PoE

PoE works by overlaying a DC voltage on the Ethernet cable pairs. The PSE (Power Sourcing Equipment — the switch or injector) detects whether the connected device is PoE-capable by measuring resistance, then negotiates the power class and delivers voltage. The PD (Powered Device — the camera, AP, or phone) draws power from the cable and uses it directly. The result: one cable from a PoE switch in a wiring closet provides both network connectivity and power to a ceiling-mounted access point, even if there's no electrical outlet nearby.

PoE standards

Which pairs carry power

Cat5e and Cat6 Ethernet cables contain four twisted pairs (eight wires). PoE uses two methods to deliver power. Mode A places DC voltage on the same pairs used for data — pairs 1/2 and 3/6 in 10/100 Mbps Ethernet. Mode B uses the spare pairs (4/5 and 7/8) that carry no data in 10/100 Mbps links. 802.3af and 802.3at switches may implement Mode A, Mode B, or both; PDs must accept power from either mode. 802.3bt (PoE++) uses all four pairs simultaneously to deliver the higher power levels — both Mode A and Mode B pairs carry power at the same time, which is why PoE++ requires all pairs to be intact and is not compatible with passive wire pairs being used for anything else.

Powered device classes

PoE devices identify their power requirements to the PSE using classification. During the detection phase, the PD presents a specific current signature (classes 0–8) that tells the switch how much power to allocate. Class 0 is unclassified (up to 15.4W allocated). Classes 1–3 allocate progressively less power for devices that need less. Classes 4–8 were introduced with 802.3bt for higher-power devices. The switch allocates the class's maximum power from its budget when a device connects, even if the device draws less — this matters for planning total PoE switch budget since the switch reserves power based on class, not actual draw.

PoE switch vs PoE injector vs midspan

A PoE switch has PoE built into some or all of its ports — the most common deployment for structured installations. A PoE injector is an inline device placed between an existing non-PoE switch and a single PoE device, adding power to one Ethernet run without replacing the switch. A midspan (patch panel injector) is a multi-port injector placed in a rack between a non-PoE switch and a patch panel, adding PoE to multiple ports at once. Injectors and midspans are cost-effective ways to add PoE capability to existing non-PoE infrastructure without replacing switches.

Common PoE devices

The most common powered devices in enterprise and prosumer networks: wireless access points (Wi-Fi 5 APs require PoE/PoE+, Wi-Fi 6E APs often require PoE++ Type 3), IP cameras (fixed cameras typically 5–10W, PTZ cameras with motors up to 25W), VoIP phones (3–7W, well within 802.3af), smart switches and small network appliances, door access control readers, and digital signage players. In home networks, Wi-Fi 6 access points like the UniFi U6 Pro require 802.3at (PoE+) and are a common reason home users upgrade from basic unmanaged switches to PoE+ managed switches.

PoE switch budget

PoE switches have a total power budget — the maximum watts they can deliver across all ports simultaneously. A 24-port switch might have a 370W budget. If every port connects a 15.4W PoE device, that's 370W total — the switch can power all 24. But if devices require PoE+ (30W each), the same switch can only power 12 devices at full load. Always verify the switch's total PoE budget against the sum of your devices' power requirements, not just the per-port rating. Many budget switches advertise "PoE on all ports" but have a total budget insufficient to power all ports simultaneously at rated wattage.

Passive PoE and its risks

Passive PoE (common with cheap wireless equipment from vendors like Ubiquiti's older lines, MikroTik, and many IP camera brands) delivers a fixed voltage — typically 24V or 48V — on the spare pairs without any detection or negotiation handshake. Because there is no 802.3af detection phase, passive PoE injectors will apply voltage to any device connected to them, including non-PoE devices. Connecting a standard laptop or non-PoE switch port to a passive PoE injector can damage or destroy the device's network interface. Always verify whether a device requires passive PoE or IEEE standard PoE before connecting — they are not interchangeable.

PoE and cable length

The 100-metre maximum cable length for Ethernet is unchanged with PoE — the same physical limit applies. However, power delivery is affected by cable resistance, which increases with length. A 100-metre run of Cat5e has approximately 9.4 ohms of resistance per conductor; carrying 600mA (typical for a 15W device), this produces a voltage drop of around 5.6V. A switch delivering 48V at the port may deliver only about 44V at the end of a 100-metre run. This is well within tolerance for most PoE devices, which accept a range of input voltages. For very long runs with high-power devices, verify the device's minimum input voltage specification against the expected voltage drop.

Frequently Asked Questions

What is the difference between PoE, PoE+, and PoE++?

PoE (802.3af) delivers up to 15.4W — enough for phones and basic cameras. PoE+ (802.3at) doubles to 30W for Wi-Fi 6 APs and PTZ cameras. PoE++ (802.3bt) reaches 60W (Type 3) or 100W (Type 4) for high-power APs, thin clients, and laptops.

Can PoE damage a non-PoE device?

No — IEEE-compliant PoE switches perform a resistance detection handshake before delivering power. Non-PoE devices don't respond and receive no power. Passive PoE injectors (non-standard) skip this check and can damage devices — always use 802.3af/at/bt compliant equipment.

Does PoE reduce Ethernet cable speed?

No. Power runs at DC, far below the gigahertz frequencies used for data. A PoE Gigabit port transmits data at the same speed as non-PoE. Very long cable runs may deliver slightly below rated power due to cable resistance, but data speed is unaffected.