Somebody has proposed cameras, access points and phones for the building, and every one of them is described as PoE powered. That is good news, because no electrician is needed at each device location, but it raises questions worth answering before the switches are ordered: how much power a cable can carry, which devices need which grade, and how to make sure the switch does not run out of budget on the day the last camera goes live.
What is happening inside the cable
Power over Ethernet, usually shortened to PoE, sends low-voltage direct current down the same twisted pair cable that carries the network data. The switch, or a midspan injector sitting between the switch and the device, is the power sourcing equipment. The camera or access point at the far end is the powered device. Before any meaningful power flows, the two negotiate. The switch applies a small detection voltage, looks for a specific resistance signature that says a PoE device is present, then reads a classification that tells it roughly how much the device intends to draw. Plug in an ordinary laptop and nothing happens, which is what makes standards-based PoE safe on a general purpose port.
The current travels on the same pairs as data, on the spare pairs, or on all four pairs at once for the highest grades. Whichever method is used, the device separates power from signal internally and the network traffic is unaffected. Because it is direct current at a modest voltage, PoE falls into the low-voltage category that a structured cabling contractor can install without the trades required for line voltage.
The standards and their power classes
Three IEEE standards define the mainstream grades, and each is usually described by the power available at the switch port and the power guaranteed at the device after cable losses.
The original standard, IEEE 802.3af, provides 15.4 W at the port and guarantees 12.95 W at the device. It comfortably runs a desk phone, a basic fixed camera or a simple access point. IEEE 802.3at, often called PoE+, raises that to 30 W at the port and 25.5 W at the device, which covers most pan-tilt-zoom cameras, dual-band access points and cameras with heaters or infrared illuminators. IEEE 802.3bt is the four-pair standard and comes in two types: Type 3 delivers 60 W at the port and 51 W at the device, and Type 4 delivers 90 W at the port and 71.3 W at the device. Those grades run large access points, video intercom panels, small displays, thin clients and door controllers that in turn power their own locks and readers.
Beyond the standards you will meet passive PoE, which simply pushes a fixed voltage without negotiation. It exists in some wireless backhaul gear and should never be mixed casually with standards-based devices, since there is no handshake to prevent damage.
What PoE typically powers
The obvious loads are security cameras, wireless access points and desk phones, and they are why PoE became the default for low-voltage systems. Less obvious loads are increasingly common. Access control door controllers accept PoE and pass regulated power on to card readers, request-to-exit sensors and electric strikes. Video intercom entry panels, paging speakers, clocks, occupancy sensors and small signage displays all ship in PoE variants. Because the device is powered from the telecom room, a single uninterruptible power supply on the switch keeps every camera and door controller running through a mains outage, something impossible when each device has its own plug-in adapter in the ceiling. Our network setup and infrastructure work is planned around exactly that arrangement.
Planning the switch budget
Every PoE switch has two limits: the maximum per port and the total power budget shared across all ports. A switch marketed as 24 ports of PoE+ does not necessarily supply 30 W to all 24 at once. The datasheet lists a total budget, and if the sum of what the devices request exceeds it, the switch starts refusing power to the lowest priority ports. Planning means listing every device, taking its real maximum draw from its documentation rather than the class it advertises, adding a margin for cameras whose heaters and infrared come on together on a January night, and comparing that sum against the switch budget with room to add devices later.
Cable quality feeds directly into that arithmetic. Power lost as heat along the run grows with resistance, so long runs, undersized conductors and copper-clad aluminium cable all waste budget and can push a device below its operating threshold. Solid copper cable of a recognised category, terminated properly and kept within the 100 metre channel, is what the standards assume. Bundling many high-power runs together raises their temperature, so the highest grades benefit from smaller bundles and larger conductors, a point covered on our network cabling and infrastructure page.
Common questions
Can PoE damage a device that does not support it?
Not with standards-based PoE. The switch only delivers meaningful power after it detects the resistance signature of a compliant powered device, so a laptop or printer plugged into a PoE port simply receives data. Passive PoE injectors are the exception, because they push voltage without checking, so keep them to the equipment they were supplied with and off shared patch panels.
How far can PoE travel?
As far as the Ethernet channel itself, which is 100 metres of cable including patch cords. Power arrives at the far end reduced by losses along the cable, and the standards account for that when they state a lower guaranteed figure at the device than at the port. Extenders exist for longer runs, though a fibre link with a small local switch is often the tidier answer.
Do I need PoE++ for everything?
No. Most desk phones, fixed cameras and modest access points run happily on 802.3af or 802.3at power. The 802.3bt grades matter for high-power access points, pan-tilt-zoom cameras with heaters, intercom panels, small displays and door controllers feeding multiple locks. Buying a switch with a few high-power ports and a sensible total budget is usually wiser than paying for 90 W on every port you will never use.
What happens to PoE devices in a power outage?
They stop, unless the switch is on an uninterruptible power supply. That is one of the strongest arguments for PoE: a single battery unit in the telecom room keeps every camera, access point and door controller alive for the duration of its runtime, and the switch can shed lower priority ports first if the battery runs low. Scattered plug-in adapters offer no such protection.
If you are specifying a new system or adding devices to an existing one, the switch budget and cabling deserve the same attention as the cameras themselves. Ask us about network setup and infrastructure for the building, or get in touch with your device list and we will work out what the switches and cabling need to be.
