Voltage Drop Calculator
Check whether a cable run is too long for the current it carries.
Your numbers
Your results
Loop resistance (Ω)
Loop resistance (Ω)
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Voltage drop (V)
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Drop as % of supply
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Power lost in cable (W)
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Voltage Drop Calculator by TheCalcTools
How the Voltage Drop Calculator works
Enter the load current, the one-way length of the cable, its conductor area and material.
Current flows out and back, so the loop length is twice the run length.
Resistance is resistivity × length ÷ area, and the drop is current × resistance.
The formula
R = ρ × 2L ÷ AΔV = I × R
Worked example
Using the default values shown in the calculator:
| Current (A) | 16 |
|---|---|
| One-way cable length (m) | 20 |
| Conductor area (mm²) | 2.5 |
| Conductor | Copper |
| Supply voltage (V) | 230 |
- Loop resistance = 1.7241 × 10⁻⁸ × 40 ÷ 2.5 × 10⁻⁶ = 0.2759 Ω.
- Voltage drop = 16 × 0.2759 = 4.414 V.
- That is 1.92% of 230 V, and 70.6 W is lost as heat.
Result:
| Loop resistance (Ω) | 0.2759 |
|---|---|
| Voltage drop (V) | 4.414 |
| Drop as % of supply | 1.92% |
| Power lost in cable (W) | 70.6 |
Frequently asked questions
What drop is acceptable?
Many wiring codes aim for 3–5% at the load. Check the rules that apply where you are.
How can I reduce the drop?
Use a thicker conductor, shorten the run or raise the supply voltage.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.