How it’s calculated
Each conductor’s resistance is R = ρL/A. Current flows out and back, so a single-phase (or DC) circuit drops voltage across two conductor lengths. In a balanced three-phase circuit the line-to-line drop is √3 times the drop in one conductor. Resistivity is corrected for temperature with ρ = ρ₂₀[1 + α(T − 20 °C)].
Assumptions: resistance only (reactance and power factor neglected, which is reasonable for smaller conductors and short runs), solid-conductor cross-section, uniform temperature. Stranded and large conductors, conduit material and power factor change the real drop; NEC Chapter 9 Table 9 gives AC impedance values.
Example: 16 A at 120 V over 100 ft of 12 AWG copper at 20 °C. 12 AWG = 0.0808 in = 3.31 mm²; R per conductor = 1.68 × 10⁻⁸ × 30.48 ÷ 3.31 × 10⁻⁶ = 0.155 Ω, so Vd = 2 × 16 × 0.155 = 4.95 V (4.1%). At 75 °C the drop rises to about 6.0 V (5.0%).
The National Electrical Code’s informational notes suggest limiting voltage drop to about 3% for a branch circuit and 5% for feeder plus branch combined. Conductor ampacity is a separate, mandatory check.
Frequently asked questions
What is an acceptable voltage drop?
A widely used guideline (NEC informational notes) is 3% on a branch circuit and 5% total for feeder plus branch. These are recommendations for efficiency and performance, not ampacity limits.
Should I use one-way or round-trip length?
Enter the one-way distance from the source to the load. The calculator applies the factor of 2 (single-phase) or √3 (three-phase).
Why does aluminum drop more voltage?
Aluminum’s resistivity (2.65 × 10⁻⁸ Ω·m) is about 1.6 times copper’s (1.68 × 10⁻⁸ Ω·m), so an aluminum conductor needs to be about two AWG sizes larger for the same drop.
Can I use this to choose a wire size?
Only for the voltage-drop part. Wire size must also satisfy ampacity, temperature rating, overcurrent protection and local code. Have an electrician or engineer confirm.
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Sources
- University Physics Vol. 2, 9.3 Resistivity and Resistance (Table 9.1) — OpenStax
- Copper Wire Tables (NBS Handbook 100) — U.S. National Bureau of Standards (NIST)
- Electric Circuits II (Kuphaldt), 7.2 Three-phase Power Systems — Workforce LibreTexts
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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