Work out the PoE budget before the switch is on the truck.
Count the powered ports by class, compare against the switch supply, and see what is left. Free, no signup, and it uses the same allocation rule WireFlow applies inside a diagram.
Budgets by PSE class allocation, which is how a switch actually apportions its supply
802.3af, 802.3at, and both 802.3bt types, with each class's device-side maximum shown
Tells you how many more ports of a given class still fit in the headroom
Flags oversubscription instead of quietly rounding it away
The number that trips people up is the gap between what a device pulls and what its port reserves. A PTZ camera that idles around 12 W still sits on an 802.3at port, and the switch has to be able to hand that port its full 30 W the moment the camera pans, heats, or powers a peripheral. Budget against the class allocation and the rig works. Budget against the idle figure and it works right up until several devices get busy at once.
That is the rule this calculator applies: every powered port reserves the PSE allocation for its class, 15.4 W on 802.3af, 30 W on 802.3at, 60 W on 802.3bt Type 3, and 90 W on Type 4.
Where the budget hides
Two details decide whether a switch that looks fine on paper is fine in the rack. The first is whether the PoE budget is shared across every port or split per line card, because a chassis switch can have plenty of total budget and still starve one blade. The second is that some switches subtract the full class allocation while others track real consumption and allocate dynamically, which changes the arithmetic completely.
Both are documented per model rather than per standard, so read the switch's own datasheet before committing a design. This calculator takes the conservative reading, which is the one that leaves you with a working rig rather than an explanation.
Frequently asked questions
Should I budget PoE by actual draw or by class?
By class, for planning. A switch must be able to deliver what each port advertises, so the allocation is what consumes the budget. Actual draw matters for thermal load and for metering during a show, not for deciding whether the switch fits the rig.
How much power does an 802.3at port reserve?
30 W at the switch. The device on the far end is guaranteed at most 25.5 W, and the difference is cable loss. Both figures are shown per class in the calculator.
Is the PoE budget shared across all ports?
Usually on a fixed-configuration switch, but not always on a chassis or stacked switch, where the budget can be allocated per line card or per member. Check the model's documentation, since a switch with adequate total budget can still run a single blade out of power.
What happens if I go over the PoE budget?
Behaviour is vendor-specific. Many switches deny power to ports past the limit, often by port priority, so the last device to boot is the one that stays dark. Some shut down lower-priority ports mid-show if demand rises. Neither is a failure mode you want to discover at a load-in.