Use our Voltage Drop Calculator to quickly estimate line losses for copper or aluminum conductors, see percent voltage drop, and predict the voltage available at your load. Ideal for electricians, engineers, solar designers, HVAC pros, and DIYers who need fast, defensible results.
What is voltage drop and why it matters
Voltage drop is the reduction in electrical potential as current flows through a conductor with resistance. Every wire has some resistance, and over distance this causes usable voltage at the load to fall. Excessive drop can make motors run hot, lights dim, electronics misbehave, and systems fail inspections. Many standards and best practices recommend limiting voltage drop to around 3% for branch circuits and 5% total for feeders plus branch circuits, though exact limits may vary by code and application.
How the Voltage Drop Calculator works
The calculator applies material-specific resistivity for copper or aluminum and adjusts for temperature. It accounts for system phase with the appropriate path factor: twice the one-way length for single-phase and the square-root-of-three factor for balanced three-phase circuits. By entering your load current, conductor size, system voltage, and run length, you can immediately see:
- Voltage drop in volts
- Percentage voltage drop relative to your system
- Estimated voltage at the load
Inputs you provide
- System phase: single-phase or three-phase
- Conductor material: copper or aluminum
- Run length: one-way distance in feet or meters
- Conductor size: cross-sectional area in mm²
- Load current in amperes
- System voltage in volts
- Optional conductor temperature in °C (defaults to 20°C)
Key assumptions and method
We calculate resistance from conductor cross-sectional area and material resistivity, then scale it by temperature using a standard temperature coefficient. For single-phase circuits, the current travels out and back, so the effective path is twice the one-way length. For balanced three-phase systems, the line-to-line drop is modeled using the ?3 factor. The resulting voltage drop is then compared to the system voltage to determine the percent drop and to estimate the terminal voltage at the load.
When to upsize conductors
If the percentage voltage drop exceeds your design target, consider one or more of the following:
- Increase the conductor cross-sectional area (larger wire size)
- Shorten the run or relocate equipment if possible
- Use copper instead of aluminum where appropriate
- Reduce load current by improving efficiency or diversity
- Verify connections and temperature ratings to minimize additional losses
Practical tips for accurate results
- Measure the one-way distance carefully; avoid underestimating conduit routing.
- Use the actual operating temperature if known; higher temperatures increase resistance.
- Convert AWG to mm² accurately; small errors in area can meaningfully affect drop.
- Check local code guidance for acceptable voltage drop thresholds.
- For long feeders or sensitive equipment, aim for lower voltage drop than the maximum allowed.
Example use case
Suppose you have a 230 V single-phase load drawing 18 A over a 40 m run of copper cable sized at 6 mm². Enter the values and the calculator will compute the conductor resistance based on copper resistivity at your chosen temperature, then apply the 2× path factor for single-phase. You’ll see the voltage lost in the cable, the percent drop, and the expected voltage at the equipment terminals. If the percent drop is higher than your target (e.g., 3%), try increasing to 10 mm² and compare the results.
Why use this Voltage Drop Calculator
Our tool is designed for clarity and speed while following widely accepted electrical engineering principles. By letting you switch between copper and aluminum, pick phase, and enter precise geometry, you can iterate designs in seconds. Use it during estimating, design reviews, field troubleshooting, or when validating contractor proposals. It’s a practical way to reduce callbacks, improve performance, and meet project specifications.