Guide
PoE budget explained
Short answer: add up the maximum watts of every device on the switch, keep the total under 80 % of the switch’s PoE budget, and check that each port supports the standard its device needs (PoE, PoE+ or PoE++).
Two limits on every PoE switch
A PoE switch has a total PoE budget (e.g. 370 W for all ports together) and a limit per port set by the PoE standard the port supports. A design must respect both: a 16-port switch with a 120 W budget cannot power 16 cameras at 12 W each, even though every port could.
The PoE standards
| Standard | Name | At the switch port | At the device |
|---|---|---|---|
| 802.3af | PoE (Type 1) | 15.4 W | 12.95 W |
| 802.3at | PoE+ (Type 2) | 30 W | 25.5 W |
| 802.3bt Type 3 | PoE++ | 60 W | 51 W |
| 802.3bt Type 4 | PoE++ | 90 W | 71.3 W |
The difference between the switch port and the device is the power lost in up to 100 m of cable.
How to calculate it
1
List the devices
Every camera, reader, access point, phone and intercom on the switch.
2
Use the maximum watts
From each data sheet – with IR on, heater on and PTZ moving, not the typical value.
3
Add them up
The total is the PoE load of the switch.
4
Apply the 80 % rule
Choose a switch whose budget is at least the load ÷ 0.8.
5
Check each port
Devices over 15.4 W need PoE+ ports; over 30 W need PoE++ ports.
Example
8 dome cameras at 6.5 W, 4 bullet cameras with IR at 12.95 W, 1 PTZ at 25.5 W and 3 access points at 13 W: 52 + 51.8 + 25.5 + 39 = 168.3 W. With the 80 % rule the switch needs at least 211 W of PoE budget, 16 PoE ports, and PoE+ on the PTZ port.
Why keep 20 % free
- Cameras draw more at night (IR) and in the cold (heaters).
- Devices draw a peak when they start – after a power cut all of them start at once.
- You will add a camera later.
Questions
Is the PoE budget the same as the switch’s power supply?
No. The power supply also runs the switch itself; the PoE budget is the part available for the ports – check it on the data sheet.
Can I use an injector instead?
Yes – a PoE+ or PoE++ injector powers one device, for example a PTZ on a switch without PoE++ ports.
Related
Let GITECHY Design do the math
Cable lengths, PoE and power budgets, batteries, voltage drop and camera coverage – on every design. One flat price per company.