Voltage Drop Calculator
Enter a circuit's current, cable length and cross-section, and material to see the voltage drop in volts and as a percentage.
Voltage Drop Calculator
Results
- Voltage drop
- 8.60 V
- Voltage drop (%)
- 3.74%
- Voltage at load
- 221.40 V
- One-way resistance
- 0.4300 Ω
Above the commonly recommended 3% limit for a single circuit run.
How to calculate voltage drop in a cable run
Enter a circuit's current, cable length and cross-section, and material to see the voltage drop in volts and as a percentage.
The formula
- Formula
V_drop = path_factor × (ρ × L / A) × I
Single-phase vs. three-phase
Single-phase circuits use a round-trip path factor of 2 (current flows out and back through the same total length of conductor). Three-phase circuits use √3 instead, reflecting how the line-to-line voltage relates to the per-phase current in a balanced load.
Why voltage drop matters
Too much resistance over a long or thin cable run wastes energy as heat and can starve equipment of the voltage it needs to run correctly. A commonly cited guideline (matching most electrical codes' recommendation) is to keep drop under 3% for a single circuit run, 5% total from source to the farthest load.
Privacy
Runs locally in your browser. No data is sent to the server.
FAQ
What's an acceptable voltage drop?
3% or less for a single branch circuit is a common guideline; the calculator flags anything above that.
Copper or aluminum — which has less drop?
Copper, for the same cross-section — it has roughly 60% of aluminum's resistivity, so a copper run of the same gauge drops noticeably less voltage.
Is the length I enter one-way or round-trip?
Enter the one-way distance from source to load; the calculator applies the round-trip (or three-phase) factor itself.