PoE Power Classes 0 to 8 Explained

Every standardized PoE link begins with a negotiation in which the powered device tells the power sourcing equipment how much power it needs. That negotiation resolves to a class number between 0 and 8. The class determines how much power the PSE reserves from its budget, which in turn determines how many devices a switch can actually run. Understanding classes is the difference between a power budget that works on paper and one that works in the rack.

Detection Comes First

Before classification, the PSE must confirm that something safe to power is attached. It applies a low probing voltage, typically between 2.8 V and 10 V, and looks for the characteristic 25 kΩ signature resistance that every compliant PD presents. Only when a valid signature is found does the PSE proceed. This step is why you can safely plug a laptop or a non-PoE printer into a PoE port: without the signature, no operating voltage is ever applied.

The Class Table

Classification happens next, in the 15.5 V to 20.5 V window. The PD draws a specific current and the PSE reads that current as a class. The PSE-side figure is what the switch subtracts from its budget; the PD-side figure is what the device is guaranteed to receive at the far end of a worst-case 100 m channel.

Class	PSE reserves	Guaranteed at PD	Type	Typical devices
0	15.4 W	0.44-12.95 W	1	Legacy and unclassified PDs
1	4.0 W	3.84 W	1	VoIP handsets, sensors
2	7.0 W	6.49 W	1	Fixed IP cameras, simple APs
3	15.4 W	12.95 W	1	Dome cameras, single-radio APs
4	30 W	25.5 W	2	Wi-Fi 6 APs, cameras with heaters
5	45 W	40 W	3	Multi-radio APs, small displays
6	60 W	51 W	3	PTZ cameras, thin clients
7	75 W	62 W	4	PoE lighting, video bars
8	90 W	71.3 W	4	Laptops, digital signage

Class 0 and the Cost of Not Classifying

Class 0 is the default a PSE assigns when a device presents a valid detection signature but no meaningful classification current. The PSE has no information, so it assumes the worst and reserves the full 15.4 W. A rack full of class 0 devices that each draw 4 W will exhaust a switch's budget at roughly a quarter of its real capacity. If you are designing a PD, implementing proper classification costs almost nothing and pays for itself the first time a customer fills a switch.

One Event, Two Events, Many Events

802.3af used a single classification event. 802.3at introduced 2-event classification: the PSE repeats the classification pulse, and a Type 2 PD that observes the second event knows it is attached to a PSE capable of 30 W, while a Type 1 PD simply ignores the repetition. 802.3bt extends this with longer sequences that let the PSE and PD converge on classes 5 through 8 and establish whether the PD is single- or dual-signature. The mechanism is deliberately layered so each generation degrades gracefully against older equipment.

When LLDP Takes Over

Physical-layer classification is coarse. Link Layer Discovery Protocol power negotiation runs at layer 2 once the link is up and lets the PD request a precise wattage in 0.1 W steps, then renegotiate as its needs change. An access point can boot conservatively, bring up its radios, and ask for more only if a second radio is enabled. The catch is timing: LLDP cannot run until the link is established, so the PD must survive on its physical-layer allocation long enough to get there. A device that requests class 4 and then negotiates 19.5 W over LLDP returns 10.5 W to the switch budget, but only where LLDP power management is enabled - on many switches it is off by default.

Budgeting With Classes

To size a switch, add the PSE-side reservation for every port you intend to populate, not the PD-side figure and not the measured draw. Then compare that total with the switch's system power budget, which is almost always lower than the sum of its per-port maximums. A 24-port switch rated class 4 on every port would need 720 W of PoE budget to deliver on that promise; most 24-port models ship with 370 W to 400 W, which is ample for a realistic mix but not for twenty-four simultaneous 30 W loads. Where the mix is uncertain, Autoclass or LLDP is what turns a theoretical shortfall into a working installation.