How Power over Ethernet budgets really work
The number that runs out is the switch's total budget, and it is usually much smaller than the port count implies.
Power over Ethernet carries electricity and data on the same cable, which is why cameras, phones and access points can go where there is no socket. The planning question is always the same: will the switch power everything you are about to plug into it?
The standards
- 802.3af (PoE) — 15.4 W at the switch, about 12.95 W at the device. Phones, simple cameras, sensors.
- 802.3at (PoE+) — 30 W at the switch, about 25.5 W at the device. Most access points, pan-tilt-zoom cameras, better displays.
- 802.3bt Type 3 — 60 W at the switch, about 51 W at the device.
- 802.3bt Type 4 — 90 W at the switch, about 71 W at the device. Laptops, small displays, heated outdoor enclosures.
Why two numbers
Cable turns some of the power into heat, and the loss grows with distance. The switch has to supply more than the device consumes, which is why every standard quotes a pair of figures.
Budget against the switch figure. A device consuming 25.5 W costs 30 W of budget, not 25.5.
The budget is not the port count
This is the mistake worth avoiding. A 48-port switch advertising 30 W a port almost never has 1,440 W to give — it has a power supply, commonly 370 or 740 W, and that is the real limit.
When the budget runs out, ports stop delivering power. Which ones depends on priority configuration, and the moment it happens is usually not when you plug in the last device but when everything restarts together after a power cut and all of it draws at once.
Class allocation and negotiation
A switch reserves power by the class a device declares, not by what it actually draws. A phone declaring class 3 has 15.4 W reserved even if it only ever uses 6.
Where both ends support LLDP power negotiation, the device can ask for less and the switch can release the difference. That helps considerably, and it depends on both ends supporting it — so budgeting against the class allocation is the safe assumption and anything better is a bonus.
Frequently asked questions
How many PoE+ devices can a 370 W switch power?
About twelve at the full 30 W class allocation, regardless of how many ports it has. The power supply is the limit, not the port count.
Why does the standard quote two different wattages?
Cable loses power as heat over distance, so the switch supplies more than the device receives. Budget against the switch figure.
What happens when the PoE budget is exceeded?
Ports stop delivering power, usually the lowest priority ones. It commonly happens when everything restarts together after an outage rather than when the last device is added.
Does a device always use its full class allocation?
No, and the switch reserves it anyway unless LLDP power negotiation is in play at both ends. Budget against the class figure and treat negotiation as a bonus.
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