Power over Ethernet (PoE) lets a single Cat5e/Cat6 cable carry both data and DC power to a device — no separate power brick, no outlet needed near the ceiling where your AP or camera lives. It sounds simple until you're standing on a ladder with a PoE camera that won't boot, or you've bought a "PoE switch" that can't actually power what you plugged into it. Here's what actually matters.
The three PoE standards
There are three IEEE standards in the wild, and mixing them up is the #1 cause of "why won't this device power on" tickets:
- 802.3af (PoE) — up to 15.4W delivered at the switch port, ~12.95W usable at the device after cable loss. Fine for basic APs, IP phones, small cameras.
- 802.3at (PoE+) — up to 30W delivered, ~25.5W usable. Covers most modern Wi-Fi 6 APs, PTZ cameras, and higher-power devices.
- 802.3bt (PoE++ / UPOE) — split into Type 3 (up to 60W) and Type 4 (up to 100W). Needed for things like large digital signage, some AI-capable cameras, or powering thin clients over Ethernet.
A PoE+ switch will happily power an 802.3af device — the standards are backward compatible — but not the other way around. If a device negotiates PoE+ and your switch only offers PoE, it'll either refuse to power up or run in a degraded/reboot-loop state. Always check the device's power class, not just what the switch box says.
Power budget: the number that actually matters
Every PoE switch has a total PoE power budget (e.g., "370W PoE budget" on a 48-port switch), separate from the per-port maximum. This is where people get burned buying used gear:
- A switch can have 48 ports each capable of 802.3at (30W), but a shared budget of only 370W. That's enough for roughly 12 ports running at full PoE+ power — not 48.
- Budget is allocated dynamically on most switches (LLDP power negotiation), but some older or cheaper units allocate the maximum class per port statically, which exhausts the budget much faster with fewer devices.
- Always add up your real device draw (APs are usually 9-22W depending on model/radios, PTZ cameras can hit 25-30W, basic IP phones are 3-7W) and compare against the switch's rated PoE budget, not port count.
When buying a used enterprise switch specifically for PoE, look up the exact model's PoE budget in the datasheet — sellers rarely list it, and it varies wildly even within the same product line (e.g., a Catalyst 2960X-48FPD-L has a much bigger budget than a 2960X-48LPS-L).
Injectors vs. PoE switches
If you only need to power one or two devices and your existing switch isn't PoE-capable, a PoE injector is the cheaper fix — it sits inline on the cable run and injects power without touching your existing switch. Good for a single AP or camera drop.
Once you're past 3-4 PoE devices, a proper PoE switch is worth it: centralized power budget management, port-level PoE status/power draw visibility via CLI or SNMP, and the ability to remotely power-cycle a hung device (power inline never / power inline auto on Cisco, or the equivalent toggle on the web UI) instead of walking over to unplug it.
Cable length and power loss
PoE power loss over cable is real but often overstated for home lab distances. At 100m (the Ethernet spec limit) you'll lose roughly 10-20% of injected power to resistance, more with cheap/thin-gauge cable. Practically:
- Under 30m: negligible loss, don't worry about it.
- Near the 100m limit with a high-power PoE+ device: budget for it, or use Cat6/Cat6a instead of Cat5e — the thicker copper loses less.
- Never run PoE over damaged, kinked, or bundled-tight cable runs — the extra heat from resistive loss in a bad cable is a (minor but real) fire risk, and it's the first thing to check when a far-end device is flaky.
Common gotchas when buying used PoE gear
- Passive PoE vs. standards-based PoE are not the same thing. Passive PoE (common on cheap Ubiquiti-style gear) just applies constant voltage with no negotiation — plugging a passive PoE injector into an 802.3af/at device without checking pinout and voltage compatibility can fry it. Don't mix passive PoE gear with standards-based 802.3af/at/bt equipment unless you've confirmed voltage and pinout match.
- Test PoE ports before relying on them. A switch that powers up fine can still have individual ports with dead PoE controllers — this is one of the most common "half-working" faults in secondhand gear. Plug a known-good PoE device into every port you plan to use before deploying.
- Check for PoE watchdog / auto-recovery features. Useful for remote APs — the switch detects a non-responsive PoE device and power-cycles the port automatically, saving you a truck roll.
If you're speccing out a PoE deployment for APs, cameras, or phones and want to buy switches with known-good, tested PoE budgets rather than gambling on untested auction listings, resellers like IT and Office list tested access points and PoE-capable networking gear with the specs already verified.
What's the largest PoE power budget you've had to plan around, and did you hit any surprises calculating it?
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