PoE: Power Down the Network Cable, and When It's Worth It

One obstacle appears every time an access point goes on a ceiling or a camera goes into the corner of a warehouse: the best spot for the device almost never has a power socket.

PoE, Power over Ethernet, resolves that simply. The power travels down the network cable that had to be run there anyway.

How it works

A LAN cable contains eight wires. On 100 Mbps Ethernet only four carry data; the other four sit idle. PoE uses those idle wires, or, at gigabit and above, rides the power on the same wires as the data, at voltages that don't interfere with it.

Wiring layout and cable quality are covered in the guide to choosing LAN cable, and with PoE that quality matters more than usual: thinner conductors lose more voltage over the length of the run.

Every PoE pairing has two roles:

• The power source: a PoE switch or injector, which supplies it.
• The powered device: an access point, camera, or IP phone, which draws it.

Before supplying power, the source runs a short check: it applies a small voltage and waits for a specific signature from the far end. If there's no signature, a plain laptop, say, power is never sent. That is what makes standards-based PoE safe to plug into anything.

Three standards, and how to choose

The numbers look cryptic, but they say one thing: how many watts are available.

• 802.3af (PoE): about 15 W at the source, roughly 12.95 W actually arriving. Enough for IP phones, fixed cameras, and simple access points.
• 802.3at (PoE+): about 30 W at the source, roughly 25 W arriving. This is the common choice today: Wi-Fi 6 access points, cameras with motors and heaters, small displays.
• 802.3bt (PoE++): 60 to 100 W. For heavy devices: high-end multi-radio access points, cameras with illumination, and some displays.

The gap between watts at the source and watts arriving isn't avoidable waste, it's loss along the cable, already accounted for in the standard. The figure to use when choosing devices is the one that arrives, not the one at the source.

What to avoid: passive PoE

Alongside those three standards sit cheap injectors sold as "passive PoE". They push voltage down the cable unconditionally, with no check, often at non-standard voltages such as 24 V.

Two consequences are real. Devices not designed for it can be damaged, and the voltage may not suit standards-based PoE gear, which then simply won't power up. Unless you're matching a specific pair from one manufacturer that requires it, passive PoE isn't worth what it saves.

Working out the power budget

The most common mistake with PoE switches isn't picking the wrong standard. It's missing one figure: the total power budget.

An 8-port PoE+ switch does not mean 30 W on all eight ports. The number that decides is listed separately, "PoE budget 60W", for instance. That switch can deliver 30 W, but to two ports at once.

Calculating it is straightforward:

1. Add up the draw of every device you'll connect.
2. Add roughly 25 per cent headroom. Devices draw more at startup, and requirements almost always grow rather than shrink.
3. Choose a switch with a budget above that figure.

For scale: four Wi-Fi 6 access points at 20 W each come to 80 W, or about 100 W with headroom. A switch with a 65 W budget will give up on the fourth device, usually by quietly refusing to power it rather than by saying so.

If the switch is also your network's backbone, the other selection criteria are in the guide to choosing a network switch.

When PoE actually earns its cost

PoE isn't an upgrade worth buying by default. It solves a specific problem, and outside that problem the extra cost doesn't return.

Worth it when:

• The device sits where there's no socket: ceilings, poles, exterior walls. This is the main reason, and often the only one needed.
• There are several devices, spread out: multiple access points across a building. The layout is covered in the guide to venue Wi-Fi capacity.
• Devices need restarting remotely. A managed PoE switch can cut and restore power to one port, meaning a ceiling access point can be restarted without a ladder.
• Everything must stay up through a power cut. One UPS in the rack powers the switch, and the switch powers everything at the far end of each cable. Far tidier than a UPS per location, see the guide to a UPS for your router.

Not worth it when:

• There's one device and a socket beside it.
• The goal is simply fewer cables on a desk.
• Budget is tight and would do more good on a better access point. Between a good access point with an ordinary adapter and an ordinary access point with PoE, the first is almost always the one you notice.

Problems that come up

The device doesn't power on at all

Check in this order: that the standards match (an 802.3at device on an 802.3af source will be underfed), whether the switch's power budget is already consumed by other ports, then try a different port to separate a faulty port from a faulty device.

It powers on, then keeps restarting

This is almost always insufficient power rather than a fault. The device starts on its initial draw, asks for more once its radios come up, and the source can't supply it. The symptom repeats at regular intervals, a pattern much like those in the guide to router indicator lights.

It powers on but the network is slow

Suspect the cable. A long run of thin conductors can carry enough power to start a device without being good enough for gigabit. Swapping the cable is the quickest test, and the guide to fixing slow Wi-Fi covers the next steps if it turns out not to be that.

Heat in the rack

A PoE switch delivering serious power produces far more heat than an ordinary one. Treat it like a router: give it space, see the guide to router overheating.

PoE removes one installation constraint: needing a socket where the device lives. If you don't have that constraint, PoE offers little.

If you do use it, two numbers decide everything, the standard, where 802.3at covers nearly everything today, and the switch's total power budget, which must be worked out from the number of devices rather than the number of ports.

Frequently asked questions