Voltage Drop Calculator
Calculate voltage drop from wire size, current, and distance
Frequently Asked Questions
Single-phase: VD = (2 × K × I × D) ÷ CM. Three-phase: VD = (1.732 × K × I × D) ÷ CM. Where K is the conductor material constant (12.9 for copper, 21.2 for aluminum, at 75°C), I is the current in amps, D is the one-way circuit distance in feet, and CM is the conductor's circular mil area (from NEC Chapter 9, Table 8).
The NEC recommends keeping voltage drop to 3% or less for branch circuits, and 5% total (combined feeder and branch circuit) for reasonable equipment efficiency — these are recommendations for good design, not hard code violations, but exceeding them can cause dimming, motor overheating, or reduced equipment lifespan.
Because the formula's factor (2 for single-phase, √3 for three-phase) already accounts for the round-trip current path — single-phase current must travel out and back (hence ×2), while three-phase's geometry uses the √3 factor instead. Enter only the one-way distance from source to load.
Use whichever matches your actual conductor material — copper (K=12.9) has meaningfully lower resistance than aluminum (K=21.2) of the same gauge, so an aluminum run needs a larger wire size to hit the same voltage drop target.