PoE Standards Explained: 802.3af, 802.3at and

The single biggest cause of PoE not working is a wattage mismatch between the power sourcing equipment and the powered device. The switch port or injector supplies power according to one IEEE standard; the device at the other end of the cable expects a different one. The result is a device that won’t power on, powers on intermittently, or reboots in a loop as the port cuts power to protect itself. Understanding the four defined PoE standards — and knowing how to identify which one your hardware uses — solves the majority of PoE problems before they happen.

The Three Main PoE Standards

IEEE has defined three standards that cover PoE, with the most recent (802.3bt) splitting into two types. Each standard is backwards compatible with the previous one from the PSE side — a PoE++ switch can power a PoE device — but the reverse is not true. The table below covers everything you need to compare them at a glance.

A few things to note from this table. First, the difference between PSE wattage (what the switch or injector outputs) and PD wattage (what reaches the device) is due to resistive losses in the cable. A longer cable run or lower-quality cable increases this loss. The IEEE standards define minimum PD power assuming worst-case cable conditions within the 100m structured cabling limit.

Second, 802.3bt (both Type 3 and Type 4) requires all four cable pairs to carry power — this is why Cat5e or better is essential for PoE++ deployments. Cat5e supports 4-pair PoE, but Cat6 or Cat6A is recommended for longer runs at higher wattage to keep resistive losses within acceptable limits.

Third, backwards compatibility at the PSE is a design requirement of the standards. A PoE++ switch port will correctly detect, classify, and power a PoE (802.3af) device, supplying only the power the device negotiates for. The intelligence is in the negotiation process.

How PoE Negotiation Works

Before a PoE port delivers full power, it goes through a structured negotiation sequence. This is what distinguishes active (standards-compliant) PoE from passive PoE, and it’s what makes active PoE safe to use without worrying about damaging non-PoE equipment.

The process works as follows:

1. Detection — the PSE applies a small probe voltage (between 2.8V and 10V) to the port. A valid PD presents a characteristic signature resistance of 25kΩ. If the PSE detects this resistance, it proceeds. If not — a standard Ethernet device won’t present this signature — power delivery is blocked.
2. Classification — the PSE ramps the voltage to a classification level and the PD signals its power class (0 through 8) by drawing a specific current. Class 0 is the default (max 15.4W). Classes 1–8 cover progressively higher power budgets up to 99.9W for Class 8 under 802.3bt.
3. Power-up — once classification is complete, the PSE enables full voltage (44–57V DC) on the port and the PD begins drawing power.
4. Ongoing monitoring — the PSE continuously monitors the port. If the connected device exceeds its negotiated power class, the PSE will shut down the port to protect itself and the device. If the device is unplugged, the PSE detects the loss of load and cuts power within a defined time window.

LLDP (Link Layer Discovery Protocol) and CDP (Cisco Discovery Protocol) can supplement this process in enterprise environments, allowing devices to negotiate higher power allocations than the hardware classification alone might allow — a useful capability when a device’s physical power class doesn’t accurately reflect its actual needs at full load.

When a PoE port shuts down under load, the most common cause is the device drawing more power than its negotiated class permits. This can happen with poorly designed or non-compliant PDs that under-report their power class during negotiation then draw more than agreed at runtime. It also happens when a switch’s overall power budget is exhausted — individual ports may cut out as the switch prioritises others.

PoE vs PoE+ — Does It Matter for Your Device?

In practical terms, the difference between 802.3af and 802.3at matters a great deal for specific categories of device. For others, it’s irrelevant.

Basic fixed-position IP cameras, VoIP desk phones, and simple wireless access points (single-radio, consumer or small-office grade) are almost always within 802.3af’s 12.95W PD budget. You can safely power these from any 802.3af-capable port or injector.

Enterprise wireless access points — particularly tri-band models, those with external antenna connectors, or those running Bluetooth and Zigbee radios alongside Wi-Fi — frequently require 802.3at (PoE+). Ubiquiti’s UniFi U6 Pro, for example, requires 802.3at. Connecting it to an 802.3af-only port will typically result in the AP powering on in a reduced mode or not at all, depending on the firmware. Always check the spec sheet: look for the “Power Method” or “PoE Input” field. It will state either the IEEE standard (802.3af, 802.3at) or the PoE class number, or sometimes just the wattage consumption at maximum load — compare this against the PD wattage column in the table above.

