What PoE Is
Power over Ethernet (PoE) delivers DC electrical power over the same twisted-pair cable that carries data. This eliminates the need for a separate power adapter and AC outlet at the device location — an access point mounted on a ceiling, an IP camera above a doorway, or a VoIP phone on a desk can all receive both data and power through a single Ethernet cable run to a PoE-capable switch or injector. The power sourcing equipment (PSE) is the switch or injector; the powered device (PD) is the access point, camera, or phone.
PoE Standards Compared
| Standard | IEEE Designation | Max Power at PSE | Max Power at PD | Pairs Used | Common Devices Powered |
|---|---|---|---|---|---|
| PoE | 802.3af (2003) | 15.4 W | 12.95 W | 2 pairs | VoIP phones, basic IP cameras, older access points |
| PoE+ | 802.3at (2009) | 30 W | 25.5 W | 2 pairs | PTZ cameras, 802.11n/ac access points, video phones |
| PoE++ (Type 3) | 802.3bt (2018) | 60 W | 51 W | 4 pairs | 802.11ax/Wi-Fi 6 access points, LED lighting, thin clients |
| PoE++ (Type 4) | 802.3bt (2018) | 90 W | 71.3 W | 4 pairs | High-power access points, small displays, laptops, PTZ cameras with heaters |
The difference between PSE power and PD power is lost as heat in the cable resistance. A 100-meter Cat6 run at full 90W PoE++ load can dissipate 15–20 watts as heat along the cable — this is why cable bundling and temperature ratings matter at high PoE wattages.
How PoE Negotiation Works
When a PoE switch port detects a connected device, it applies a small detection voltage to check for the resistance signature of an 802.3af/at/bt compliant PD (typically a 25 kΩ signature resistor). If the device responds correctly, the switch begins power delivery at the base level and negotiates higher power via LLDP (Link Layer Discovery Protocol) if the device requests it. Passive PoE (non-standard) injectors skip this negotiation entirely and apply voltage whether or not the connected device is a PoE device — passive PoE can damage non-PoE equipment and should be avoided except when explicitly required by a specific device.
Cable Requirements for High-Power PoE
Low-power PoE (802.3af, 15.4W) works reliably on Cat5e. As power increases, cable quality becomes more critical:
- Cat5e: adequate for PoE and PoE+ (up to 30W) on runs within the 100-meter limit with bare copper conductors.
- Cat6 or Cat6A: required for PoE++ Type 3 and Type 4 (60–90W) to keep conductor temperature rise within safe limits, particularly in bundled runs where cables cannot dissipate heat individually.
- Bare copper only: CCA (copper-clad aluminum) cable has higher resistance than solid copper and heats more under PoE current. At high PoE wattages, CCA cable can exceed safe operating temperatures. Always use solid bare copper cable for PoE installations.
- Bundle derating: TIA-568 requires reducing the number of PoE cables in a bundle when running at 802.3bt power levels, because bundled cables cannot shed heat as effectively as individual runs. Check the TIA-568-C.2-1 amendment for specific derating tables based on bundle size and PoE class.
Frequently Asked Questions
Can I plug a non-PoE device into a PoE port?
Yes, safely — with standards-compliant 802.3af/at/bt PoE. The switch applies a low detection voltage first and only delivers full power after confirming the device presents a PoE signature. A non-PoE device (a regular computer, a printer) will not present the PoE signature, so the switch sees no PD, delivers no power, and the Ethernet data link operates normally. The only risk is with passive PoE injectors, which apply voltage unconditionally — never connect non-PoE devices to passive PoE ports.
My PoE access point keeps rebooting — could the switch port be underpowering it?
Yes, this is a common problem. If a Wi-Fi 6 access point requires 25W (PoE+) but is connected to a switch port that only delivers 15.4W (PoE 802.3af), the access point will power up, begin initializing, draw more current than the port can supply, and reset — repeating in a boot loop. Check the access point's power requirement in its datasheet and verify the switch port's PoE class in the switch management interface. Also check the switch's total PoE power budget: switches have a combined power limit across all ports, and a switch with many powered devices may throttle individual ports when the budget is exhausted.
What is the difference between a PoE switch and a PoE injector?
A PoE switch has PoE circuitry built into each port — data and power originate from the same device. A PoE injector (also called a PoE adapter or midspan) is an inline device: Ethernet data enters from a regular (non-PoE) switch, the injector adds power to the cable, and the combined data+power cable continues to the PoE device. Injectors are useful when you have a non-PoE switch and need to power one or a few PoE devices without replacing the switch. A single-port injector is the cheapest solution for one device; a multi-port midspan powers several devices. For new installations powering many PoE devices, a PoE switch is simpler and often more cost-effective.