Network wiring notes - Power over Ethernet

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For other network related things, see:

Also:


PoE is useful to install devices where finding wall power would take more wire and/or more installation bother.


In the rarly days of PoE, there were a bunch of different non-standard solutions, with varying details, incompatibilities, and some required electrical knowledge and/or had some risk of damaging devices.


These days, it's usually IEEE standard PoE, which is a little pricier than the simplest variation, but

means you don't need to be an electrical expert to get it right
avoids (potentially damaging) incompatibilities between varying proprietary implementations
means no significant power will be drawn until both sides negotiate and agree
means you can mix standard PoE and non-PoE with little to no risk


Standardized PoE

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

For typical ethernet, you are looking for IEEE 802.3af, 802.3at, 802.3bt:

  • IEEE 802.3af
From 2003
defines four choices, inserting up to 15.4 W of DC power (48V, 100mA to 400mA), expect to get no more than 13W (dissipation, etc).
  • IEEE 802.3at, a.k.a. PoE+
From 2009, updates the earlier one with some specifics.
adds one more choice, inserting up to 30W, expect to get 25W of that
  • IEEE 802.3bt, a.k.a. PoE++ (or 4PPoE, which seems to also be a name used for pre-standard implementations?(verify))
Two revisions some years apart, together introduced four more choices, type 3 inserting up to ~60W (expect to get ~50W of that), and later type 4 up to ~100W (expect to get ~70W of that)
also added support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T(verify)


They are backwards compatible in this order, and each adding higher-power modes and a few features.


See also

Ghetto fix PoE

For simple DIY, you might consider standard-based PoE is expensive overkill, and even the potentially safer passive PoE is also bother.


Consider a microcontroller with Ethernet. Such projects are almost invariably fine using 100Mbit speeds, meaning you're got two pairs not being use, 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.

In theory a single 24 AWG is good for maybe 2A, but stay under 1A for peace of mind. If you use both both unused pairs, it still lets you carry ~2A pretty safely.

You'll get a little voltage drop over a long wire, so on more than a few meters, do some V=IR calculation. A supply with a few volts more, and a regulator on the receiving end, can make a lot of sense for some setups.

Yes, you will get a little from these power pairs because the wires are so close - which ought not to be much assuming a dozen volts DC and the noise is mostly switching noise that ought not to be very strong.