Voltage Drop Calculator
Computes volts lost and percent drop from length, current, and AWG resistance for copper or aluminum.
Limitation: Educational estimate only.
How to Use the Voltage Drop Calculator
Enter the fields below, then review the educational estimate.
Enter source voltage and load amps.
3% planning target
Many educational guides aim for about 3% drop on branch circuits—confirm your project’s actual requirements.
What This Calculator Estimates
Uses fixed Ω/kft AWG tables and the classic 2-wire or √3 three-phase multiplier.
Worked Examples
12 AWG copper, 50 ft, 10 A, 120 V
Inputs
Result
VD = 2·50·10·1.588 / 1000.
Safety first
De-energize circuits before measuring. When in doubt, stop and consult a licensed electrician.
Frequently Asked Questions
Still have questions about this calculation?
Try the CalculatorFormulas & Assumptions
Transparent educational arithmetic. Not NEC design software or a wiring permit.
Formula
Single-phase / DC
VD = (2 · L · I · R) / 1000
Three-phase
VD = (√3 · L · I · R) / 1000
Percent
% = 100 · VD / Vsource
Scientific Background
Resistance values are educational handbook-style Ω/kft at ~25 °C. Temperature, terminations, and parallel conductors are not modeled. Tables are not a substitute for NEC Chapter 9.
Sources & review
Voltage Drop Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.
Last reviewed
July 22, 2026
Reference
physics.nist.gov
https://physics.nist.gov/cuu/Constants/index.htmlReference
nist.gov
https://www.nist.gov/pml/owm/wire-gauge-and-current-carrying-capacityFree Calculator Hub methodology
Free Calculator Hub
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See our methodology and corrections policy.