Power over Ethernet Distance Calculator

How it works

Voltage drop budget = V(PSE) − V(PD,min) · Max length = drop ÷ (2 × current × Ω per metre)

PoE sends DC power down the same twisted pairs that carry data, and the copper’s resistance drops some voltage along the way — the longer the run and the higher the current, the bigger the drop. The device keeps working only while the voltage arriving stays above its minimum, so the length limit is the voltage budget divided by the current times the loop resistance (out and back). Ethernet itself is capped at 100 m for signalling reasons, so even when the voltage math allows more, 100 m is the practical ceiling; past that you need a PoE extender or fibre.

Worked example

A 48 V source feeding a device that needs at least 37 V has a 11 V budget. At 0.35 A over 24 AWG copper (0.094 Ω/m), the loop drops 11 = 0.35 × 2 × 0.094 × L, giving L ≈ 167 m on voltage alone — but the 100 m Ethernet limit applies, so plan for 100 m.

Frequently asked questions