PoE (Power over Ethernet) is a set of IEEE 802.3 standards that deliver DC power and data over the same twisted-pair Ethernet cable, so a single RJ45 connection both connects and powers a device. In network hardware, PoE lets a switch or injector, the power sourcing equipment (PSE), supply wireless access points, IP cameras, VoIP phones and sensors, the powered devices (PD), without a separate power outlet at each location.
PoE operates at a nominal 44-57 V DC, a range classified as safety extra-low voltage, and it negotiates power before applying it. A device that does not support PoE receives no power at all, which makes standards-based PoE safe to use on ordinary Ethernet ports.
How PoE detects devices and delivers power
Standards-based PoE follows a fixed detection, classification and power-up sequence. The PSE first applies a low voltage, between about 2.7 and 10 V, and checks for a 25 kΩ signature resistance in the device. Only a valid signature tells the switch that a PoE-capable device is attached.
During classification, the PSE applies a slightly higher voltage and measures the current the device draws, which encodes its power class. The switch then enables full voltage and keeps monitoring the current; if the device disconnects or exceeds its class, the port is shut down. Devices can refine their requirement later through LLDP power negotiation, which allows more precise budgeting than the class alone.
Power travels on the cable pairs as a common-mode voltage through the center taps of the Ethernet transformers, so it does not disturb the data signal. Older standards use two of the four pairs, while the newest standard can use all four pairs to deliver more power with lower current per conductor.
PoE standards and power classes
Each PoE generation raised the power available at the device. The PSE output is always higher than the power at the PD, because some energy is lost as heat in the cable over a full 100 m channel.
| Standard | Common name | Power at PSE | Power at device | Pairs used |
|---|---|---|---|---|
| IEEE 802.3af | PoE, Type 1 | 15.4 W | 12.95 W | 2 pairs |
| IEEE 802.3at | PoE+, Type 2 | 30 W | 25.5 W | 2 pairs |
| IEEE 802.3bt Type 3 | PoE++, 4PPoE | 60 W | 51 W | 4 pairs |
| IEEE 802.3bt Type 4 | PoE++, high power | 90 W | 71.3 W | 4 pairs |
Within these standards, devices declare classes 0 to 8. Class 0 is the default for devices that do not classify, classes 1 to 3 cover low-power devices up to 802.3af limits, class 4 corresponds to PoE+, and classes 5 to 8 belong to 802.3bt. Typical loads range from about 5 W for a desk phone to 20-30 W for a Wi-Fi 6 access point and 60 W or more for heated outdoor cameras or small displays.
Active PoE compared with passive PoE and other options
The term PoE is also used loosely for passive PoE, which applies a fixed voltage, often 24 V or 48 V, without detection or classification. Passive PoE is common in some outdoor wireless equipment, but it can damage devices that do not expect power, so it must never be connected to arbitrary Ethernet ports.
| Option | Negotiation | Safety with non-PoE devices | Typical use |
|---|---|---|---|
| Standard PoE (802.3af/at/bt) | Detection and classification | Safe, no power without signature | Enterprise access points, cameras, phones |
| Passive PoE | None | Risk of damage | Point-to-point wireless links |
| PoE injector | Standard or passive | Depends on the injector type | Single device on a non-PoE switch |
| Local power adapter | Not applicable | Safe | Devices near an outlet |
PoE switches differ from data-only switches only in their power supply and port circuitry. The cabling, RJ45 connectors and data rates stay the same, so PoE works with 100 Mb/s, 1 GbE and multi-gigabit ports such as 2.5 GbE. Fiber links using SFP or SFP+ modules cannot carry PoE, since glass fiber conducts no current.
Planning a PoE deployment
The central planning figure is the switch’s PoE power budget, the total wattage it can deliver across all ports at once. A 48-port switch may support PoE+ on every port but offer a budget of only 370 W, far less than 48 × 30 W. Sum the expected draw of all devices and keep a reserve for growth and inrush.
| Situation | Recommended choice | Reason |
|---|---|---|
| VoIP phones and sensors | 802.3af ports | Loads stay under 13 W |
| Wi-Fi 6 or 6E access points | 802.3at or 802.3bt ports | Full radio features may need over 25 W |
| PTZ cameras with heaters | 802.3bt Type 3 or 4 | Heating and motors exceed PoE+ |
| Many high-power ports in a bundle | CAT6 or better cabling | Lower resistance and better heat dissipation |
| One device on an old switch | Standard-compliant injector | Adds power without replacing the switch |
Cable quality affects delivered power. Higher currents heat large cable bundles, so installations with many 802.3bt ports should use CAT6 or better with solid copper conductors and follow bundle-size guidance. The 100 m channel limit of Ethernet also applies to PoE.
Common PoE problems
Most PoE faults show up as devices that reboot, stay dark or run with reduced features. The table lists typical patterns.
| Symptom | Likely cause | Fix |
|---|---|---|
| Device powers up, then reboots | Switch power budget exhausted | Set port priorities or add budget |
| Access point runs with fewer radios | Only PoE+ available for a PoE++ device | Use an 802.3bt port or injector |
| Device never powers on | Passive device on standard PoE port | Match the power type |
| Intermittent failures on long runs | Copper-clad aluminum or damaged cable | Replace with solid copper cable |
A frequent mistake is assuming that every port can deliver its maximum class at the same time. Configure port priority so that critical devices such as access points and door controllers keep power when the budget runs short.
The idea in brief
PoE lets one network cable carry both internet and electricity. Instead of running a power cord to a ceiling access point or a security camera, the installer runs only the network cable, and the switch at the other end supplies the power.
The switch first checks whether the device asks for power and how much it needs. Devices that do not ask receive nothing, so a normal laptop plugged into the same port is never harmed.
A practical scenario
A secondary school upgrades its wireless network with 40 Wi-Fi 6 access points mounted on corridor ceilings, plus 12 outdoor cameras. No power outlets exist at the mounting points, and adding them would require an electrician in every corridor.
The network team installs CAT6 cabling and two 48-port switches with 802.3at on all ports and a 740 W budget each. The access points draw about 22 W in full operation, and the four rooftop cameras with winter heaters need 802.3bt, so the team adds four 60 W midspan injectors for those cameras only. Port priority is set to high for access points.
The project avoids new electrical circuits entirely, and every device can be power-cycled remotely from the switch when it hangs.