PoE Voltage Drop Calculator — Cat5e/Cat6/Cat6a | WireStandard

Enter PoE standard (IEEE 802.3af 15.4 W / at 30 W / bt Type 3 60 W / Type 4 100 W), cable category (Cat5e, Cat6, Cat6a), and run length to compute voltage drop, power loss in the cable, and end-of-cable voltage at the powered device (PD). Results compare against the IEEE 802.3 minimum PD input voltage: 37 V for af/at and 41.1 V for bt.

PoE Voltage Drop Formula

VD = I × R_loop × L. Loop resistance per pair: Cat5e ≈ 0.188 Ω/m (two conductors × 0.094 Ω/m each); Cat6/6a ≈ 0.094 Ω/m per conductor, lower due to 23 AWG vs 24 AWG. IEEE 802.3af/at use 2-pair power delivery; IEEE 802.3bt (Type 3/4) uses 4 pairs in parallel, halving total loop resistance. PSE output: 48 V nominal (af), 54 V nominal (at/bt). Power loss = I² × R_loop × L.

Worked Example

802.3at PoE+ (30 W) over Cat5e, 90 m run. I = 30 W ÷ 50 V = 0.6 A. Loop R = 0.188 Ω/m × 90 m = 16.92 Ω. VD = 0.6 × 16.92 = 10.15 V. Voltage at PD = 54 − 10.15 = 43.85 V — above 37 V minimum, passes. Cable power loss = 0.6² × 16.92 = 6.09 W (20 % of input). At 100 m: VD = 11.28 V, PD voltage = 42.72 V, still passes — but 100 m is the IEEE 802.3 channel limit regardless.

Frequently Asked Questions

Why does Cat6a outperform Cat5e for PoE over long runs?

Cat6a uses 23 AWG conductors versus 24 AWG in Cat5e. The larger cross-section lowers per-meter DC resistance, so voltage drop accumulates more slowly. Over a 90 m run the difference can mean 1–2 V more at the PD, which matters for high-wattage bt devices operating near the 41.1 V minimum.

Can I run PoE beyond 100 m?

IEEE 802.3 limits an Ethernet channel to 100 m. Beyond that, PoE extenders (which regenerate the link) or a fiber run with a media converter at each end are required. Passive copper extensions beyond 100 m violate the standard and introduce unpredictable power loss.

Does cable temperature affect PoE voltage drop?

Yes. Copper resistance rises approximately 0.4 % per °C above 20 °C. TIA TSB-184-A recommends keeping the ambient temperature inside bundled cable runs at or below 60 °C; dense PoE bundles in conduit can self-heat well above that. Higher resistance means more voltage drop and more cable heat — a compounding effect worth derating for in high-density installations.