differently at each end of the link. On the PSE side — a switch, injector or network video recorder — the cable-side center taps connect to the PSE controller's output, and the jack sits in a row of other powered ports. Port density and heat dominate the selection.

On the PD side — a camera, access point, industrial sensor or HMI — the cable-side center taps feed the bridge rectifier and the PD controller. The jack only ever carries one port's current, but it must bring every powered center tap out to a pin, and it often sits in a sealed enclosure with no airflow. Whether the magnetics sit inside the jack or beside it is a separate decision, covered in our guide to integrated vs. discrete Ethernet magnetics; the ratings below apply either way.

The Three Ratings That Decide a PoE MagJack

A PoE MagJack is qualified on three ratings, and the datasheet's headline numbers rarely describe your operating point.

• Center-tap current. The winding halves and the center-tap path carry the full pair-set current, so the rating must cover the current your highest supported class draws — with margin at maximum ambient, not at room temperature.
• OCL under DC bias. Ideally the PoE current flows in opposite directions through the two winding halves and its flux cancels. Real cable and contacts are never perfectly balanced, so a net DC remains and pushes the core toward saturation. Industry-typical 100BASE-TX magnetics are specified at 350 µH or more with 8 mA of bias; at PoE current the absolute unbalance is larger, so read OCL at a PoE-level bias.
• Temperature rise. Resistive loss in the windings and center-tap path heats the part; check it against the rated ambient range, which runs to −40 to +85 °C on industrial-grade parts.

The check to run on your own schematic: find the class your PD requests or your PSE supplies, map it to its type and pair-set current in the table below, confirm whether power arrives on data pairs, spare pairs or all four, and then verify on the jack's internal schematic that every powered pair's center tap is brought out — not tied into a termination network.

IEEE 802.3bt: What Types 3 and 4 Demand

IEEE 802.3bt (2018), commonly called PoE++, moved Power over Ethernet from two powered pairs to four. Type 3 delivers 60 W at the PSE and 51 W at the PD; Type 4 delivers 90 W and 71.3 W. Classes 5 and 6 belong to Type 3, and classes 7 and 8 to Type 4.

Four-pair powering changes the magnetics requirement in kind, not just in degree. All four cable-side center taps now carry current, so a jack designed for two-pair Type 1 or Type 2 operation cannot be pressed into Type 3 or Type 4 service, even when the footprint matches. The thermal load also rises with total power, which matters most in sealed PD enclosures.

Per-pair current at Types 3 and 4 depends on the class and the four-pair current sharing, so take it from the jack datasheet rather than extrapolating from the Type 2 figure. Most lines from an RJ45 modular jack manufacturer separate their PoE++ parts from PoE and PoE+ parts for exactly this reason.

Multi-Port PoE: Heat Adds Up in Ganged Jacks

A single powered port is rarely the thermal problem. A PSE built on 1xN ganged or 2xN stacked multi-port RJ45 connectors puts many sets of powered windings into one housing, and the heat from each adds to its neighbours. Inner ports run hottest because they are surrounded on both sides.

Derate per the multi-port datasheet rather than the single-port figure, plan airflow across the jack row, and treat the maximum supported class on every port at once as the design case — that is the condition a fully loaded switch will eventually see.

PoE Types and What the Magnetics Must Carry

Frequently Asked Questions

Is PoE+ the same as 802.3at?

Yes — PoE+ is the common name for IEEE 802.3at (2009), also designated Type 2. It delivers 30 W at the PSE and 25.5 W at the powered device, at 600 mA per pair set. PoE refers to IEEE 802.3af (Type 1), and PoE++ to IEEE 802.3bt (Types 3 and 4).

What magnetics do I need for PoE?

You need magnetics rated for your PoE type: center taps brought out for every powered pair, a center-tap current rating that covers the pair-set current, and OCL specified under PoE-level DC bias. Speed grade and PoE rating are separate specifications, so confirm both on the datasheet rather than assuming one implies the other.

Can a non-PoE MagJack carry PoE current?

No — a non-PoE MagJack should not be used on a powered port. Its cable-side center taps are often joined through a resistor termination network, and its windings are not rated for PoE current or the unbalance that comes with it. Standards-compliant PSEs detect a valid PD signature before applying power, but a passive injector applies voltage regardless.

Does 802.3bt need four-pair magnetics?

Yes, for Type 3 and Type 4 operation. IEEE 802.3bt powers all four pairs, so all four cable-side center taps carry current and must be brought out and rated accordingly. A two-pair PoE or PoE+ MagJack cannot substitute, even when its footprint matches.

Can I second-source a PoE MagJack without re-qualifying the board?

Often yes, if the replacement matches on more than the footprint. It must equal the original on PoE type, center-tap current rating, internal center-tap topology, OCL under bias, operating temperature range and LED configuration. An RJ45 cross-reference tool shortlists candidates by original part number, and drop-in equivalents, samples and mechanical drawings are available on request for verification.

Selecting PoE magnetics comes down to the PoE type and pair count you must support, a center-tap current rating sized for your highest class at maximum ambient, and OCL read at a PoE-level bias rather than the non-PoE headline figure. Confirm the internal center-tap topology on the schematic before layout — it is the one rating a matching footprint will not reveal.