MB Networks
TOOL-24 · Networking
PoE budget calculator
List your powered devices, check the total against your switch's PoE budget, and see the headroom before you plug in one more access point.
Total draw
0W
Budget
0W
Headroom
0W
Enter each device's actual PoE class draw, not its idle power — the switch reserves budget per-class at negotiation time regardless of what the device actually pulls moment to moment.
Why the switch reserves more than a device actually uses
PoE allocation happens by negotiated class, not real-time measured draw — a Class 4 device reserves 30W of switch budget the instant it powers up, even if it typically draws 18W in normal operation. That headroom exists so a device can hit its peak draw (a PTZ camera's heater kicking in, an AP under full radio load) without browning out. Budget against the class rating, not typical usage.
The cable-length and power-loss factor
PoE power is delivered over the same copper as data, and resistance loss over longer cable runs means the switch actually has to push more wattage than the device receives — most switches already account for this in their published budget, but it's worth knowing if you're right at the edge of a budget and a run is unusually long (80m+).
What's the difference between 802.3af, at, and bt?
They're successive PoE standards with higher power ceilings: 802.3af (PoE) tops out around 15.4W at the source, 802.3at (PoE+) around 30W, and 802.3bt (PoE++) up to 60–90W depending on type. A device negotiates the highest standard both it and the switch port support.
Does the switch's total PoE budget mean every port gets that much?
No — the budget is shared across all active PoE ports, not per-port. A 120W switch budget with eight 15W devices connected is right at its ceiling; adding a ninth device would exceed it even though each individual port could technically supply more.
What happens if I exceed the PoE budget?
Behavior varies by switch, but commonly the switch either refuses to power the newest device negotiated, or in worse cases drops power to an already-running port to stay within budget — neither is a graceful failure, which is why checking headroom before adding a device matters.
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