Planning PoE: standards, power budget and cabling done right

The short answer: what matters when planning PoE

Power over Ethernet (PoE) supplies devices with power over the network cable. Reliable planning needs four things: the right PoE standard for each device, a switch with a sufficient power budget, cabling that copes with length and heat, and backup power for the switches. Clarify these points early and you avoid cameras that reboot at night and access points that run with reduced functions.

PoE is convenient because the camera or access point needs no power socket. But it also makes a lot of equipment depend on a few switches, so careful calculation before installation matters.

The PoE standards at a glance

The IEEE 802.3 standards define how much power a switch port (technically the PSE, power sourcing equipment) may supply and how much is guaranteed to reach the device even over 100 metres of cable. The difference covers the heat losses in the cable. The older standards use two wire pairs, while 802.3bt can use all four.

Before supplying power, standards-compliant PoE checks whether a PoE-capable device is connected at the other end and determines its power class. An ordinary laptop on the same port therefore receives no power. A device that requires 802.3bt only gets the lower power on an older port and, depending on the model, runs with restrictions or not at all. So-called passive PoE is different: it applies voltage permanently without this check, is not standards-compliant, can damage devices and should not be mixed with standard PoE.

How much power typical devices need

• IP cameras: Simple dome and bullet cameras often manage with 802.3af. Infrared illumination, heaters, motorised zoom or pan-tilt-zoom (PTZ) cameras raise the requirement, sometimes to 802.3at or 802.3bt.
• Wi-Fi access points: Current models for Wi-Fi 6, 6E and 7 often need 802.3at, some 802.3bt. With too little power, some devices switch off radios or ports. How many access points you need is a question for office Wi-Fi planning.
• IP phones: 802.3af is usually enough, while models with a large colour display or an expansion module may need 802.3at.
• Access readers and door stations: Readers themselves need little power. If the device also powers a door strike or has a camera, the requirement rises.
• Other devices: Small switches, thin clients, displays and sensors are increasingly PoE-powered too.

Calculating the switch's PoE budget correctly

Every PoE switch has a total budget: the power it can distribute across all ports combined. This budget is often lower than the sum of all ports. For example, a 24-port PoE switch with a 370 W budget cannot supply 30 W to every port at once, as that would require 720 W.

1. List all PoE devices per switch, with their standard and maximum consumption from the data sheet.
2. Add up the values and allow for cable losses. You are on the safe side with the switch-port value from the table above, for example 30 W for a PoE+ device.
3. Allow headroom of around 20 to 30 per cent for expansion and peak loads.
4. Check that the switch offers the required standard on enough ports, for example 802.3bt for PTZ cameras or new access points.
5. Set priorities: critical ports such as entrance cameras or access readers take precedence if the budget becomes tight.

Also check how the switch allocates power. Some models reserve the full power of the detected class for each port, while others go by actual consumption or negotiate via LLDP. Allocation by class is safer but uses more budget.

Cabling: the 100-metre limit and heat in cable bundles

PoE is subject to the same limit as Ethernet: a channel may be no longer than 100 metres, made up of at most 90 metres of permanently installed cable and 10 metres of patch cords in total. Longer runs may work in a test but are not standards-compliant and can become unstable under load.

At high power, heat becomes a factor: every current-carrying cable warms up, and in tightly packed bundles the heat cannot escape easily. Higher temperatures increase attenuation and can reduce the permissible length. The technical specification ISO/IEC TS 29125 provides planning rules on bundle sizes and installation methods. Cat 6A cables usually have thicker conductors than Cat 5e, so they warm up less, and shielding also helps conduct heat away. Our article on Cat 6A, Cat 7 or Cat 8 explains which category suits your building.

Power cuts: why PoE switches belong on a UPS

With PoE, cameras, access points, phones and access readers depend on a few switches. If those lose power, everything goes down at once. An uninterruptible power supply (UPS) bridges short outages and buys time for an orderly shutdown.

• Protect the whole chain: Besides the PoE switches, connect the core switch, firewall and recorder to the UPS. Otherwise the cameras keep running but nothing is recorded.
• Size it correctly: Base it on the expected peak load of the PoE devices plus the switches' own consumption and power supply losses. The switch's nominal PoE budget alone says little about this.
• Define runtimes: Decide how long which devices must keep running, for example cameras and access control longer than guest Wi-Fi. For doors, agree with fire safety and building services whether they stay open or locked during a power cut.
• Maintain and test: UPS batteries age. Plan regular tests and replacement as the manufacturer specifies.

Longer distances: PoE extenders and alternatives

If a device is more than 100 metres from the switch, a PoE extender can help. It regenerates the signal, passes the power on and so allows a further segment of up to 100 metres. Because the extender itself consumes power, less reaches the end device, and every additional device is another potential point of failure.

For greater distances, outdoor installations or links between buildings, fibre is usually the better solution: fibre to a small, locally powered PoE switch on site. A metal-free fibre cable also isolates the buildings electrically, which helps where earth potentials differ or surges occur. Some switches offer a mode for longer copper runs at a reduced data rate, but it lies outside the standard.

Conclusion: PoE is a matter of planning

PoE saves power sockets, power supplies and installation effort, but it demands careful calculation. Plan standards, budget, cabling and backup power together and the network stays reliable under full load and during power cuts. Q-entra plans network and Wi-Fi, structured cabling and video surveillance as one coordinated whole, so that every device gets the power it needs.