The Other Voltage Drop: PoE and the 100-Meter Budget

Voltage drop did not stay in the electrical room. Every PoE camera, access point and phone receives its power through 0.5 mm-ish data conductors — 24 or 23 AWG — and the same Vd = 2ILR/1000 physics applies, just at 48 V and fractional amps.

The numbers

Cat5e (24 AWG) measures roughly 9.4 Ω per 100 m per conductor. PoE splits current across pairs, but a full 100 m run still eats real power: standard 802.3af delivers 15.4 W at the source and guarantees only 12.95 W at the device — the missing 2.5 W is cable drop, baked into the spec. PoE+ (30 W → 25.5 W) and PoE++ (60/90 W classes) budget even more for the copper.

Where installs go wrong

• Marginal devices at full length: a 25 W PTZ camera on a 95 m Cat5e run with a budget PoE+ switch port is living on the edge — cold weather (heater on) tips it into brownout reboots.
• Thin patch cords and couplers: 28 AWG slim patch leads and back-to-back couplers add disproportionate resistance per meter.
• CCA cable: copper-clad aluminum has ~40% more resistance than solid copper Cat6 — it fails PoE budgets first and standards always.

The fixes

Cat6 (23 AWG) cuts loop resistance ~20% — buy it for anything powered. Keep powered runs under 80 m where layout allows, count the device's worst-case draw (heaters, IR), and for true distance problems run 240 V out to a midspan switch instead — at which point you are back on the main calculator, where the volts are bigger and the math is friendlier.