Network wiring notes - Power over Ethernet

For other network related things, see: Networking basics - General · Layer 2 mostly · Layer 3 (IP mostly) Firewalling and other packet stuff Also: Network wiring notes - 8P8C / RJ45 Network wiring notes - Power over Ethernet Wireless communication IO and wired communication More technical VPN notes

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 and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, 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)

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

• https://en.wikipedia.org/wiki/Power_over_Ethernet

Ghetto fix PoE

Consider a DIY project with Ethernet. Such projects are almost invariably fine using 100Mbit speeds, meaning you're got two pairs not being used, of what is usually 24-gauge (sometimes 26-gauge wire).

You can generally get away with using the others for other things, and if you do not insert those pairs into the 8P8C plug on either end you run no 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 crosstalk from these power pairs because the wires are so close, and carrying power is more current (so more field) than that cable is designed for, and depending on what you power you may have some switching noise.