PTZ cameras (pan-tilt-zoom), which include motors and often heaters for outdoor use, routinely exceed 802.3af’s budget. A PTZ camera drawing 20–25W at full load needs 802.3at. A PTZ with a built-in heater for sub-zero operation might push that further still.

Active PoE vs Passive PoE and Why Standards Matter

Everything described above applies to active, standards-compliant PoE. Passive PoE is a different category entirely: it applies a fixed voltage to the cable at all times with no detection, classification, or negotiation. There is no safety handshake. Whatever is plugged into the output port receives voltage immediately.

Passive PoE is predominantly found in older Ubiquiti hardware (which used a proprietary 24V passive system) and in budget networking equipment from manufacturers that want to avoid IEEE licensing. The risks are real: connecting a passive 48V PoE injector to a device that expects 24V, or connecting a passive injector to a standard Ethernet device not expecting any power, can cause hardware damage.

For a full explanation of the differences and guidance on mixing active and passive PoE hardware safely, see active vs passive PoE explained.

How to Check What PoE Standard Your Device Needs

Don’t guess. Checking takes two minutes and prevents wasted time and potentially damaged hardware:

1. Find the product’s official spec sheet — not the product listing on a retailer’s website, which is often incomplete. Go to the manufacturer’s site and download the datasheet or specifications document.
2. Look for the power input section. This may be labelled “Power Method”, “PoE Input”, “Power Supply”, or similar. It will state one of: the IEEE standard (802.3af / 802.3at / 802.3bt), the PoE class number (Class 0 through 8), or the DC voltage and wattage range.
3. If only a wattage is listed, compare it against the PD column in the table above (12.95W for af, 25.5W for at, 51W for bt Type 3, 71.3W for bt Type 4). Choose the lowest standard whose PD wattage exceeds the device’s stated consumption.
4. Check whether the device supports active or passive PoE. The spec sheet will usually state “IEEE 802.3af/at compliant” for active PoE. If it says “passive PoE” or “24V passive”, it needs a passive injector — not an active one — unless the device explicitly supports both modes.
5. Check the data rate requirement while you’re there. If the device is gigabit-capable or connects to a gigabit switch, confirm your injector or switch port is also gigabit rated. A Fast Ethernet injector will cap throughput at 100Mbps.

What Happens If the Standards Don’t Match?

The outcome depends on which direction the mismatch goes:

• PSE standard is lower than PD requirement (most common problem) — the PSE negotiates a power class that doesn’t meet the device’s actual draw. Typically the device will either fail to power on, power on briefly and then shut down as it tries to draw more than the port allows, or power on in a degraded mode with some features disabled. The PSE port may log an overcurrent event and shut down. No damage occurs in this scenario — the PSE’s current limiting protects both devices.
• PSE standard is higher than PD requirement — not a problem. A PoE++ switch powering an 802.3af device will detect and classify the device correctly and supply only the wattage the device negotiates for. The higher capability of the port is simply unused.
• Passive PoE voltage mismatch — this is where damage can occur. A passive 48V injector connected to a device expecting 24V passive PoE will overvoltage the device’s power input circuitry. Conversely, a 24V passive injector connected to a device rated for 48V active PoE may not trigger the active detection sequence, leaving the device unpowered or delivering insufficient voltage. In either case, passive PoE mismatches can cause permanent hardware damage, which the IEEE negotiation process exists specifically to prevent.
• Active PoE connected to a non-PoE device — provided the PSE is genuinely IEEE-compliant, this is safe. The detection stage will not find a valid PD signature and will not enable power delivery. If the PSE is a passive injector masquerading as active, all bets are off.

Getting the PoE standard right before you buy your hardware takes minutes and avoids the most common class of PoE failures entirely. Check the spec sheet, match the standard, confirm active vs passive, and verify your cable can handle the power class you’re deploying. For a broader overview of PoE technology including injectors, switches, and common applications, return to the complete PoE guide.