A worked example
A 120V circuit carrying 20A through 50 feet of 12 AWG copper drops about 3.95V — just over the NEC-recommended 3%.
Frequently asked questions
Why does voltage drop matter?
A significant voltage drop means the load receives less voltage than the supply provides — motors run slower and hotter, lights dim, and sensitive electronics may malfunction. Long wire runs and high current are the main culprits.
Why does the NEC recommend keeping drop under 3%?
It's a practical guideline balancing safety, efficiency, and cost — beyond 3%, equipment can underperform noticeably and energy waste from resistive heating in the wire becomes significant. It's a recommendation, not a hard code violation in most cases.
Why is copper preferred over aluminum?
Copper has lower resistivity than aluminum — for the same gauge, a copper wire drops less voltage. Aluminum wiring was common in residential construction in the 1960s-70s but fell out of favor partly due to connection reliability issues, though it's still widely used for large feeders and utility lines.
This calculator is for general guidance only — always consult the National Electrical Code and a licensed electrician for any actual wiring project.