PoE Standards Comparison: 802.3af vs 802.3at vs 802.3bt
A clear breakdown of Power over Ethernet standards and when to use each one.
Power over Ethernet has evolved significantly since its introduction. Understanding the differences between 802.3af, 802.3at, and 802.3bt helps you choose the right infrastructure for your devices.
The Three Main Standards
| Standard | Common Name | Max Power (PSE) | Max Power (PD) | Year |
|---|---|---|---|---|
| 802.3af | PoE | 15.4W | 12.95W | 2003 |
| 802.3at | PoE+ | 30W | 25.5W | 2009 |
| 802.3bt Type 3 | PoE++ | 60W | 51W | 2018 |
| 802.3bt Type 4 | PoE++ | 100W | 71W | 2018 |
Note: The difference between PSE (Power Sourcing Equipment) and PD (Powered Device) power accounts for cable losses.
PoE Budget Calculator Plan your power requirements across all standards802.3af: The Original Standard
The first PoE standard delivers up to 12.95W to the device. This covers:
- Basic IP phones
- Simple wireless access points
- Basic IP cameras (no heater or PTZ)
- Wall clocks and sensors
- Card readers and simple IoT devices
If your device needs less than 13W, 802.3af is sufficient and available on nearly all PoE switches.
802.3at: The Workhorse
PoE+ doubles the available power to 25.5W at the device. This enables:
- Dual-radio enterprise access points
- PTZ cameras (pan, tilt, zoom)
- Video phones with large displays
- IP cameras with heaters
- Thin clients and small displays
- Door access systems with electric locks
802.3at is the most commonly deployed standard for enterprise networking today.
802.3bt: High Power Applications
The newest standard comes in two types. Type 3 delivers up to 51W, while Type 4 reaches 71W at the device.
Type 3 (51W) devices:
- WiFi 6E tri-band access points
- Multi-sensor cameras
- Video conferencing endpoints
- Digital signage displays (small)
Type 4 (71W) devices:
- Laptops and all-in-one computers
- Larger displays and TVs
- PTZ cameras with heaters and wipers
- LED lighting fixtures
- Point-of-sale systems
Cable Requirements
Higher power standards have stricter cable requirements:
| Standard | Pairs Used | Minimum Cable | Notes |
|---|---|---|---|
| 802.3af | 2 | Cat5 | Mode A or B |
| 802.3at | 2 | Cat5 | Mode A or B |
| 802.3bt Type 3 | 4 | Cat5e | All pairs required |
| 802.3bt Type 4 | 4 | Cat5e | Cat6 recommended |
802.3bt uses all four pairs, which means you cannot use pairs for other purposes (like a second device).
Backward Compatibility
PoE standards are backward compatible through negotiation:
- An 802.3at switch can power 802.3af devices
- An 802.3bt switch can power 802.3at and 802.3af devices
- An 802.3af switch cannot provide enough power for 802.3at devices
The powered device and switch negotiate through a classification process. The device receives only the power class it requests, not the maximum the switch can provide.
Power Classes
| Class | Max Power at PD | Standard |
|---|---|---|
| 0 | 12.95W | 802.3af |
| 1 | 3.84W | 802.3af |
| 2 | 6.49W | 802.3af |
| 3 | 12.95W | 802.3af |
| 4 | 25.5W | 802.3at |
| 5 | 40W | 802.3bt |
| 6 | 51W | 802.3bt |
| 7 | 62W | 802.3bt |
| 8 | 71W | 802.3bt |
Choosing the Right Standard
Go with 802.3af when:
- Deploying basic IP phones
- Simple access points with low user counts
- Budget is the primary concern
- All devices need less than 13W
Go with 802.3at when:
- Deploying enterprise wireless
- Installing PTZ or outdoor cameras
- You want headroom for future devices
- Mixed device types in your deployment
Go with 802.3bt when:
- Deploying WiFi 6E access points
- Powering laptops or displays
- LED lighting over Ethernet
- High-end video conferencing systems
Budget Considerations
When planning switch purchases, remember that the total PoE budget matters more than per-port maximum. A 24-port 802.3at switch might have a 370W budget—enough for all ports at 15W, but only half at 30W.
Calculate your actual load, add 20% headroom, and ensure your switch budget covers it. Don't assume every port can deliver maximum power simultaneously.