How to use the Voltage Drop Calculator
The voltage drop calculator estimates the voltage lost along a copper wire.
Open the Voltage Drop Calculator →
What you need to enter
- Source voltage (V)
- Current (A)
- One-way length (ft)
- Wire gauge (AWG)
- Phase
The formula
Single-phase: drop = 2 × length × current × resistance per 1000 ft ÷ 1000. Three-phase uses √3 instead of 2.
Worked example
Using the default values in the calculator:
- Source voltage (V): 120
- Current (A): 20
- One-way length (ft): 100
- Wire gauge (AWG): 1 AWG
- Phase: Single-phase / DC
| Voltage drop | 0.5 V |
| Percent drop | 0.41% |
| Voltage at load | 119.5 V |
| Rating | Within the 3% guideline |
Tips for better results
- Keep drop under about 3%.
- Thicker wire lowers the drop.
Common mistakes to avoid
- Using round-trip length incorrectly.
- Ignoring local code.
Please note: Results are estimates for general use. For safety-critical work such as electrical or building projects, check with a qualified professional and local codes. See our Disclaimer.