A Guide to Industrial Power over Ethernet

Power over Ethernet (PoE) is a family of IEEE 802.3 standards that send DC power and data down the same twisted-pair Ethernet cable, from a power sourcing equipment (PSE) port on a switch, injector or computer to a powered device (PD) such as a camera, wireless access point, panel PC or edge computer. One cable of up to 100 metres replaces a data run and a separate mains or DC supply, and the PSE only switches power on after it has detected a compliant device at the far end.

Three standards set the power levels. 802.3af (PoE) supplies up to 15.4 W per port, 802.3at (PoE+) up to 30 W, and 802.3bt (PoE++) up to 60 W or 90 W by using all four pairs. The device always receives less than the port supplies, because some power is lost as heat in the cable, so a deployment is sized on the figure at the device and on the switch or computer’s total PoE budget, not on the per-port maximum alone.

This guide is the reference view for industrial projects: the IEEE standards and classes side by side, how detection and classification work, how to size a PoE budget, active versus passive PoE, the PSE and PD roles, what changes in a cabinet, a vehicle or on a pole, and how far PoE can reach. For short answers on single topics, see our FAQs on what PoE is, PoE vs PoE+ vs PoE++ and how much power PoE delivers.

Every PoE standard is an amendment to IEEE 802.3, and each newer one stays compatible with devices built to the older ones. The table is the quick reference: power is quoted at the PSE port and at the PD after the worst-case 100 m of cable.

The 2018 standard is the one most often misquoted. Type 3 covers classes 5 and 6, at 45 W and 60 W from the PSE, and it can also power older class 0–4 devices over two pairs. Type 4 covers classes 7 and 8, at 75 W and 90 W, and always uses all four pairs. Some datasheets round the Type 4 figure to 100 W, which is a marketing number rather than an IEEE one; the standard caps the PSE at 90 W and the device at 71.3 W.

Before 802.3bt, several switch makers sold pre-standard 60 W schemes, and those devices do not always interoperate with standard Type 3 equipment. If an older device is labelled with a proprietary scheme rather than an IEEE type and class, check it against the PSE before you order. The PoE vs PoE+ vs PoE++ FAQ linked above covers the names in more detail.

Why the device gets less than the port

The PSE figure includes the power lost in the cable, which turns into heat along the run. 802.3af assumes a worst-case loop resistance of 20 Ω, the figure for older Category 3 cable, which costs up to about 2.45 W at 350 mA. 802.3at and 802.3bt assume Category 5e or better at about 12.5 Ω, which is why a 30 W port leaves 25.5 W at the device and a 90 W port leaves 71.3 W. Shorter runs lose less, but the standards size every link for the full 100 m, and so should a power budget.

PoE is safe to plug into any Ethernet device because the PSE does not apply full voltage until it has checked what is on the other end. The sequence takes a fraction of a second and runs every time a cable is connected or a device restarts.

That handshake is what the IEEE calls active PoE, and it is why a PoE switch can share a rack with ordinary devices. It is also the step that passive PoE leaves out, which is covered in the section on active vs passive PoE below.

Data and power share the same wires because the power is DC and the data is not. Ethernet magnetics in every port couple the data signal across a transformer, and the PSE applies the DC voltage between the centre taps of two pairs, so it appears as a common-mode voltage the data receivers never see. On a Gigabit link, which uses all four pairs for data, the power simply rides on top.

Mode A, Mode B and four-pair power

Two-pair PoE can use either set of pairs. Mode A puts power on the pairs on pins 1–2 and 3–6, the same pairs that carry data on a 10/100 link. Mode B uses pins 4–5 and 7–8, the spare pairs on 10/100. A standards-compliant PD must accept either, with either polarity, so the choice belongs to the PSE. Our FAQ on PoE Mode A vs Mode B has the full pin map.

802.3bt adds four-pair power and, with it, two kinds of device. A single-signature PD presents one signature across both pair sets and is powered as one load. A dual-signature PD presents a separate signature on each pair set, so each half can be classified and powered on its own. Most cameras and access points are single-signature; dual-signature designs tend to appear in devices with two independent power rails. The cable must be at least twisted pair to Category 5e for 802.3at and 802.3bt.

A PoE class is the power level a device asks for during classification. The PSE reserves that amount from its budget, so the class decides how many devices a switch or computer can power, not just whether one device will run.

Class is a ceiling, not a measurement. A class 4 camera may draw 12 W on a mild day and 25 W when its heater and IR illuminator are on, and a PSE that allocates by class will reserve 30 W for it either way. Many managed PSEs can instead negotiate the real figure with the device over LLDP, which 802.3at introduced as data link layer classification, and 802.3bt adds Autoclass, where the device draws its maximum briefly at start-up so the PSE can measure it. Both free up budget, and both are worth checking for on a datasheet when the switch is near its limit.

When a PSE runs out of budget, it refuses or removes power from the lowest-priority port. Managed industrial PoE switches let you set that priority per port, so a door controller stays on while a monitor goes dark, and unmanaged models usually fix it by port number.

Per-port figures say what one device can have. The PoE budget says what all of them can have at once, and it is the number that most often limits an industrial build. The table works through a typical roadside cabinet.

On that load, an eight-port switch with a 240 W budget, such as the Antaira LMP-1204G-SFP-bt with four 90 W PoE++ ports and four PoE+ ports, has room for the cameras, the access point and a later addition. A computer with four PoE+ ports and a 50 W budget would not, even though each of its ports can deliver 25.5 W on its own.

