Power over Ethernet (PoE) sends electrical power and network data over the same twisted-pair Ethernet cable. A PoE-capable switch, or an injector placed between a switch and a device, powers equipment such as desk phones, wireless access points, IP cameras and door card readers, so those devices don’t need their own electrical outlet. It is defined by IEEE standards that have raised the available power over time.
At a glance
- One Ethernet cable carries both data and power; the device needs no separate power supply or outlet.
- Power comes from PoE switches or midspan injectors, called power sourcing equipment (PSE); the devices drawing it are powered devices (PDs).
- IEEE standards set the limits: roughly 15 W per port (802.3af), 30 W (802.3at, PoE+) and 60 or 90 W (802.3bt) at the switch, with less arriving at the device.
- Runs are limited to the usual 100 meters for copper Ethernet.
- The real planning constraints are each switch’s total power budget, the power class each device needs, and backup power for the closet.
What problem it solves
Wired phones, cameras, access points and sensors need both a network connection and power. Without PoE, each one needs an electrical outlet nearby, which means an electrician, building permits in some cases, and devices that can only go where power already is. That is a poor fit for ceiling-mounted access points, cameras on exterior walls or card readers beside doors.
PoE removes that second cable. The low-voltage Ethernet run installed for data also powers the device, so installation is simpler and devices can be placed where coverage is best. Because power comes from the network closet, it can also be backed up centrally: a UPS on the PoE switches can keep VoIP phones, cameras and Wi-Fi up during a short outage. And because the switch controls each port’s power, IT can often power-cycle a frozen device remotely instead of sending someone to a ladder.
How it works
Detection and classification. When a device is plugged into a standards-based PoE port, the switch first checks for a valid PoE signature before applying power. The device then signals which power class it needs. Devices that don’t support PoE are not powered, and simply get data.
Delivery. The switch supplies power over the cable’s wire pairs alongside data. Older standards use two of the four pairs; 802.3bt can use all four to deliver more power. Some power is lost in the cable, so the device receives less than the switch sends.
The standards.
- IEEE 802.3af (2003): up to 15.4 W per port at the switch, enough for desk phones and simple cameras.
- IEEE 802.3at, PoE+ (2009): up to 30 W, common for modern access points and pan-tilt-zoom cameras.
- IEEE 802.3bt (2018): Type 3 up to 60 W and Type 4 up to 90 W, for high-end access points, displays, lighting and some building systems.
Some vendors sell proprietary higher-power variants, and “passive” PoE injectors skip the detection step entirely; neither is interchangeable with the IEEE standards, so check compatibility.
Power budget. Each PoE switch has a total power budget shared across its ports, and it is often less than the per-port maximum times the number of ports. A 48-port switch may not be able to power 48 devices at full PoE+ at once. Switch software reports what each port draws and can prioritize critical ports when the budget runs short.
Cabling. PoE follows normal Ethernet distance limits of 100 meters. At higher power levels, large bundles of cable can warm up, so structured cabling design guidance for bundle size and cable category matters more for 802.3bt deployments.
When it matters for buyers
- Opening or refitting an office. Planning PoE switches, cabling and closet power together is cheaper than retrofitting outlets device by device.
- Rolling out Wi-Fi, cameras or access control. Newer access points and multi-sensor cameras may need PoE+ or 802.3bt; older switches may not supply enough.
- Replacing switches. Count devices, add up their power classes and compare against each candidate switch’s total budget, not just its port count.
- Planning for outages. Decide which PoE devices must stay up and size UPS runtime for the switches that power them, including the PoE load.
- Smart building projects. Lighting, sensors and displays on PoE shift some electrical work into the IT budget and onto the network team.
Questions to ask vendors
- Which PoE standard and class does each device need, and what does it draw in normal use versus peak?
- What is the total PoE budget per switch, and how many devices can it power at full class at once?
- Does the switch support power prioritization per port, and remote power-cycling of a device?
- Will the proposed cabling category and bundle sizes handle the power levels we plan to run?
- What UPS capacity and runtime are needed for the switches, including their PoE load?
- Are any devices using proprietary or passive PoE, and are they compatible with our switches?
We cover switch, cabling and access point design in our wired and wireless LAN infrastructure overview.
How it differs from passive PoE
Standards-based PoE (802.3af, at and bt) negotiates before it powers anything: the switch detects a compatible device, learns its power class and only then supplies power, within defined limits. Passive PoE, common in low-cost injectors and some outdoor radios, sends a fixed voltage down the cable whether or not the device on the other end expects it. That can damage equipment that isn’t designed for it, and there is no common standard for voltage or pin use. When a product says “PoE,” check whether it means the IEEE standard or a passive or proprietary variant.
Problems this helps with
- New cameras and access points need power where there are no outlets
- Electricians are needed for every device we add to the ceiling
- Not sure our switches can power everything we plan to plug in
- Phones and cameras go dark when the power blips
