PoE Standards Explained: IEEE 802.3af vs 802.3at vs 802.3bt

Power over Ethernet (PoE) lets a single Category-rated twisted-pair cable carry both Ethernet data and DC power to a device. Because there is one global family of standards governing how that power is negotiated and delivered, understanding the IEEE 802.3 specifications is the foundation for selecting any PoE switch, injector, splitter, or powered device. This article compares the three ratified generations of standardized PoE and the four power Types they define.

Why a Standard Matters

Standardized (or "active") PoE is built around a negotiation handshake between the power sourcing equipment (PSE) and the powered device (PD). Before any meaningful power is applied, the PSE detects a valid PD by looking for a characteristic 25 kΩ signature resistance, classifies how much power the device needs, and only then ramps up to operating voltage. This sequence prevents a non-PoE device from being damaged when plugged into a PoE port. Standards also guarantee interoperability: a compliant PD from one vendor will work with a compliant PSE from another.

IEEE 802.3af (Type 1) - The Original PoE

Ratified in 2003, IEEE 802.3af defined the first interoperable PoE. The PSE sources up to 15.4 W per port, while approximately 12.95 W is guaranteed at the PD after worst-case cable losses. The PSE output voltage range is 44-57 V DC, the PD must operate down to 37 V, and current is limited to roughly 350 mA per pair. Type 1 power travels over two pairs, using either Alternative A (the data pairs) or Alternative B (the spare pairs). Cat3 or better cabling is sufficient for Type 1, though Cat5e is standard practice today.

IEEE 802.3at (Type 2) - PoE+

The 2009 amendment, commonly branded PoE+, roughly doubled available power. A Type 2 PSE delivers up to 30 W, with about 25.5 W usable at the PD. To carry the higher load, the PSE voltage range tightens to 50-57 V and per-pair current rises to 600 mA. Like Type 1, PoE+ uses two pairs and Cat5 or better. PoE+ became the workhorse for pan-tilt-zoom cameras, dual-radio wireless access points, and video VoIP phones.

IEEE 802.3bt (Types 3 and 4) - PoE++ / 4PPoE

Ratified in 2018, IEEE 802.3bt introduced four-pair power (4PPoE) and two new Types. Type 3 sources up to 60 W at the PSE with about 51 W at the PD; Type 4 sources up to 90 W with roughly 71.3 W at the PD. Both 802.3bt Types raise the PSE voltage floor to 52-57 V. Type 3 can use two or four pairs, while Type 4 mandates all four pairs and permits up to 960 mA per pair. By energizing all eight conductors, 802.3bt spreads current across more copper, reducing per-pair heating even as total power climbs. The standard also added classes 5 through 8 and improved support for low-standby IoT loads.

Side-by-Side Comparison

Parameter	802.3af (Type 1)	802.3at (Type 2)	802.3bt (Type 3)	802.3bt (Type 4)
Common name	PoE	PoE+	PoE++ / 4PPoE	PoE++ / Hi-PoE
PSE output power	15.4 W	30 W	60 W	90 W
Power at PD	12.95 W	25.5 W	51 W	71.3 W
PSE voltage range	44-57 V	50-57 V	52-57 V	52-57 V
PD voltage range	37-57 V	42.5-57 V	41.1-57 V	41.1-57 V
Max current per pair	350 mA	600 mA	600 mA	960 mA
Pairs used	2	2	2 or 4	4 (mandatory)
Min cabling	Cat3 / Cat5	Cat5	Cat5	Cat5e+
Power classes	0-3	4	5-6	7-8

Power Classes and Backward Compatibility

During classification, the PD declares a class that tells the PSE how much power to reserve. Classes 0-3 belong to Type 1, Class 4 to Type 2, Classes 5-6 to Type 3, and Classes 7-8 to Type 4. Critically, all generations are backward compatible: a Type 4 switch will correctly power a legacy 802.3af camera, supplying only the power that the older PD requests. This protects existing investments while allowing networks to scale up to high-power endpoints.

Choosing the Right Type

Match the standard to the heaviest endpoint you expect to power. Fixed IP cameras and basic phones are comfortable on Type 1. PTZ cameras, Wi-Fi 6 access points, and video phones generally need Type 2. High-density Wi-Fi 6E/7 radios, pan-tilt-zoom domes with heaters, LED lighting, and thin clients increasingly require Type 3 or Type 4. As a manufacturer of PoE splitters and custom power modules, we design products to the exact standard tier each deployment demands, ensuring the negotiated handshake, voltage, and current all align with the endpoint's true requirements.