What Is PoE? Power Over Ethernet and PoE Switches Explained

Cameras, access points, IP phones, and door controllers all need two things: a network connection and power. Running both a data cable and a power outlet to every one of those locations is expensive and often impossible, which is exactly the problem Power over Ethernet was built to solve.

PoE delivers electrical power over the same twisted pair cable that carries data. One cable, one jack, one device, and no electrician required for each ceiling mount. It is a small idea with an outsized effect on how buildings are wired.

How Power Travels Along the Cable

An Ethernet cable has four twisted pairs. Fast Ethernet uses two of them for data and leaves two idle, so early PoE sent power down the spare pairs. Gigabit Ethernet uses all four pairs for data, so PoE in that case puts power on the center tap of the data transformers, sharing the pairs with the signal. The standard allows either method, and the device does not care which one the switch uses.

The important part is that the power source negotiates before it sends real power. A compliant switch first applies a low detection voltage and checks for a signature resistance, which tells it that a real PoE device is connected. That signature is why plugging a PoE port into a non PoE device is normally safe, and why an unknown device that fails the handshake receives no power at all.

The PoE Standards

The device receives slightly less than the port supplies, because some power is lost in the cable itself. The difference grows with current and cable length, which is why a 30 W port might deliver around 25.5 W to a device at the far end. Check the device’s requirement against the delivered power, not the port’s label.

What a PoE Switch Adds

A PoE switch is an ordinary switch whose ports can source power. Two numbers describe it: the per port maximum, which follows the standards above, and the total power budget, which is the shared pool for all ports. The budget is what limits you. A 24 port switch with a 370 W budget cannot run 24 devices at 30 W, because that would need 720 W. It can run about 12, or a larger mix of low power devices.

Planning also means leaving headroom. Devices draw a surge when they first power on, and a budget that is exactly full will drop a device when something else reboots. A useful rule is to keep 20 to 25 percent of the budget free.

The budget is also a single point of failure. If the switch loses power, every device it powers loses power too, which is why access points on a floor often justify a UPS on the switch and a second switch with its own power feed.

Injectors, Splitters, and Midspans

Not every switch is PoE capable, and not every environment justifies replacing one. A PoE injector adds power to a single run by sitting inline between a non PoE switch and the device. It is a fine fix for one camera or one access point and a poor strategy for a whole floor.

A midspan is the same idea at scale, a dedicated device that sits between the switch and many endpoints and injects power on each link. Splitters do the reverse, taking a PoE run and separating power from data for a device that needs both but is not PoE aware. These are useful tools, and they all add a device to the physical path that belongs in your documentation, because the next person tracing a cable will otherwise wonder why an odd box sits between the switch and the wall.

Documenting the Powered Side

PoE ports are the one place where a network record has to carry an electrical fact, not just a logical one. For each powered port, the useful records are the device it feeds, the PoE class or wattage it draws, and the switch it hangs on. When an access point drops, the question is often whether the port is still delivering power, and that answer is faster when the port’s role is recorded rather than guessed.

Keeping port assignments on the switch record itself, as device inventory and cable tracking in Obelinf do, means a PoE port carries its device, its cable, and its power profile in one entry. For the physical side of the same picture, the rack power calculator helps confirm that the switch feeding those ports has the headroom to do it.