(PoE) Power over Ethernet
A family of IEEE standards that delivers DC power and data over the same twisted-pair Ethernet cable, removing the need for a separate power supply at the device. Common with VoIP phones, IP cameras, wireless access points, smart lighting, and a growing set of IoT endpoints.
How it works
Power flows from a PSE (Power Sourcing Equipment — typically a PoE switch or midspan injector) to a PD (Powered Device). Before energizing the line, the PSE sends a low-voltage detection signal to confirm the device is PoE-compatible, then negotiates a power class so it doesn’t over- or under-deliver. Nominal line voltage is 48 V DC (range 44–57 V), with a max cable run of 100 m.
Three power-delivery modes:
- Mode A — power on data pairs (1/2, 3/6); used by 802.3af/at.
- Mode B — power on spare pairs (4/5, 7/8); requires 4-pair cabling.
- 4PPoE / 4-pair — power on all four pairs; required for 802.3bt Types 3 & 4.
PoE types and specs
| Standard | Year | Common name | Type | Max @ PSE | Min @ PD | Typical use |
|---|---|---|---|---|---|---|
| IEEE 802.3af | 2003 | PoE | Type 1 | 15.4 W | 12.95 W | VoIP phones, basic IP cameras, low-power APs |
| IEEE 802.3at | 2009 | PoE+ | Type 2 | 30 W | 25.5 W | PTZ IP cameras, dual-band Wi-Fi APs, thin clients |
| IEEE 802.3bt | 2018 | PoE++ / 4PPoE | Type 3 | 60 W | 51 W | Wi-Fi 6 APs, video bars, small displays |
| IEEE 802.3bt | 2018 | PoE++ / 4PPoE | Type 4 | 90–100 W | 71.3 W | Laptops, large displays, PoE LED lighting, thin-client PCs |
PD power classes
Each PD signals a class so the PSE allocates the right budget:
- Class 0 — 0.44–12.95 W (default if no class advertised)
- Class 1 — 0.44–3.84 W
- Class 2 — 3.84–6.49 W
- Class 3 — 6.49–12.95 W
- Class 4 — 12.95–25.5 W (802.3at)
- Class 5–6 — up to 51 W (802.3bt Type 3)
- Class 7–8 — up to 71.3 W (802.3bt Type 4)
Passive PoE (non-standard)
Some vendors (notably Ubiquiti) ship “passive PoE” — typically 24 V or 48 V DC sent over the spare pairs without any negotiation. Cheaper and simpler, but not interoperable with standards-compliant PoE: plugging a passive injector into a non-PoE device can damage it.
Cabling
- Cat5e is the minimum for 802.3af/at.
- Cat6 / Cat6a is recommended for 802.3bt to handle the higher current and reduce heat in bundled runs.
- Voltage drop and bundle heating become real concerns past ~60 W per cable.
Quick history
- 1997 — PowerDsine pioneers commercial “midspan” power injection.
- 2003 — IEEE ratifies 802.3af (PoE), the first standard. 15.4 W ceiling, two-pair power.
- 2009 — IEEE ratifies 802.3at (PoE+). 30 W, supports growing AP and PTZ-camera demand.
- 2018 — IEEE ratifies 802.3bt (PoE++). Adds Type 3 (60 W) and Type 4 (90 W+) using all four pairs, opening the door to PoE-powered laptops, displays, and lighting.
- 2020s — PoE becomes a substrate for “smart building” infrastructure: lighting grids, access control, and Wi-Fi 6/6E APs increasingly assume Type 3+ availability.
See also
https://www.digikey.com/reference-designs/en/power-management/poe-power-over-ethernet
PoE compatible Magjacks
PoE compatible Ethernet transformers
Choosing PoE ethernet transformer for gigabit speed