Running both data and power over a single Ethernet cable sounds simple — and it is, once you understand the standards. But the alphabet soup of IEEE 802.3af, 802.3at, and 802.3bt can get confusing fast. Each standard delivers different power levels, uses different negotiation protocols, and has different cable requirements.
Let's sort through them so you can pick the right PoE standard for your next design.
The Core Idea Behind PoE
Power over Ethernet sends DC power alongside data over standard Cat5e/Cat6 cables. The power is injected by Power Sourcing Equipment (PSE) — typically a PoE switch or midspan injector — and extracted by a Powered Device (PD) at the other end.
The key constraint: the PSE can't just dump voltage onto the cable. It has to negotiate with the PD, confirm the device actually needs power, and deliver the right amount. This prevents damage to non-PoE Ethernet devices that happen to be connected.
PoE Standards at a Glance
| Parameter | 802.3af (PoE) | 802.3at (PoE+) | 802.3bt Type 3 (4PPoE) | 802.3bt Type 4 |
|---|---|---|---|---|
| PSE output power | 15.4W | 30W | 60W | 90W |
| PD input power | 12.95W | 25.5W | 51W | 71.3W |
| PSE voltage range | 44–57V | 50–57V | 50–57V | 52–57V |
| Max current per pair | 350 mA | 600 mA | 600 mA | 960 mA |
| Pairs used | 2 (or 4) | 2 (or 4) | 4 | 4 |
| Classification | Physical layer + optional data | Physical + data layer | Data layer | Data layer |
| Cable requirement | Cat3 minimum | Cat5e recommended | Cat5e minimum | Cat5e minimum |
| Year published | 2003 | 2009 | 2018 | 2018 |
Why PD Power Is Less Than PSE Power
Notice the PD always receives less power than the PSE outputs. That's because power is lost in the cable. A 100-meter Cat5e cable has roughly 12.5Ω of loop resistance per pair. At 350 mA (802.3af), that's about 1.5W lost per pair. The standards account for worst-case cable length.
For 802.3bt Type 4, the PSE delivers 90W but the PD only sees 71.3W — nearly 19W lost in the cable. That's why proper cable gauge matters at higher power levels.
How Power Is Delivered: Alternative A and B
There are two methods for carrying power over Ethernet cables:
Alternative A (Mode A): Power is delivered over the data pairs (pins 1,2 and 3,6). The PSE applies a common-mode DC voltage on these pairs, which doesn't interfere with the differential data signals. This is the more common method and is used by most PoE switches.
Alternative B (Mode B): Power is delivered over the spare pairs (pins 4,5 and 7,8) in 10/100 Mbps connections. In Gigabit Ethernet, all four pairs carry data, so Alternative B uses phantom power on those same pairs.
Ethernet Pinout with PoE
| Pin | 10/100 Mbps | Gigabit | Alt A Power | Alt B Power |
|---|---|---|---|---|
| 1 | TX+ | BI_DA+ | Positive | — |
| 2 | TX- | BI_DA- | Positive | — |
| 3 | RX+ | BI_DB+ | Negative | — |
| 4 | Spare | BI_DC+ | — | Positive |
| 5 | Spare | BI_DC- | — | Positive |
| 6 | RX- | BI_DB- | Negative | — |
| 7 | Spare | BI_DD+ | — | Negative |
| 8 | Spare | BI_DD- | — | Negative |
Most modern PoE switches support both Alternative A and B and auto-detect which one the PD expects.
PoE Negotiation: How Devices Talk
PoE isn't just "apply voltage and go." There's a multi-stage negotiation process:
1. Detection
The PSE probes the cable with a low voltage (2.8–10V) and measures the PD's signature resistance. A valid PoE PD presents a 25kΩ signature resistance. If the PSE doesn't see this signature, it assumes a non-PoE device is connected and doesn't apply power.
2. Classification
After detection, the PSE optionally classifies the PD to determine how much power to budget:
| Class | PD Power Range | Max PD Power |
|---|---|---|
| 0 | Default (unclassified) | 12.95W |
| 1 | 0.44–3.84W | 3.84W |
| 2 | 3.84–6.49W | 6.49W |
| 3 | 6.49–12.95W | 12.95W |
| 4 | 12.95–25.5W | 25.5W (PoE+ only) |
| 5 | 40W | 40W (bt) |
| 6 | 51W | 51W (bt) |
| 7 | 62W | 62W (bt) |
| 8 | 71.3W | 71.3W (bt) |
Classification helps PSE power budgeting — a 24-port PoE switch needs to know it can't deliver 15.4W on all ports simultaneously if its power supply is only rated for 200W.
