PoE power classes are a negotiation, not a label. The connected device signals which class it belongs to, the source equipment reserves that much from its budget, and power then flows. What reaches the device is always less than what the source sent, because the cable itself consumes some of it as heat. Both numbers are legitimate; they are just measured at different ends.
What classification actually accomplishes
Before delivering power, source equipment performs a detection step to confirm that a powered device is present rather than an ordinary computer that would be damaged. Once detection succeeds, the device presents a signature indicating its class, and the source reserves that allocation from its total budget. This reservation is what stops a switch from promising more than it can deliver: if the next device’s class would exceed the remaining budget, the switch declines to power it rather than browning out everything. Devices can also negotiate more precisely through data-layer protocols after the link comes up, allowing an allocation that better matches actual consumption than the coarse classes allow. Detection uses a low voltage and looks for a specific resistance signature, which is what allows a switch to share a port between powered devices and ordinary computers without risking either.
Why the device gets less than the switch sends
Copper has resistance, and current flowing through it dissipates power along the way. The standards account for this by specifying a higher figure at the source than at the powered device, with the difference representing worst-case cable loss over the maximum permitted channel. On a short run with good cable the loss is small and the device receives nearly what was sent. On a long run with thin conductors it is much larger. This is why two identical cameras on the same switch can behave differently: one is on a short run in the same room and the other is at the far end of a long horizontal cable, receiving noticeably less. Data-layer negotiation can also be revisited while the link is running, so a device that needs more power when a heater or motor engages can request it rather than being locked to its startup allocation.
The budget that catches people out
A switch has a total power budget that is usually much less than the number of ports multiplied by the maximum per-port class. Filling every port with high-class devices will exhaust it. When it runs out, behavior depends on the switch: better ones refuse to power additional devices and log the event clearly, while poorer ones may shed power from lower-priority ports. Configure port priorities deliberately so that the devices you care about survive, and monitor the reserved and consumed figures rather than assuming. Also note that reservation is based on class, so a switch can report its budget exhausted while actual consumption remains modest. Conductor size and run length are the two variables under your control here, and specifying larger conductors for long powered runs recovers a meaningful part of what the cable would otherwise waste.
Devices that need more than a class provides
High-consumption equipment such as pan-tilt-zoom cameras with heaters, multi-radio access points, and powered displays can exceed what older source equipment delivers. Options include a switch supporting a higher-power standard, a midspan injector rated for the device, or a local power supply at the device with the network cable carrying data only. When choosing, check that the source equipment and the device implement the same standard revision, because a device expecting a higher-power standard connected to older source equipment will typically fall back to lower power and operate in a reduced mode, which owners often misread as a fault rather than a negotiated outcome. Midspan injectors sit between the switch and the device and are useful for adding power to an existing installation, though they add another connector pair and another device to power and monitor.
Non-standard injectors and why to avoid them
Cheap injectors exist that apply voltage to the cable without performing detection or classification at all. They can damage equipment that was never meant to receive power, and they provide no protection when a device is disconnected or a cable is faulted. Because they skip negotiation, they also give you no visibility into consumption. Use source equipment that implements the standard properly, from a manufacturer that states which revision it complies with, and that carries recognized safety testing marks. The same applies to passive splitters and adapters of unclear provenance. The cost difference is small relative to the equipment at risk. Monitoring reserved power alongside consumed power is worth configuring, because a switch can refuse a new device while its actual consumption looks comfortable, and the reason is invisible without both figures.
What to check before you act
- Compare the switch’s total budget against the sum of device classes.
- Confirm source and device implement the same standard revision.
- Set port priorities so critical devices survive budget exhaustion.
- Avoid injectors that apply voltage without detection and classification.
Common questions
Why does my camera report low power on a switch that has capacity?
Either the switch implements an older standard than the camera expects, or cable loss on a long run has reduced what arrives. Check both.
Does plugging a normal computer into a PoE port damage it?
Standards-compliant equipment performs detection first and will not energize a port without a valid signature. Non-standard injectors offer no such protection.
The practical takeaway
Plan the budget from device classes, not from measured consumption, and remember that long runs deliver less than short ones. Use standards-compliant source equipment so that detection and classification actually protect the devices on the other end.
References and further reading
- NFPA: National Electrical Code coverage of powered communications cabling
- CPSC: electrical equipment and accessory safety
- Department of Energy: network equipment energy considerations
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
