Calculate the voltage drop over a wire run.
Calculated instantly in your browser.
How do you calculate voltage drop over a wire run?
Voltage drop = factor × current × wire resistance, where the factor is 2 for single-phase and √3 for three-phase, and resistance = resistivity × length ÷ area. Long, thin runs raise resistance and waste energy as heat. For example, 20 A over 30 m of AWG 12 copper, single-phase, drops about 6.25 V — roughly 5% on a 120 V supply.
Understanding your result
Long runs and thin wire increase resistance, which drops voltage and wastes energy as heat. Keeping the drop under about 3% for branch circuits ensures equipment gets enough voltage to work properly.
Formula and method
Voltage drop = factor × current × wire resistance, where the factor is 2 for single-phase and √3 for three-phase, and resistance = resistivity × length ÷ area.
Worked example
20 A over 30 m of AWG 12 copper, single-phase, drops about 6.25 V — roughly 5% on a 120 V supply.
How to use this tool
- Enter the current and one-way length.
- Choose the AWG, material and phase.
- Optionally add the source voltage for a percentage.
Common mistakes to avoid
- Entering the round-trip length instead of one-way.
- Ignoring drop on long, thin or aluminium runs.
About the Voltage Drop Calculator
The Voltage Drop Calculator works out how much voltage is lost along a wire run from the current, length, wire size, material and whether the system is single or three-phase.
Who should use this tool
Electricians, solar installers and electronics builders.
Benefits
- Voltage drop in volts and percent.
- Single and three-phase support.
- Copper or aluminium conductors.
- Flags the 3% guideline.
Practical use cases
- Sizing wire for a long run.
- Checking drop on a solar or battery cable.
- Avoiding under-voltage at the load.
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Frequently asked questions
What is an acceptable voltage drop?
A common guideline is under 3% for a branch circuit and under 5% total from source to load.
Do I enter one-way or round-trip length?
One-way length. The calculator accounts for the return path using the single- or three-phase factor.