Calculated Result
Results appear here after you press calculate.
Result breakdown
Plotly graph
The chart shows how delivered voltage, voltage drop, and cable loss change with one-way cable length.
Calculator
Use one-way cable length. The tool calculates electrical loop resistance automatically.
Example data table
These sample cases are generated with the same formulas used by the calculator.
| Scenario | Profile | Length | Cable | Load entry | Delivered voltage | Voltage drop | Cable loss | Status |
|---|---|---|---|---|---|---|---|---|
| Loading example calculations... | ||||||||
Formula used
The model assumes DC resistance and balanced current sharing across active conductors.
1) One conductor resistance at 20°C
R20 = ρ × L / A
2) Temperature-adjusted conductor resistance
RT = R20 × [1 + α × (T − 20)]
3) Effective loop resistance
Rloop = 2 × RT for 2-pair delivery
Rloop = 1 × RT for 4-pair delivery
4) Voltage drop
Vdrop = I × Rloop
5) Delivered voltage
Vdevice = Vsource − Vdrop
6) Cable loss
Ploss = I² × Rloop
7) Power mode solution
P = I × (Vsource − I × Rloop)
R × I² − V × I + P = 0
When you choose load power mode, the calculator solves the quadratic equation so current, voltage drop, and delivered voltage remain internally consistent for the selected cable resistance.
How to use this calculator
- Choose a common profile or keep the setup fully custom.
- Select whether you want to calculate from load power or line current.
- Enter source voltage, one-way cable length, AWG, material, pair count, and temperature.
- Set the minimum device voltage to reflect your endpoint requirement.
- Optionally enter custom conductor resistance when using vendor or measured cable data.
- Press Calculate voltage drop to show results above the form.
- Review delivered voltage, drop, loss, efficiency, and maximum estimated length.
- Use the CSV or PDF buttons to export your calculated summary.
FAQs
1) What affects PoE voltage drop the most?
Cable length, conductor size, material, pair count, source voltage, and device load matter most. Longer, thinner, warmer, or higher resistance cable creates more drop and heat.
2) Why does 4-pair delivery usually drop less?
Four-pair delivery shares current across more conductors. That lowers effective loop resistance and reduces voltage drop compared with the same cable carrying the same total power on two pairs.
3) Should I enter one-way length or total loop length?
Enter one-way installed cable length only. The calculator automatically converts that distance into the full electrical loop resistance needed for a PoE drop estimate.
4) Why can temperature change the answer?
Conductor resistance rises as cable temperature rises. Warmer cable increases voltage drop, reduces delivered voltage, and raises line loss under the same load.
5) Is this exact for every switch and device?
No. It is a planning calculator. Real ports, connectors, startup surge, patch panels, cable quality, and device input behavior can shift results from the estimate.
6) Can I estimate custom cable types?
Yes. Turn on custom conductor resistance and enter one-conductor ohms per kilometer. That lets you model unusual cable builds, vendor data, or measured resistance values.
7) Why does the chart stop at some lengths?
That usually means the requested power can no longer be sustained at longer distances with the chosen voltage and cable resistance. The discriminant becomes invalid, so the plot ends.
8) Does a lower delivered voltage always mean failure?
Not always. Failure depends on the powered device input requirement. Compare the calculated delivered voltage with your device minimum operating voltage for a more realistic pass or fail check.