Definition
Voltage drop is the reduction in electrical potential as current flows through a conductor due to the conductor's resistance. Excessive voltage drop causes inefficient operation, overheating, and equipment malfunction. The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for feeders and 5% total (feeder + branch circuit). Voltage drop increases with distance, current load, and decreases with larger wire gauge (smaller AWG number).
Formula
For single-phase circuits: VD = (2 x K x I x D) / CM. For three-phase: VD = (1.732 x K x I x D) / CM. K = resistivity (12.9 for copper, 21.2 for aluminum), I = current in amps, D = distance one way in feet, CM = circular mils of conductor. Keep voltage drop under 3% for branch circuits and 5% total for optimal performance.
Example
Voltage Drop Calculation Example
A 240V, 20-amp load at 150 feet with #10 copper wire (K=12.9, CM=10,380): VD = (2 x 12.9 x 20 x 150) / 10,380 = 7.5 volts. Percentage drop = 7.5 / 240 = 3.1%. This slightly exceeds the 3% recommendation-using #8 wire (CM=16,510) reduces the drop to 1.9%, meeting code requirements.