What the standard defines
Power over Ethernet delivers current to a device along the same balanced cabling that carries its data, so that a ceiling-mounted access point or a camera needs one cable rather than two. It was published as IEEE 802.3af in 2003 and has since been folded into the base 802.3 standard, where its behaviour is designated Type 1.
The heart of it is a handshake, and the handshake is the reason the whole scheme is safe. Before applying power, the source — the standard calls it power sourcing equipment — looks for a particular electrical signature at the far end. Only when it finds one does it classify the device, learn roughly how much power it expects, and energise the pairs. Anything without the signature simply never gets powered.
Two figures are published, and both matter. The source delivers up to 15.4 W at its port. The standard guarantees the powered device a lower figure — 12.95 W — because the run of cable between them dissipates the difference as heat. Type 1 uses two of the four pairs, and defines classes 0 to 3 through which a device declares its appetite.
What it does not promise
It does not promise that the wattage at the switch arrives at the device. That is the entire meaning of the two published numbers, and a device specified near the upper figure is one to check the cable length on.
It does not promise a total. A switch may name Type 1 on every port and still publish a budget well below the sum of them, at which point ports are powered in order until the budget runs out.
It does not promise to run a device built for more. Equipment specified for a higher type will not settle for Type 1; some models start in a reduced mode and say so on the data sheet, and others do not start.
And it does not describe every injector on sale. A passive injector that simply puts a voltage on a pair, with no signature check and no classification, is doing something else entirely.
What it works alongside
A Type 1 device works from a Type 2, Type 3 or Type 4 source — the negotiation resolves it downwards, which is why a newer switch powers older equipment without configuration. It runs over Category 5e cabling and better, alongside the data on the same pairs, so a powered link is not a slower link.
The questions that come up before an order
Will plugging a laptop into a Power over Ethernet port damage it?
The standard is built so that it does not. A compliant source checks for a valid signature at the far end and only energises the pairs once it finds one, which is why a standards-based port is safe to plug an ordinary device into. Passive injectors that apply a voltage without that exchange are outside the standard.
Can every port on a switch deliver its full class at the same time?
Not necessarily. The per-port class and the unit’s total power budget are two separate published figures, and on many switches the budget is less than the port count multiplied by the class. Both are on the data sheet.
Last reviewed 9 September 2026