PoE power budget calculator

The port current is I = P_pse / 48, and the loop resistance is R_loop = R_single·length for 2-pair mode (the source and return legs each run over one twisted pair, two conductors in parallel) or R_single/2·length for 4-pair. The cable loss is I²·R_loop. Power at the PD is the port budget minus the cable loss, and the voltage at the PD is 48 − I·R_loop. For 802.3at (30 W) over 100 m of 24 AWG solid cable this gives 0.625 A, 8.4 Ω, 3.28 W of loss and about 26.7 W at 42.8 V at the device.

The theoretical limit is the length at which the cable loss eats the entire budget, but the practical limit is much shorter because the PD still needs a working voltage. With a 26.72 W target on 24 AWG solid cable the tool resolves back to 100 m; pushing the target higher shrinks the allowable run. Real budgets are set by PD class power and derating, not the raw port rating.

802.3bt uses all four pairs in parallel instead of just two, which halves the loop resistance the current travels through. Although the port current rises (up to 1.875 A for Type 4), the reduced loop resistance means the volt-drop per metre stays comparable — which is exactly how 60 W and 90 W budgets survive a 100 m cable.

Always budget against the PD class power, which is lower than the port maximum — an 802.3at switch port can source 30 W but a Class 3 PD only draws about 13 W. The port rating sets the headroom of the budget; the class power sets what the cable actually carries. Add derating for ambient temperature, splitters and legacy equipment on top.