Voltage Drop Calculator
Calculate voltage drop across a cable run from length, current and conductor size to check it meets AS/NZS or NEC limits, free in your browser.
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Minimum 0
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the current, one-way cable length and conductor resistance (mΩ/m).
- Enter the supply voltage for the percentage.
How it works
Drop = 2 × length × current × resistance (the 2 accounts for the return path). Percentage = drop ÷ supply voltage.
Worked example
20 A, 30 m, 7.41 mΩ/m, 230 V. ≈ 8.9 V drop (3.87%).
Common mistakes
- Using the round-trip length — enter the one-way run; the formula doubles it.
- Guessing resistance — use the cable's rated mΩ/m for its size and material.
Frequently asked questions
How much voltage drop is acceptable?
A common guide is 3% for the branch circuit and 5% total including the main; check the wiring rules that apply where you are.
How do I reduce voltage drop?
Use a larger conductor, shorten the run, or raise the supply voltage — drop rises with current and length and falls with cross-section.
What's an acceptable voltage drop?
Many standards aim for 5% or less total. This is a guide only and not a substitute for a licensed electrician's design.
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Cite this tool
Zerdly. (2026). Voltage Drop Calculator. Retrieved from https://www.zerdly.com/tools/voltage-drop-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.