- PoE sends low-voltage DC power over the same twisted-pair cable as the network data.
- Standards range from 15.4 W (802.3af) to 90 W (802.3bt Type 4) at the switch.
- Some power is lost in the cable, so the device gets less than the switch sends.
- Bundled PoE cables warm up, and the NEC limits bundle sizes for power-carrying data cables.
Power over Ethernet lets one category cable carry both a network connection and the power to run the device on the end. Security cameras, Wi-Fi access points, door stations, phones and even some lighting use it. One cable means fewer receptacles, simpler installs, and central backup power at the network closet.
How it works
The power source (PSE), usually a PoE switch or an injector, puts a DC voltage of roughly 44 to 57 V on the cable’s pairs. Before sending power, it checks for a signature from the device (PD) so it doesn’t send power to a laptop or other equipment that isn’t expecting it. The device can also tell the switch what power class it needs.
The standards
| Standard | At the switch (PSE) | At the device (PD) | Pairs used |
|---|---|---|---|
| 802.3af (Type 1) | 15.4 W | 12.95 W | 2 |
| 802.3at (Type 2, “PoE+”) | 30 W | 25.5 W | 2 |
| 802.3bt (Type 3) | 60 W | 51 W | 2 or 4 |
| 802.3bt (Type 4) | 90 W | 71.3 W | 4 |
The gap between the two columns is the worst-case loss in up to 100 meters of cable. Shorter runs lose less, but designs assume the worst case.
Things that matter in a real building
- Switch power budget. A switch may have 24 PoE ports but not enough total power to run every port at full output. Add up the devices.
- Distance. Standard Ethernet, and PoE with it, is designed for 100 meters (328 feet) per link. Longer runs need an extender or a fiber link with local power.
- Heat in bundles. Each cable warms a little as it carries power. In large bundles, especially at higher power levels, that heat adds up. The NEC limits cable size and bundle counts for power-carrying data cables, and some cables are marked “LP” (limited power) for this use.
- Quality terminations. Poorly made connections can heat up or arc when a high-power device is unplugged.
Backup power
Because every device is fed from the network closet, a single UPS there can keep cameras, access control and Wi-Fi running during a short outage. Size it from the actual PoE draw plus the switch itself.
Sources and further reading
- IEEE 802, Practical PoE tutorial (July 2017)
- NFPA free access to codes and standards (read the NEC online)
Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.