Is your switch's PoE budget enough?

What this is about

Eight PoE ports don't mean eight fully powered ports. What counts is the PoE budget: the total power the switch's power supply can deliver over Power over Ethernet. PoE sends power over the same cable as the data. The switch is the source (PSE), and the camera or access point is the powered device (PD).

Every device announces a class from 0 to 8 when it powers up. The class sets how much power the switch has to provide at the port: 15.4 W for class 3, 30 W for class 4, up to 90 W for class 8. The device receives less, because the cable eats part of it. That's why the budget counts port watts.

The standard also caps what a port can deliver. IEEE 802.3af (Type 1) stops at 15.4 W, 802.3at (Type 2) at 30 W, and 802.3bt at 60 W (Type 3) and 90 W (Type 4). From class 5 upwards, all four wire pairs are needed.

Many switches allocate their budget by actual draw rather than by class. Don't count on it. We calculate with the class values, the cautious case, and suggest keeping 20 percent of the total budget free (load ÷ 0.8). With a 100 W load, that means a 125 W budget, of which 25 W stay free. That 20 percent is our rule of thumb, not part of any standard. If you don't know a device's class, enter the data sheet figure in watts at the port.

How we calculate

• Reserved = sum of count × port power of the class (PSE side). Class 0 counts like class 3, at 15.4 W.
• Headroom = the switch's PoE budget − reserved. If it goes negative, the calculator warns you.
• Recommended budget = reserved ÷ (1 − 0.20): 20 percent of the total budget stays free, not 20 percent on top. Example: a 100 W load gives a 125 W budget, of which 25 W stay free.
• Limits per switch type: Type 1 up to class 3 (15.4 W), Type 2 up to class 4 (30 W), Type 3 up to class 6 (60 W), Type 4 up to class 8 (90 W). Devices above that are flagged.
• Example with the preset values: 3 cameras in class 3, 2 access points in class 4 and one phone in class 2 reserve 113.2 W. With headroom, look for a switch with a budget of at least 141.5 W.
• Not included: the switch's own consumption, the vendor's allocation mode, cable length and quality, and passive PoE, which isn't part of the standard.