PoE standards and power classes: 802.3af vs at vs bt
Every PoE class with the exact wattage the switch supplies and the wattage the device is guaranteed to receive — plus typical draw for the equipment you actually hang on a network. Values are from the IEEE standards, so they do not drift with vendor firmware.
Need the arithmetic done for you? Use the free PoE budget calculator — add your devices and it totals the draw and checks switch headroom.
The four power tiers
| Standard | Ratified | Pairs used | Supplied | Guaranteed at device |
|---|---|---|---|---|
| 802.3af (PoE) | 2003 | 2 pairs | 15.4 W at the switch port | 12.95 W at the device |
| 802.3at (PoE+) | 2009 | 2 pairs | 30 W at the switch port | 25.5 W at the device |
| 802.3bt Type 3 (PoE++) | 2018 | 4 pairs | 60 W at the switch port | 51 W at the device |
| 802.3bt Type 4 (PoE++) | 2018 | 4 pairs | 90 W at the switch port | 71.3 W at the device |
Power classes 0–8
A device signals its class during negotiation, and the switch reserves that much of its budget — whether or not the device ever draws it.
| Class | Standard | Type | Switch supplies | Device receives | Typical use |
|---|---|---|---|---|---|
| 0 | 802.3af | Type 1 | 15.4 W | 12.95 W | Default when no classification is signalled |
| 1 | 802.3af | Type 1 | 4.0 W | 3.84 W | Very low power devices |
| 2 | 802.3af | Type 1 | 7.0 W | 6.49 W | IP phones, simple sensors |
| 3 | 802.3af | Type 1 | 15.4 W | 12.95 W | Wi-Fi 5 APs, fixed cameras |
| 4 | 802.3at | Type 2 | 30 W | 25.5 W | Wi-Fi 6 APs, PTZ cameras |
| 5 | 802.3bt | Type 3 | 45 W | 40 W | Wi-Fi 6E APs, video phones |
| 6 | 802.3bt | Type 3 | 60 W | 51 W | Multi-radio APs, small displays |
| 7 | 802.3bt | Type 4 | 75 W | 62 W | High-power APs, thin clients |
| 8 | 802.3bt | Type 4 | 90 W | 71.3 W | Maximum PoE, laptops/displays |
Typical draw by device
Ranges, not promises — a vendor's outdoor model with a heater can sit at the top of its band all winter. Always check the datasheet for anything you are buying in volume.
| Device | Typical draw | Usual class |
|---|---|---|
| Wi-Fi 5 (802.11ac) access point | 10–15 W | Class 3 (af) |
| Wi-Fi 6 (802.11ax) access point | 15–25 W | Class 4 (at) |
| Wi-Fi 6E access point | 25–35 W | Class 4–5 (at/bt) |
| Wi-Fi 7 (802.11be) access point | 30–50 W | Class 5–6 (bt) |
| IP desk phone | 3–7 W | Class 1–2 |
| Fixed IP camera | 5–12 W | Class 2–3 |
| PTZ camera with heater | 15–30 W | Class 4–5 |
| Outdoor AP with heater | 25–50 W | Class 4–6 |
Three things that catch people out
Aggregate budget, not port count
A 24-port switch advertising 370 W cannot run 24 Class 4 devices (that would need 720 W). The total budget is what limits you, and it is usually the number people forget.
Reserved vs actual draw
The switch reserves power based on the class a device signals. An AP that idles at 9 W still holds a Class 4 reservation of 30 W, so your budget fills faster than a wattmeter suggests.
Redundancy maths
If two stacked switches share a load and one fails, the survivor needs to carry everything. Sizing each to ~50% utilisation is what makes that survivable.
Common questions
What is the difference between 802.3af, 802.3at and 802.3bt?
They are successive PoE standards with increasing power. 802.3af (PoE) delivers up to 15.4 W from the switch port over two pairs. 802.3at (PoE+) raises that to 30 W, still over two pairs. 802.3bt (PoE++) uses all four pairs and defines Type 3 up to 60 W and Type 4 up to 90 W. Each is backward compatible — a Class 3 device works fine in a bt port.
Why is the power at the device lower than at the switch?
Copper cable loses power as heat over its length. The standards account for this by specifying two numbers: the PSE figure is what the switch port supplies, and the PD figure is what the device is guaranteed at the far end of 100 m of cable. For 802.3af that is 15.4 W in and 12.95 W out — roughly a 16% loss budget.
How do I size a PoE switch budget?
Add the class-based draw of every device, then leave headroom — plan around 20% spare so a failed switch in a stacked pair or a future AP does not push you over. Remember that a switch advertising, say, 370 W total cannot deliver maximum power on every port simultaneously; the aggregate budget is the constraint, not the per-port rating.
Does cable category affect PoE?
Yes, through resistance. Cat5e is permitted for all PoE types, but thinner conductors (higher AWG) and longer runs increase loss and heat. For Type 3 and Type 4 installations, Cat6 or better with larger conductors runs cooler, which matters when many powered cables share a bundle or conduit.
Sources: IEEE 802.3af-2003, IEEE 802.3at-2009 and IEEE 802.3bt-2018. Device draw figures are typical ranges compiled from published vendor datasheets and vary by model, radio count, and environmental options such as heaters.
Total your PoE budget in a minute
Add each device, get total draw, per-port class and whether the switch has headroom. Free, no signup.