PoE Budget Calculator (802.3af, at, bt)

A PoE switch has a total power budget, often far less than 30 W on every port at once. This calculator adds up what your devices need the way switches do: each IEEE 802.3 class reserves a fixed amount at the port (15.4 W for class 3, 30 W for class 4, up to 90 W for class 8). It also shows what the devices actually receive, the budget left and the power lost in the cable.

How to use this tool

1. Enter the switch's PoE budget from its datasheet.
2. List the devices, one line per kind: a count and a class (class 3), a standard (802.3at) or a measured power in watts.
3. Pick the switch's port standard so devices that need more per port are flagged.
4. Optionally add the cable length, gauge, port voltage and temperature to estimate cable loss.

Formula

allocated = Σ count × PSE power of the class; remaining = budget − allocated; cable loss = I²R with I = (V − √(V² − 4·R·P)) ÷ (2R)

PSE / device power per class: 0 = 15.4 / 12.95 W, 1 = 4 / 3.84, 2 = 7 / 6.49, 3 = 15.4 / 12.95, 4 = 30 / 25.5, 5 = 45 / 40, 6 = 60 / 51, 7 = 75 / 62, 8 = 90 / 71.3 W. R is the DC loop resistance: one conductor's resistance × length for 2-pair power (two wires out, two back), half that for 4-pair (classes 5–8). P is the device draw and V the port voltage.

Example

Eight class 3 cameras reserve 8 × 15.4 = 123.2 W. On a 120 W switch that is 3.2 W over budget, although the cameras receive at most 8 × 12.95 = 103.6 W. A 25.5 W access point at the end of 100 m of 24 AWG cable at 50 V loses about 2.7 W in the cable, so the port supplies about 28.2 W.