How to Calculate Wire Gauge by Voltage Drop
Selecting the correct wire gauge requires satisfying two conditions: current-carrying capacity (ampacity) and allowable voltage drop. This calculator computes the minimum copper cross-section based on voltage drop, then recommends the next standard size up.
Formula (single-phase / DC)
Current (A) = Power ÷ Voltage / Required cross-section (mm²) = 2 × Length × Current × 0.0175 ÷ Allowable drop (V)
NEC recommends a maximum 3% voltage drop for branch circuits. The run length entered here is one-way; the calculator automatically doubles it for the round trip. This tool is for reference only — always have a licensed electrician verify your wire sizing before installation.
Frequently Asked Questions
Enter the one-way distance. The calculator multiplies by 2 internally to account for the full return path.
It's a theoretical reference. Factors like conduit fill, ambient temperature, and load characteristics can affect the actual wire selection, so always confirm with a qualified electrician.