FIELD GUIDE • UPDATED SEPTEMBER 2026
Voltage Drop and Wire Sizing
Estimate circuit loss and select a conductor that meets the design target.
Voltage drop increases with current, distance and conductor resistance. A long branch circuit can have acceptable ampacity but still deliver poor equipment performance because too much voltage is lost along the run.
Field method
For a simple two-wire circuit, use the one-way distance and account for the outgoing and return path. Three-phase calculations use a different relationship. Copper and aluminum have different resistance, and conductor temperature changes resistance.
Select a design target appropriate for the equipment and project. The NEC includes informational guidance commonly associated with branch circuits and feeders, while equipment specifications or project documents may impose stricter limits.
Common mistakes
Example: a 20-amp, 120-volt load 150 feet away may need a larger copper conductor than ampacity alone suggests. The WireWise finder compares common sizes until the estimated drop falls below the chosen percentage.
Common mistakes include entering round-trip distance when the calculator already doubles it, sizing only from breaker rating, ignoring motor starting conditions, and assuming voltage-drop recommendations override terminal or ampacity rules.
Start with the informational notes associated with NEC Articles 210 and 215, then verify conductor ampacity, terminals, equipment instructions and local design requirements.