Voltage Drop Calculator
NEC-based. Enter your run details — get drop in volts & %, with 3%/5% guidance.
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How it works
VD = 2 × K × L × I ÷ CM (1φ) VD = √3 × K × L × I ÷ CM (3φ)
- K = 12.9 for copper, 21.2 for aluminum (NEC Chapter 9, Table 8).
- L = one-way length in feet. CM = circular mils of the conductor.
- NEC 210.19(A) informational note: keep branch-circuit drop ≤ 3%, and feeder + branch combined ≤ 5% for reasonable efficiency.
- If drop exceeds 3%, the tool suggests the next larger wire size.
- Note: for 2/0 AWG and larger on AC circuits, NEC Chapter 9 Table 9 refines the calculation with reactance (XL); this tool uses the industry-standard K-factor simplification (same approach as major manufacturer calculators).