What PoE standards and classes exist, and how much power does each deliver?
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802.3af (PoE) gives 15.4 W at the switch port and guarantees 12.95 W at the device. 802.3at (PoE+) gives 30 W and 25.5 W. 802.3bt uses all four pairs: Type 3 gives 60 W and 51 W, Type 4 gives 90 W and 71.3 W. The gap is what the cable can lose over 100 m. The switch first detects a valid signature resistance, then reads the device's class, then applies about 48 V.
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Classes
| Class | Standard | Power at the switch (PSE) | Power at the device (PD) |
|---|---|---|---|
| 0 | af | 15.4 W | 12.95 W |
| 1 | af | 4.0 W | 3.84 W |
| 2 | af | 7.0 W | 6.49 W |
| 3 | af | 15.4 W | 12.95 W |
| 4 | at (PoE+) | 30 W | 25.5 W |
| 5 | bt Type 3 | 45 W | 40 W |
| 6 | bt Type 3 | 60 W | 51 W |
| 7 | bt Type 4 | 75 W | 62 W |
| 8 | bt Type 4 | 90 W | 71.3 W |
Power-up sequence
- Detection: the switch applies a low voltage and looks for a signature resistance of about 25 kilohms. A normal NIC does not present it, so it never gets power.
- Classification: the device draws a set current that tells the switch its class.
- Power on: the switch applies roughly 44 to 57 V and keeps monitoring the load.
- Optional refinement: CDP or LLDP-MED lets the device ask for an exact wattage, so the switch can reserve less than the class maximum.
Cisco names
UPOE is Cisco's 60 W four-pair PoE that came before 802.3bt. UPOE+ is Cisco's 90 W offering aligned with 802.3bt Type 4.
They will ask next
What uses each level?
Phones and simple sensors fit in 802.3af. Most Wi-Fi 6 and 6E APs, and PTZ cameras, want PoE+ or more. Wi-Fi 7 APs, displays, thin clients and LED lighting push into 802.3bt.
Why does class 0 cause budgeting waste?
A class 0 device gives no class information, so the switch reserves the full 15.4 W even if the device draws 3 W, unless CDP or LLDP refines the request.
Traps
- Quoting the switch figure as what the device receives. The device gets less because of cable loss.