Power over Ethernet: Standards, Power Budgets, and Troubleshooting

Power over Ethernet delivers electrical power and data over the same twisted pair cable, so a camera, access point, or phone needs one connection instead of two. The value is not the cable saved — it is that the device can be placed where there is no outlet, and that its power comes from a UPS-backed switch in a wiring closet rather than from whatever socket happens to be nearby.

The standards

802.3af, the original, provides 15.4 W at the source and about 12.95 W at the device after cable loss. Enough for a phone, a basic access point, or a fixed camera.

802.3at, called PoE+, raises this to 30 W at the source and roughly 25.5 W delivered. This covers pan-tilt-zoom cameras, higher-capacity access points, and small displays.

802.3bt defines two further types: Type 3 at up to 60 W and Type 4 at up to about 100 W at the source, delivering roughly 51 W and 71 W respectively. These use all four pairs rather than two, and they support laptops, larger displays, and LED lighting.

Note the pattern: the source figure and the delivered figure always differ, because resistance in the cable dissipates power as heat over the run. Exam questions sometimes give one and ask for the other, and specifying equipment against the source figure is a common real-world mistake.

Negotiation and the power budget

A PoE switch does not simply energize its ports. It first detects a valid signature resistance on the connected device, confirming something designed to receive power is present rather than a laptop that would be damaged. It then classifies the device to learn how much power it requires, and only then supplies it. Devices can also negotiate more precisely using LLDP, which lets a device request exactly what it needs instead of reserving a whole class.

The power budget is where deployments go wrong. A switch has a total PoE budget that is almost always less than the sum of all ports at maximum. A 24-port switch advertising PoE+ on every port rarely has 720 W available; 370 W is a common figure, which supports roughly twelve devices at full PoE+ draw.

Exceeding the budget produces behavior that looks like a fault: ports stop delivering power, usually the highest-numbered or lowest-priority ones, and devices reboot in a loop. Calculate the real total draw before deployment, leave headroom, and use per-port power priority so that if the budget is reached, a decorative display loses power rather than a security camera.

Cable and distance

The 100 metre limit for Ethernet applies to PoE too, and power loss increases with length — a device at 95 metres receives measurably less than one at 10 metres. For higher-power classes, run Cat6 or better rather than Cat5e; the thicker conductors lose less.

Heat matters at the higher power levels. Large bundles of cables carrying 60 W or more each will warm noticeably in a conduit, and standards specify bundle limits for this reason. Avoid flat or copper-clad aluminium cable entirely for PoE; the resistance is higher and the fire risk is real.

Troubleshooting a device that will not power up

Work in this order. Check whether the switch port is PoE-capable at all — on many switches only some ports are. Check the switch's remaining power budget and whether other ports were recently added. Confirm the standard matches: an 802.3bt device on an 802.3af port will not get what it needs, and may not power on at all.

Then check the cable. Damaged pairs, poor terminations, and runs over 100 metres all cause intermittent power as well as data errors. A device that boots and then reboots under load is the classic marginal-power symptom, and rising interface error counters alongside it point at the cable rather than the switch — the diagnostic path described under CRC and frame check sequence errors. A toner and probe locates the run, and a cable certifier proves whether it is in spec.

Finally, check for a passive injector in the path. Passive PoE devices apply voltage without negotiation and can damage equipment that was not expecting it.

Power planning

PoE concentrates power draw in the wiring closet, which changes the UPS calculation: the switch now carries the load of every device attached to it. Size the UPS accordingly, and remember that PoE devices stay up exactly as long as the switch does — which is an availability advantage if the closet is protected and a single point of failure if it is not. Monitoring per-port and total draw, the same discipline applied to managed PDUs, catches budget problems before they cause outages.

Exam angles

Know 802.3af at 15.4 W, 802.3at (PoE+) at 30 W, and 802.3bt Types 3 and 4 at 60 W and 100 W at the source, with lower figures delivered. Understand detection and classification before power is applied. Recognize the switch power budget as the constraint that matters in deployment, and be ready for a troubleshooting scenario where a device fails to power on or reboots — budget exhaustion, standard mismatch, or cable length are the answers.