Voltage Drop Calculator
Calculate voltage drop for single-phase and three-phase circuits using the NEC K-method. Enter the load current, one-way length, conductor size, and material to see voltage drop in volts and percentage. NEC FPNs in 210.19(A) and 215.2(A) recommend keeping branch-circuit drop under 3% and the combined feeder + branch drop under 5%.
Voltage Drop
7.90 V
Drop Percentage
6.58%
Voltage at Load
112.10 V
✗ Exceeds 5% NEC recommendation — increase conductor size
How the calculation works
The K-method approximates voltage drop using:
Single-phase: VD = (2 × K × I × L) / CM Three-phase: VD = (1.732 × K × I × L) / CM
K = 12.9 for copper, 21.2 for aluminum (typical resistivity at operating temperature). I = load current (A). L = one-way length (ft). CM = conductor cross-sectional area (circular mils, from NEC Chapter 9 Table 8).
Test your knowledge
Practice voltage drop and conductor sizing with these tests:
Educational tool only. Verify against the current NEC. Not a substitute for a licensed engineer.