PoE cable voltage drop calculator
Long Ethernet runs lose voltage to cable resistance. Plug in your cable type, PoE standard and run length to instantly see the voltage at the device — and whether it stays above the IEEE-mandated minimum.
- Source voltage
- 50.0 V
- Current draw
- 600 mA
- Voltage drop
- 12.43 V (24.9%)
- At device
- 37.57 V
- Result (min 42.5 V)
- FAIL — under spec
Why voltage drop matters for PoE
PoE devices have a minimum operating voltage. If too much is lost across a long cable, the device may reboot, refuse to power on, or behave erratically when IR/heaters kick in.
100 m isn't always safe
The 100 m limit is for data integrity, not power. A 90 W PoE++ device on a long Cat5e CCA run can fail the voltage spec well before 100 m. Always check both.
Plan it visually
Use SightPlanr to draw cable runs on satellite imagery and watch the planner warn you when a run gets too long.
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