IEEE 802.3bt Explained: Type 3, Type 4 and 90W PoE

IEEE 802.3bt, ratified in 2018, is the third generation of standardized Power over Ethernet. It roughly triples the power available to a device compared with 802.3at and, for the first time, makes powering all four pairs of a twisted-pair cable a standardized behaviour rather than a vendor extension. If you are specifying equipment for PTZ cameras, Wi-Fi 7 access points, PoE lighting or thin clients, 802.3bt is the specification that matters.

What 802.3bt Actually Changed

Earlier PoE generations pushed power down two of the four pairs in the cable. 802.3bt energises all four, which halves the effective loop resistance and cuts conduction losses for a given delivered wattage. On top of that it adds two new power Types, extends the classification ladder from five classes to nine, and introduces mechanisms that let a PSE reclaim power a device is not actually using. The standard also formalises operation alongside 2.5G, 5G and 10GBASE-T, which the older documents never addressed.

Type 3: 60 W at the Source

Type 3 covers classes 5 and 6 and allows a PSE to source up to 60 W per port, guaranteeing 51 W at the powered device after worst-case channel losses. Classes 5 and 6 require 4-pair operation, while a Type 3 PSE must still support the lower classes for backward compatibility. Most enterprise switches marketed as "PoE++" or "60W PoE" are Type 3 devices.

Type 4: 90 W and the 100 W Question

Type 4 adds classes 7 and 8, taking the PSE up to 90 W per port with 71.3 W guaranteed at the PD. Type 4 is always 4-pair. You will sometimes see "100 W PoE" in marketing copy; that figure refers to the theoretical maximum a Type 4 port can source into the cable under ideal conditions, not to a standardized guarantee. When you compare datasheets, compare the PD-side number, because that is the power your device can actually consume.

Parameter	Type 1 (802.3af)	Type 2 (802.3at)	Type 3 (802.3bt)	Type 4 (802.3bt)
Classes	0-3	4	5-6	7-8
Max PSE power per port	15.4 W	30 W	60 W	90 W
Guaranteed PD power	12.95 W	25.5 W	51 W	71.3 W
Pairs used	2	2	2 or 4	4
PSE output voltage	44-57 V	50-57 V	50-57 V	52-57 V
Minimum cabling	Cat3 / Cat5	Cat5e	Cat5e	Cat5e

Single-Signature and Dual-Signature PDs

802.3bt defines two ways a powered device can present itself on four pairs. A single-signature PD shows one detection signature and one classification signature across both pairsets, and the PSE treats it as a single load. A dual-signature PD presents an independent signature on each pairset, so the two halves can be detected, classified and powered separately. Dual-signature designs exist mainly for devices that internally split power between two subsystems, and for certain PoE lighting fixtures. The distinction matters when you buy a midspan or a splitter: a PSE that only supports single-signature PDs will not correctly power a dual-signature load.

Autoclass and Power Reclamation

Classification tells the PSE which class a device belongs to, but a class is a band, not a measurement. A class 6 device is allocated 60 W even if it never draws more than 38 W, and in a fully loaded switch that wasted allocation is expensive. 802.3bt adds Autoclass, in which the PSE measures the PD's actual peak draw during a short window after power-up and then reserves only that amount plus a margin. On a 24-port switch feeding mixed loads, Autoclass can free enough headroom to avoid a second power supply.

Backward Compatibility

An 802.3bt PSE will correctly detect, classify and power 802.3af and 802.3at devices; the detection and classification sequences were extended, not replaced. The reverse is not true. Plugging a class 8 device into an 802.3at switch produces either no power at all or a device that boots into a reduced-function mode, depending on how the PD firmware handles an under-powered supply. Many PTZ cameras and multi-radio access points behave exactly this way, which is a frequent source of "the camera works but the heater and the zoom motor do not" support tickets.

Specifying Type 3 and Type 4 Equipment

When you write a specification, pin down five things: the Type and class per port, whether the per-port maximum can be sustained on every port simultaneously or only on a subset, the total system power budget, whether the PSE supports single- and dual-signature PDs, and whether Autoclass and LLDP negotiation are implemented. A switch advertising "90 W PoE" that can only deliver it on two of twenty-four ports is a legitimate product, but it is not the product most buyers think they are getting.

Cabling deserves its own line in the specification. Four-pair power at Type 3 and Type 4 levels pushes considerably more total current through a bundle, and the resulting temperature rise limits large installations far more often than any single port's rating does. Cat6 or Cat6a with a documented maximum bundle size is the safe default.