How PoE power is allocated
A PoE switch (the PSE) has a total power budget, set by its power supplies, that it shares between ports. During classification each powered device (PD) announces a class, and many switches reserve the class maximum for that port whether the device uses it or not. Planning with class values is therefore the safe approach.
| Class | Standard | At the PSE port | At the device | Typical devices |
|---|---|---|---|---|
| 1 | 802.3af | 4.0 W | 3.84 W | Basic sensors |
| 2 | 802.3af | 7.0 W | 6.49 W | Desk IP phones |
| 3 | 802.3af (PoE) | 15.4 W | 13.0 W | Fixed cameras, small APs |
| 4 | 802.3at (PoE+) | 30 W | 25.5 W | Wi-Fi 6 APs, PTZ cameras |
| 6 | 802.3bt Type 3 | 60 W | 51 W | Wi-Fi 7 APs, thin clients |
| 8 | 802.3bt Type 4 | 90 W | 71.3 W | Displays, heated cameras, lighting |
The difference between the PSE and PD columns is the worst-case loss in 100 m of cable. The calculator uses the PSE value, because that is what the switch subtracts from its budget.
Worked example
A 48-port switch with a 370 W PoE budget feeds 8 Wi-Fi APs (class 4) and 4 cameras (class 3):
8 × 30.0 W = 240.0 W 4 × 15.4 W = 61.6 W total = 301.6 W → 81.5% of 370 W
It fits, with 68.4 W of margin, but it is above the 80% planning threshold. The recommended budget is total / 0.8 = 377 W, so the next AP added should push you to a second power supply or a larger PoE model.
Why keep 20% spare
- Devices draw peak power at boot, during IR night mode (cameras) or when radios ramp up.
- Losing one of two redundant power supplies may cut the PoE budget in half; check the vendor's "power budget with N+1" table.
- When the budget is exceeded, the switch denies power by port priority. Low-priority ports go dark without warning.
Checking on the switch
! Cisco IOS XE show power inline show power inline gi1/0/1 detail interface gi1/0/1 power inline port priority high # Junos (EX) show poe controller show poe interface # MikroTik /interface ethernet poe print /interface ethernet poe monitor ether2
Tips
- Use LLDP-MED power negotiation where both sides support it: the switch then allocates the real requirement instead of the class maximum.
- Use the custom row for devices that publish a measured maximum (for example 12 W for a specific AP model).
- Long or thin cable runs (CCA, 26 AWG patch cords) increase loss and heat; 802.3bt Type 4 especially needs solid copper Cat6A in bundles.