Computers show the gap between per-port and total power most clearly. The Neousys NRU-52S series has four 802.3bt ports that can each deliver up to 90 W to one camera, but its PoE budget is 50 W across the ports when passively cooled, or 144 W with the optional fan kit. The Sintrones IBOX-602P-IP66 supplies 15.4 W on each of its two M12 ports within a 32 W budget. Neither is a limitation once it is planned for; both are reasons to add up the devices before choosing the computer. Our white paper on PoE power budgeting for multi-camera edge AI works through larger examples.

Input voltage changes the budget

A PSE has to produce 44–57 V at the port, so most PoE switches and cards expect a 48–55 V DC supply, and the PoE budget is limited by what that supply can deliver. Switches with a built-in boost converter accept a lower input and step it up, but the budget usually falls with the voltage. The Antaira LNP-0500G-bt-24, for example, gives a 180 W PoE budget from a 24–55 V DC supply and 90 W from 12 V DC. Size the power supply for the PoE budget plus the switch or computer’s own draw, and add the losses of the supply itself.

Active PoE is the IEEE kind described above. Passive PoE applies a fixed voltage, commonly 24 V or 48 V and sometimes 12 V, to the cable whenever the injector is switched on, with no detection, no classification and no disconnect. It persists because it is cheap and simple, and some outdoor radios and small devices are built for it. Our FAQ on active vs passive PoE goes further.

Feeding passive 48 V into a port that is not built for it can damage the port, and an IEEE device on a 24 V passive injector will usually not start at all. In an industrial network, where devices from several makers share switches and the people who maintain it change over the years, specifying IEEE-compliant PoE end to end removes a whole class of site faults. Where a passive device has to stay, give it its own matched injector and label the cable.

Every PoE link has one PSE and one PD, and the same piece of equipment can be either. A switch is usually a PSE, a camera is usually a PD, and computers can be both. Our FAQ on PoE PD vs PSE explains the terms, and the table shows where each role turns up in an industrial system.

Do I need a PoE switch or a PoE injector?

A PoE switch is the better choice when several devices need power, because one budget, one supply and, on managed models, one place to monitor and restart ports cover them all. A PoE injector suits one or two devices on an existing non-PoE switch, or a single device that needs more power than the switch can give. Injectors are also called midspans, and on 10/100 links they usually use the spare pairs, Mode B.

Choosing between managed and unmanaged PoE switches follows the same logic as any other switch, with two extras: managed models let you see each port’s draw and power-cycle a hung camera remotely. Our guide to managed vs unmanaged switches sets out the wider choice.

Computers on both sides of the link

As a PSE, a PoE computer powers and connects a set of cameras or sensors directly, so a vision or surveillance node needs no separate switch. That is the common pattern in industrial NVR systems and in NVIDIA Jetson PoE systems running AI on the camera feeds, and our guide to PoE in surveillance and NVR systems covers it in depth. Board-level designs can start from PoE single board computers, and existing computers can gain ports through Ethernet and PoE frame grabbers.

As a PD, the computer itself runs from one cable, which removes the need for a local power supply in a ceiling, on a pole or in a sealed enclosure. The power available sets the class of computer: 25.5 W at 802.3at suits a low-power processor and a small screen, while 802.3bt opens the way to more capable systems. Our FAQ on whether a computer can be powered by PoE sets out the limits, and PoE panel PCs and PoE monitors put a screen on the same single cable. We configure, image and test every PoE computer and panel PC with its power settings and devices in mind before it ships.

The IEEE standards are the same in a factory as in an office. What changes is everything around them: the temperature, the supply, the electrical noise, the vibration and how hard it is to reach a failed device.

Temperature and budget interact. Many PoE switches and computers reach their full budget only within part of their rated range, or with a fan fitted, so read the derating notes as well as the headline figure. The same applies to cables: a bundle of 90 W runs in a closed tray heats up, and ISO/IEC and TIA guidance on bundled PoE cabling recommends larger conductors or smaller bundles where many high-power links share a route. Our FAQ on the industrial temperature range explains the common ratings.

Vibration is the other industrial difference. An RJ45 plug held by a plastic clip can work loose on a vehicle or a machine, so rail and vehicle PoE uses M12 connectors, X-coded for Gigabit and above and D-coded for 100 Mbit/s. Our guide to M12 connectors covers the codings, and M12 Ethernet switches bring the same sealed connectors to the network side. On rolling stock, look for computers designed to EN 50155.

Applied examples on our site include PoE computers running ANPR barrier control and embedded systems with PoE in a smart parking scheme.

PoE follows Ethernet’s own limit: 100 m per link, made up of up to 90 m of fixed cable and 10 m of patch leads. Beyond that, the data link may still work, but the voltage at the device falls and neither the data nor the power is guaranteed.

Cable category matters for power as well as speed. Category 5e is the minimum for 802.3at and 802.3bt, and the larger conductors often used in Category 6 and 6A cable, typically 23 AWG, run cooler at high power. Our FAQs on Cat5e vs Cat6 and Cat6 vs Cat6A compare them, and single-mode vs multimode fibre covers the fibre side of a long run. For single-pair links on machines and vehicles, see our white paper on single-pair Ethernet.