How the voltage drop calculator works
Enter actual current, one-way distance, supply voltage and conductor material/size. The model applies the return-path multiplier and a listed 75°C resistance. Compare the unrounded percentage to your target, then try supported sizes from smallest to largest.
Single-phase / two-wire DC: ΔV = 2 × I × R × L / 1,000. Balanced three-phase: ΔV = √3 × I × R × L / 1,000. Drop % = 100 × ΔV / supply voltage.
Worked example
A 120 V single-phase circuit carries 20 A over 100 ft one way on #12 uncoated copper. R = 1.98 Ω/1,000 ft. ΔV = 2 × 20 × 1.98 × 100 / 1,000 = 7.92 V. Drop = 6.60%; receiving voltage = 112.08 V. For a 3% target the first listed size meeting this resistance model is #8 copper: 3.112 V, or 2.59%. Check that size’s ampacity separately.
NEC references
NEC 2023: 210.19; Chapter 9 Table 8. These are reference identifiers, not reproduced code text. Read our methodology and verification limits.
Limitations
No parallel conductors, mixed return paths, plated/solid substitutions, unbalanced three-phase or AC impedance design. The model assumes power factor 1 and zero reactance. 3% branch and 5% combined feeder/branch values are efficiency guidance, not universal pass/fail rules. A loss equal to or greater than the supply is rejected.
For estimation only. Results compare loads under the selected NEC method; they do not guarantee permit approval or code compliance. Verify with your local adopted code and a licensed electrician.
FAQ
Is length one way or round trip?
Enter one-way distance. The model includes the return conductor for single-phase and two-wire DC.
What does the three-phase voltage mean?
Use line-to-line supply voltage for a balanced three-phase circuit.
Does the suggested wire size meet ampacity?
It meets only the selected voltage-drop target under this model. Use the wire-size calculator and check installation requirements.
Why might another tool differ?
A tool that uses power factor, reactance, steel raceway or another temperature can produce a different result. Compare models before comparing numbers.
Does zero current give zero drop?
Yes. It also makes the smallest listed size pass the voltage-drop target, but that does not establish a safe conductor for your circuit.
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