PoE Standards 802.3af / at / bt and Powered-Device (PD) Design: How Many Watts Do You Actually Get?
The three PoE standards (802.3af / at / bt) deliver very different power — and the PD never receives as much as the PSE rating. This guide clarifies the af/at/bt power and cabling differences, the powering handshake, the PD-side power front-end, and uses the WIZnet WIZPoE-P1 module as the fast PD solution.
Three PoE standards: af / at / bt power
PoE carries power and data on the same Ethernet cable. The source-side (PSE) power rises each generation: 802.3af (PoE, Type 1) about 15.4W per port; 802.3at (PoE+, Type 2) about 30W; 802.3bt (PoE++, Type 3) about 60W and Type 4 about 90W (the IEEE figure; some implementations cite up to ~100W). The number of pairs differs too: af/at use 2 pairs, while bt Type 3/4 use all four pairs at once, halving the per-conductor current and reducing cable loss so higher power is feasible; the power classes also extend from Class 0–4 to Class 0–8.
Why the PSE rates 30W but the PD gets 25.5W
A commonly mixed-up point: the PD does not receive as much as the switch (PSE) is rated for. Power travels up to 100m of twisted pair, and the cable resistance dissipates part of it as heat (I²R loss). So what IEEE guarantees is the power the PD is sure to get in the worst case: af = 12.95W, at = 25.5W, bt Type 3 = 51W, Type 4 = 71.3W (about 71W). When you budget the power supply, use this PD-side set of numbers as the ceiling, not the PSE-side 15.4 / 30 / 60 / 90W.
The powering handshake: it won’t fry a non-PoE device
PoE does not blindly apply 48V and risk frying a non-PoE device — there are two handshake stages first. Detection: the PSE applies a low probe voltage (about 2.7–10V) and checks for the PD’s signature resistance of about 25kΩ to confirm a compliant PD is present. Then classification: the PSE applies a slightly higher voltage to read the PD’s class signature (Class 0–4 for Type 1/2, Class 0–8 for Type 3/4, with LLDP negotiation for the highest power) so it allocates the right budget, and only then applies the full operating voltage. A device without the correct signature resistance is never powered.
The PD-side power front-end: three blocks
1) Separate the data path from the power path: at the RJ45, the data signal must couple through the LAN magnetics to the PHY, while power is tapped from the data pairs (Mode A) or spare pairs (Mode B) — note that a PD power module like the WIZnet WIZPoE-P1 usually does not include the data-path magnetics, so you must provide a magnetics-equipped RJ45 yourself. 2) The PD interface: the input needs a rectifier bridge (Mode A/B and polarity are not fixed, so auto-bridging is required) plus a PD interface controller that presents the ~25kΩ signature resistance, provides the class signature and protection — this is the key to being recognized by the PSE and getting the right power. 3) An isolated DC-DC: convert the ~37–57V on the line (typically a flyback) down to the load rail (5V / 3.3V / 12V) with safety isolation.
Selection, and the WIZPoE-P1 shortcut
Start by sizing the power budget: take the PD-side available watts (af 12.95W / at 25.5W / bt 51 / 71.3W), subtract the DC-DC conversion loss (another ~10–25%), and what remains is the truly usable power; if you are near the ceiling, step up to the next standard. Low power (about 5W/5V) is fine on basic 802.3af, and the fastest route is a ready-made PD power module: the WIZnet WIZPoE-P1 is an 802.3af PD module with a 41–61V input, a 5V output of about 7W (the datasheet rates up to ~8–9W), 70–90% efficiency, a built-in rectifier bridge plus isolated DC-DC, and Mode A/B support — packaging the PoE-to-5V front-end for you (remember the data-path magnetics still have to be added separately). JLink Technology is an authorized WIZnet distributor; contact us for WIZPoE-series specs, samples and pricing.
Products mentioned
WIZPoE-P1 PoE Powered Device Module (8W)
WIZPoE-P1 is a WIZnet IEEE 802.3af/at standard PoE Powered Device (PD) module that draws power directly from the Ethernet cable, outputting 5V at up to 8W — no separate power cable needed. Suitable for IP cameras, industrial sensors, APs, access control devices, and other PoE-powered applications.
W5500 Hardwired TCP/IP Ethernet Controller
W5500 is WIZnet's flagship Ethernet controller with a unique hardwired TCP/IP stack, enabling direct network connectivity without software TCP/IP implementation. Connects to the host MCU via SPI, features 32KB TX/RX buffer, supports up to 8 sockets — the most widely adopted Ethernet chip in the Arduino/Raspberry Pi ecosystem.
Need these components or design help?
JLink Technology provides parts, datasheets, and engineering support.
Request a Quote