How to Calculate Voltage Drop
What is Voltage Drop?
Voltage Drop Calculator helps contractors and tradespeople accurately size, spec, and estimate materials and systems.
Formula
VD = (2 × K × I × L) / CM where K=12.9(Cu), I=amps, L=feet, CM=circular mils
Step-by-Step Guide
- 1Enter circuit amps, one-way distance, and wire gauge circular mils
- 2Apply the formula using your project measurements
- 3Add required safety factors or waste allowances per code
Worked Examples
Input
15A, 100 ft, 12 AWG (6530 CM)
Result
VD = (2×12.9×15×100)/6530 = 5.94V = 4.95% (max 3% recommended)
Common Mistakes to Avoid
- ✕Not applying code-required safety factors or derating
- ✕Forgetting to account for waste and cut losses in material estimates
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