PoE Budget

The formula

Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.

// Switch budget is consumed at the PSE figure for the class,
// not at what the device actually draws.
totalPSE = Σ ( quantity × classPsePower )

// Switch to specify for a target headroom
requiredBudget = totalPSE ÷ (1 − targetHeadroom)
actualHeadroom = (switchBudget − totalPSE) ÷ switchBudget

// Cable loss estimate — current splits across the pairs in use
pairsEachWay  = pairsUsed ÷ 2      2-pair: 1  ·  4-pair: 2
loopResistance = length × ohmsPerMetre ÷ pairsEachWay
current        = classPsePower ÷ pseVoltage
loss           = current² × loopResistance

What it assumes

Each of these is a way the number can be wrong for your job. Read them before you buy hardware.

1. Budget is consumed at the class figure, not actual draw. A switch reserves the full class power for a port whether the device uses it or not, unless you have configured LLDP power negotiation and the device supports it. Budgeting on measured draw is how you end up with ports that will not power up.
2. Class PSE power is the standard's figure. 802.3af Class 3 = 15.4 W, 802.3at Class 4 = 30 W, 802.3bt Class 6 = 60 W, Class 8 = 90 W. These already embed worst-case 100 m channel loss between PSE and PD — which is why the PD figure is lower.
3. The cable loss figure here is an independent estimate, calculated for your stated cable and run length rather than the standard's worst case. It is a sanity check, not a substitute for the standard's guaranteed PD power. On short runs with good cable, real delivered power will exceed the guaranteed PD figure.
4. Resistance values are for solid copper at 20 °C. Copper resistance rises about 0.4% per °C. In a hot ceiling plenum at 50 °C, add roughly 12%. Copper-clad aluminium is far worse and is not standards compliant for PoE — it is listed only so you can see the penalty.
5. The 100 m (328 ft) channel limit is absolute and is a data limit, not a power limit. Exceeding it is out of specification regardless of what the power arithmetic says. A warning appears above if your stated run exceeds it.
6. Simultaneous full draw is assumed. Correct for the worst case: a cold start where every device powers up at once. Heaters in outdoor camera housings are the usual cause of a budget that passed on paper and failed in January.
7. Per-port maxima are not checked. A switch with a large total budget may still cap individual ports below the class you need. Check both numbers on the datasheet.
8. Redundant power supplies are not modelled. If the switch budget you entered depends on two PSUs, a single-supply failure may drop it below your load.