Voltage Drop Calculator & Wire Size
Calculate voltage drop, load voltage and cable loss for copper or aluminium wire in AWG or mm², and find the minimum wire gauge for a 3 % limit.
🔒 Runs entirely in your browser — nothing is uploadedHow voltage drop is calculated
Every wire has a small resistance, so some voltage is lost along the cable whenever current flows. The resistance of a conductor depends on its material, cross-sectional area, length and temperature: R = ρ × L ÷ A, where ρ is the resistivity. At 20 °C copper has a resistivity of 0.017241 Ω·mm²/m and aluminium about 0.0283 Ω·mm²/m, and both rise by roughly 0.4 % for every degree Celsius. The calculator converts AWG and kcmil sizes into square millimetres, applies the temperature correction and finds the resistance per kilometre.
For DC and single-phase AC circuits current flows out on one conductor and back on another, so the drop is Vd = 2 × L × R × I, where L is the one-way length. In a balanced three-phase circuit the return currents largely cancel, giving Vd = √3 × L × R × I. Dividing the drop by the source voltage gives the percentage, and subtracting it from the source voltage gives the voltage that actually reaches the load. The power lost as heat in the cable is I² × R summed over the current-carrying conductors.
Choosing a wire gauge
Too much voltage drop makes motors run hot, lights dim and electronics misbehave, and it wastes energy as heat. Long runs — to a garage, a well pump, an EV charger or a solar array — are where it matters most. Set the maximum drop you accept, typically 3 % for a branch circuit, and the calculator shows the smallest AWG/kcmil or metric size that meets it. The table lists every size in the same standard so you can compare the drop, resistance and margin at a glance, and copy it as CSV or Markdown.
Remember that voltage drop is only one part of sizing a cable. The conductor must also carry the current safely (ampacity), match the protective device and suit the installation method and ambient temperature required by your local electrical code. Low-voltage DC systems such as 12 V or 24 V solar and RV wiring are especially sensitive, because a single volt of drop is already several percent. All calculations run in your browser and nothing is uploaded.
How to use
- Describe the circuitChoose AC single-phase, AC three-phase or DC, the conductor material and the wire size in AWG, kcmil or mm².
- Enter the loadType the source voltage, the load current in amps and the one-way cable length in feet or metres.
- Set the limitsAdjust the conductor temperature and the maximum voltage drop you accept — 3 % is a common target.
- Check the result and wire gaugeRead the drop in volts and percent, the voltage at the load, and the smallest wire size that stays within your limit.