Voltage Drop Calculator
Free · works offline · no account · NEC 2023 tables
Size your conductors before the inspector or the equipment tells you they’re too small. OhmBase’s voltage drop calculator uses the standard K-factor circular-mil method electricians learn in apprenticeship, with NEC 2023 conductor data, and runs entirely offline on your phone.
Open the free calculator →How the math works
For single-phase circuits: VD = 2 × K × I × D ÷ CM, where K is 12.9 for copper (21.2 for aluminum), I is load amps, D is one-way distance in feet, and CM is the conductor’s circular-mil area. Three-phase circuits replace the 2 with 1.732 (√3). NEC 210.19 informational notes recommend keeping branch-circuit drop to 3% (5% max combined with the feeder).
Worked example
12 AWG copper · 20 A load · 100 ft one-way · 120 V single phase
- VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V
- Drop % = 7.90 ÷ 120 = 6.58%
- Verdict: exceeds the 5% guideline — upsize the wire or shorten the run.
FAQ
- What voltage drop is acceptable under the NEC?
- The NEC recommends (informational note, not a hard rule in most occupancies) no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. Some jurisdictions and specs make it mandatory — check your AHJ.
- Does wire length count one way or round trip?
- Enter the one-way distance. The formula’s ×2 (or ×1.732 for three phase) accounts for the return path.
- Copper vs aluminum — what changes?
- Only the K-factor: about 12.9 for copper and 21.2 for aluminum at 75°C. Aluminum drops more voltage at the same size, which is why it’s usually upsized.