Network wiring notes - Power over Ethernet

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Standardized PoE

Power over Ethernet usually refers to standardized variant, which describes a method of drawing power from data wires, and ensures this power is present to be drawn.

Specs say single devices can ask for 12W-25W, wires could do up to ~45W. (verify)


Standard PoE is good for anyone who wants a safe plug-and-play solution for things like VoIP phones, APs, IP cameras, embedded devices, where finding wall power would take more wire and more bother.

There are also some pre-standard and non-standard methods.


On standard PoE

  • IEEE 802.3af-2003
    • inserts up to 15.4 W of DC power (48V, 100mA to 400mA). Due to dissipation in long cales you may get only ~13W.
  • IEEE 802.3at-2009 updates the earlier one with some specifics.

See also https://en.wikipedia.org/wiki/Power_over_Ethernet


Both have two modes:

  • Mode A, 'phantom power', ensures power can be taken from the data lines
    • which means it can also work for gigabit
  • Mode B, uses pairs that are unused in 10/100Mbit
    • which means it won't work for gigabit

The mode is decided by negotiation(verify).


Power Sourcing Equipment (PSE) is the thing that 'injects' PoE-style power.

There are some switches that do this, but it may sometimes make more sense (e.g. cheaper) to add insertion closer to the point you need it, particularly if you have few devices that want PoE in the first place.

PSE can choose to support just one mode(verify)


PoE-powered devices should be capable of handling both modes.

Homebrew PoE

For simple DIY, standard-based PoE is expensive overkill.


Consider a microcontroller with Ethernet. Such projects are almost invariably 10/100, meaning you're got four wires (two unused pairs) of what is usually 24-gauge, sometimes 26-gauge wire.

If you do not insert those pairs into the 8P8C plug on either end, you can use them for anything while not running risk of directly connecting power into a network card. (Note that this is not how standard PoE works, not even mode B. It has negotiation and various safeties you won't be building in, and you may have to explain the dangers to everyone else, so the simplest solution is keep it completely separate from the 8P8C)

In theory 24 AWG is good for an amp or two, but for peace of mind you can use less than that, and while you're at it you could use both unused pairs.

You'll get a little voltage drop over a long wire, so do some V=IR calculation to see if for your length you need an adapter with a volt more than you need on the other end. A regulator on the receiving end can make a lot of sense for some setups. (Standard PoE does something similar)

Yes, you will get a little interference, via induced power because the wires are so close - which ought not to be much assuming a dozen volts DC.