3. Power Up
Once classified, the PSE ramps the voltage up to the operating range (44–57V) within about 400 ms. The PD has to start drawing current within a specific time window or the PSE disconnects.
4. Power Maintenance
The PSE continuously monitors the current draw. If the PD disconnects (cable unplugged) or the current drops below a minimum threshold for too long, the PSE removes power to protect the circuit.
Real Design Example: PoE-Powered IP Camera
Let's say you're designing a PoE IP camera that draws 8W. Which standard do you need?
- 802.3af can deliver up to 12.95W at the PD — sufficient ✓
- You'd classify as Class 3 (6.49–12.95W range)
- The PSE allocates 15.4W (Class 3 budget), leaving margin for cable losses
- Cable loss at 8W over 100m: roughly 1-2W depending on current draw and cable gauge
What about the voltage at the PD? If the PSE outputs 48V and the cable drops 2V:
V_PD = 48V - (R_cable × I)
I = 8W / 48V ≈ 167 mA
V_PD = 48V - (12.5Ω × 0.167A) = 48V - 2.1V = 45.9V
The PD sees about 46V — well within the valid range. Your buck converter steps that down to 5V or 3.3V for the camera electronics.
Cable Considerations
Cable quality matters, especially at higher power levels:
| Cable Type | AWG | Loop Resistance (per pair, 100m) | Max Recommended Power |
|---|---|---|---|
| Cat5e (standard) | 24 | 12.5Ω | 30W (PoE+) |
| Cat5e (high-quality) | 23 | 9.8Ω | 60W (bt Type 3) |
| Cat6 | 23 | 9.8Ω | 60W |
| Cat6A | 22-23 | 8.4-9.8Ω | 90W (bt Type 4) |
For 802.3bt Type 4 at 90W, you really want Cat6A with 22 AWG conductors. Using cheap Cat5e cable at that power level means excessive voltage drop and heating — the cable itself can become a fire hazard over long runs.
Temperature Rise in Cables
This is often overlooked. A fully loaded Cat5e cable carrying 90W PoE can heat up by 10°C or more, especially in bundled cable runs. In a conduit with 50 cables all carrying PoE+, the center cables can be 15-20°C warmer than ambient.
The TIA standard TIA-568.2-D addresses this with guidance on maximum bundle sizes and derating for elevated temperatures. If you're running PoE at scale — like a large office or data center — pay attention to this.
Active vs. Passive PoE
Active PoE follows the IEEE standards described above. The PSE and PD negotiate, classify, and manage power delivery safely.
Passive PoE (sometimes called "PoE injectors" sold cheaply online) skips the negotiation entirely. It just applies 24V or 48V to the cable, regardless of what's connected. This works with specific devices designed for it but will damage standard Ethernet equipment.
If you're building a system, use active PoE. Passive PoE is fine for hobbyist point-to-point links where you control both ends, but it has no place in professional installations.
Design Considerations for PoE Powered Devices
If you're designing a PD that receives PoE:
Use a PoE PD controller IC. Chips like the Texas Instruments TPS2378, Silicon Labs Si3404, or Maxim MAX5969 handle detection, classification, inrush current limiting, and thermal shutdown. Building this circuit from discrete components is possible but not worth the engineering time.
Include proper isolation. IEEE 802.3 requires 1500V isolation between the PoE input and the PD's circuitry. Your Ethernet transformer provides this, but the DC-DC converter after the PoE interface also needs to meet isolation requirements.
Plan your thermal budget. A PoE PD controller plus buck converter running at 25W will dissipate 1-3W as heat. In a sealed outdoor enclosure, that heat needs somewhere to go.
Add TVS diodes for surge protection. Outdoor PoE runs are lightning magnets. A TVS diode on the PoE input (before your PD controller) is cheap insurance. Littelfuse and Bourns make PoE-specific TVS arrays.
PoE Standards Timeline
| Year | Standard | Milestone |
|---|---|---|
| 2003 | IEEE 802.3af | First PoE standard, 15.4W |
| 2009 | IEEE 802.3at | PoE+, doubled power to 30W |
| 2018 | IEEE 802.3bt | 4PPoE, up to 90W, 4-pair power |
| 2023+ | IEEE 802.3bt adoption | Widespread adoption in enterprise |
The industry is moving toward higher power delivery over Ethernet. With 802.3bt Type 4 delivering up to 71.3W at the PD, you can now power thin clients, digital signage, PTZ cameras, and even small servers over a single Ethernet cable.
Planning a PoE deployment and need to size your power budget? The PoE Power Budget Calculator helps you calculate voltage drop, cable losses, and total power requirements for multi-device networks. Enter your cable length, gauge, and device power needs, and it handles the rest.