What a PoE budget is
Power over Ethernet (PoE) sends power and data over the same network cable to cameras, Wi-Fi access points, IP phones and sensors. A PoE switch has a total power budget, often much lower than its number of ports times the per-port maximum. If devices ask for more than the budget, some ports stay unpowered.
Classes and standards
Each device announces a class, and the switch reserves the corresponding power at its port:
| Class | Standard | Reserved at the switch | Available at the device |
|---|---|---|---|
| 1 | 802.3af | 4.0 W | 3.84 W |
| 2 | 802.3af | 7.0 W | 6.49 W |
| 3 | 802.3af (PoE) | 15.4 W | 12.95 W |
| 4 | 802.3at (PoE+) | 30 W | 25.5 W |
| 6 | 802.3bt (PoE++) | 60 W | 51 W |
| 8 | 802.3bt (PoE++) | 90 W | 71.3 W |
The difference is lost in the cable, up to 100 m long.
Example
Four class 3 cameras (61.6 W), two class 4 access points (60 W) and four class 2 phones (28 W) reserve 149.6 W. On a switch with a 185 W budget, that is 81% used, leaving 35.4 W: room for one more access point or two more cameras.
Allocation versus real consumption
The calculator uses class-based allocation, the worst case. Many switches can instead allocate by actual draw, or by values negotiated with LLDP, which lets more devices fit. Even so, plan for the worst case: outdoor cameras with heaters, PTZ motors and infrared lights draw their peak on cold nights, exactly when you need them.
Tips
- Keep 20% of the budget free for growth and peaks.
- Check the per-port limit too: a PoE+ switch cannot power a class 6 device on any port.
- Power the switch from a UPS so cameras and phones keep working during an outage, and include the PoE load when sizing the UPS.
- A PoE injector can power a single high-demand device without replacing the switch.