PoE power classes: IEEE 802.3af, at and bt
Every PoE class with the power the switch budgets and the power the device receives, by IEEE standard and pair count.
| Class | Standard | Switch budgets (PSE) | Device receives (PD) | Pairs |
|---|---|---|---|---|
| Class 1 | 802.3af | 4.0 W | 3.84 W | 2 |
| Class 2 | 802.3af | 7.0 W | 6.49 W | 2 |
| Class 0 | 802.3af unclassified | 15.4 W | 12.95 W | 2 |
| Class 3 | 802.3af PoE | 15.4 W | 12.95 W | 2 |
| Class 4 | 802.3at PoE+ | 30.0 W | 25.50 W | 2 |
| Class 5 | 802.3bt Type 3 | 45.0 W | 40.00 W | 4 |
| Class 6 | 802.3bt Type 3 PoE++ | 60.0 W | 51.00 W | 4 |
| Class 7 | 802.3bt Type 4 | 75.0 W | 62.00 W | 4 |
| Class 8 | 802.3bt Type 4 | 90.0 W | 71.30 W | 4 |
Which number to use
Size the switch with the PSE column. The switch reserves its class allocation for each port whether or not the device draws it, so a Class 3 camera costs 15.4 W of budget even if it only ever uses 6 W. Some switches can allocate by measured draw instead; check the switch configuration before you count on it.
The gap between the two columns is the worst-case loss in 100 meters of cable. Shorter runs lose less, and four-pair 802.3bt delivery halves the loss by splitting the current across all four pairs.
Where it goes wrong
- PTZ cameras with heaters and blowers, and IR illuminators at night, can draw at the top of their class. Budget for the class, not the brochure's typical figure.
- Midspan injectors and switches must match the standard. A Type 4 (90 W) device on an 802.3at switch will not power up fully, or at all.
- Copper-clad aluminum cable is not standards compliant and runs hotter under PoE. Use solid copper.