What Is PoE?

PoE, or Power over Ethernet, carries DC power on the same twisted-pair cable as the network data. Every PoE link has two ends: power sourcing equipment (PSE), such as a PoE switch, which supplies the power, and a powered device (PD), such as a camera, which uses it.

Standard PoE is careful about when it switches on. The PSE first applies a small test voltage to look for a PD signature on the line. If it finds one, it reads the device’s power class, and only then applies the full supply of around 48 V DC. If an ordinary laptop or printer is plugged in, nothing is detected and no power is sent, which is why standard PoE ports are safe to use with any Ethernet equipment.

The data is unaffected. Power rides on the cable pairs alongside the signal, and the device separates the two with transformers and a PoE controller.

The IEEE has published three PoE standards, each adding power. 802.3af (PoE) supplies about 15 W per port, 802.3at (PoE+) about 30 W, and 802.3bt (PoE++) 60 W or 90 W depending on type. The two older standards carry power on two of the cable’s four pairs; 802.3bt uses all four. Our PoE vs PoE+ vs PoE++ comparison sets out the classes and which devices each one suits.

The type sets what a port can deliver; the class, read from the device during start-up, tells the source how much power to reserve. Class 1 reserves 4 W at the source, Class 2 7 W, Classes 0 and 3 15.4 W, Class 4 30 W, Class 5 45 W, Class 6 60 W, Class 7 75 W and Class 8 90 W.

Some power is lost as heat in the cable, so the device always receives less than the source supplies. Plan a system against the power at the device, not the figure on the switch. A switch or computer also has a total PoE budget, which is often lower than its per-port maximum multiplied by the number of ports, so a fully loaded PoE switch may not be able to give every port its full class at once.

The same budgeting applies to a PoE computer. The Advantech MIC-712P Jetson computers, for example, have six PoE+ ports sharing a 150 W total budget, less than six ports at the full 30 W, while the IP67 MIC-715-OX in-vehicle computer powers cameras from four IEEE 802.3af ports on M12 connectors.

PoE does not need a new network. Power can be added at several points, and the right one depends on how many devices need it.

Our PoE computers and industrial PoE switches supply power to cameras and sensors directly, with no separate injectors.

Everything above describes active, or standard, PoE. Passive PoE is a non-standard alternative that applies a fixed voltage, often 24 V, to the cable with no detection step. It is cheap and simple in closed systems, but it can damage devices that are not built for it. Our active vs passive PoE FAQ explains when each turns up.

PoE is most useful wherever devices sit far from a power socket or where an electrician’s visit would cost more than the device.

PoE uses the same twisted-pair cabling as ordinary Ethernet, and the same 100 m limit applies to the run between the PSE and the device. Cat5e is the minimum for standard PoE; the comparison of Cat5e vs Cat6 cable covers the cable grades.

Power heats the copper slightly, and the effect grows with the current and the number of cables bundled together. At 802.3at it rarely matters. At 802.3bt Type 4, large bundles in warm ceilings or cable trays can run hot enough to need derating, so solid-copper Cat6 or Cat6A and looser bundling are the safer choice. Avoid copper-clad aluminum cable, which has higher resistance and loses more power as heat.

To go further than 100 m, use a PoE extender, which takes power and data in and repeats both out again, or run fiber to a remote PoE switch near the devices.

We build PoE computers as standard, with the PoE ports, power budget and storage specified to suit the cameras and devices they will run.