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Power on all four pairs, and a switch budget that becomes the real limit

802.3bt: all four pairs, and a budget to read

The amendment that took Power over Ethernet past the point where the switch could be ignored. Two new Types, current on every conductor rather than half of them, and a per-port figure that means much less than the total the unit publishes.

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What the standard defines

IEEE 802.3bt was published in 2018 and folded into the base 802.3 standard. It added Type 3 and Type 4, and with them the structural change the earlier types did not have: power delivered over all four pairs rather than two.

Each Type publishes the familiar pair of figures. A Type 3 source delivers up to 60 W at its port; a Type 4 source delivers up to 90 W. In both cases the standard states separately, and lower, what the powered device is guaranteed — the run of cable between the two takes the difference, exactly as it does at every level below.

The amendment also extends the classification range, adding higher classes through which a device declares what it expects, and it defines how a source recognises a device drawing from four pairs and how the load is shared between them. That last point is why a four-pair source and a two-pair device coexist safely on the same switch.

What it does not promise

It does not promise ninety watts at the far end. Ninety is a port figure. The device figure is published as its own number, and it is the one that has to cover what the equipment needs.

It does not promise a switch that can do it everywhere. This is the level where the per-port class becomes the less useful of the two published numbers: a unit with several Type 4 ports will have a total budget substantially below their sum, and it is the budget that decides how many devices come up. A switch that lists its budget on the data sheet is telling you the thing you need.

It does not promise the cabling will stay cool. Four-pair delivery puts current down every conductor in the sheath, and heat rise in a tightly packed bundle is addressed by the cabling standards and by how the run is installed — not by this amendment.

And it does not promise anything about the words on the carton. PoE++ and the various proprietary names are marketing; the Type number is the specification.

What it works alongside

A Type 3 or Type 4 source powers Type 1 and Type 2 devices, resolving the negotiation downwards, so an upgraded switch keeps everything already on the wall running. It is specified over Category 5e cabling and better, alongside the data, and it appears on the same eight-position modular connector as everything else on this shelf.

The questions that come up before an order

What do PoE++, UPoE and similar names refer to?

They are trade names, and different makers apply them to different things. The figures that can be compared between two data sheets are the Type number, the wattage published at the port, the wattage published at the device, and the unit’s total budget.

Why does a switch with several high-power ports publish a budget below their sum?

Because the internal supply is sized for a realistic load rather than for every port at maximum. The budget is the number that decides how many powered devices the switch actually runs; the per-port class only says what one of them may draw.

Last reviewed 9 September 2026