Text & Web Utilities
Voltage Drop Calculator
Estimate voltage drop over a wire run.
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Enter details
Result
Voltage drop
4.764 V
Round-trip conductor length
Drop at 120 V
3.97%
About the Voltage Drop Calculator
Long wire runs lose voltage to resistance, which can dim lights and stress equipment. Enter the current, one-way run length, and the wire's resistance per 1000 ft and the calculator returns the voltage drop and the percentage drop at 120 V. It helps you size conductors correctly to stay within code limits.
How to use the Voltage Drop Calculator
- 1Enter your current, wire length, and conductor resistance.
- 2Read the result instantly — it recalculates as you type.
- 3Adjust the numbers to model different scenarios.
Frequently asked questions
How is voltage drop calculated?
Drop = 2 × length ÷ 1000 × resistance-per-1000ft × current (the 2 accounts for the round-trip conductor). This tool applies it for you.
What's an acceptable voltage drop?
Codes generally recommend keeping it under 3% on a branch circuit (5% total). Larger wire lowers the drop.