Complete Guide to Voltage Drop Calculation
The Voltage Drop Calculator ensures electrical systems deliver full power by calculating loss over distance. It uses voltage, phase, current, length, material, and max drop (3–5%). Our free tool follows NEC standards for safe wiring.
What is Voltage Drop?
V = I × R, where R = ρ × L / A. Drop = (2 × I × L × ρ) / (CM × K) for single-phase. NEC limits: 3% feeder, 5% branch+feeder. Our Voltage Drop Calculator includes phase and material.
Why Calculate Voltage Drop?
Prevents:
- Dim Lights: Low voltage.
- Motor Damage: Under-voltage.
- Overheating: Undersized wire.
- Code Violation: NEC 210.19.
Pair with Wire Size and Load Calculation.
How to Calculate Voltage Drop
Formula:
- Single-Phase: VD = 2 × I × L × ρ / CM
- Three-Phase: VD = √3 × I × L × ρ / CM
ρ = resistivity (copper: 10.4, aluminum: 17.0), CM = circular mils.
Example: 240V, 20A, 100 ft, #12 Cu = 3.2V (1.3%).
Voltage Drop by Wire Size (100 ft, 20A, Copper)
| AWG | CM | Drop (V) | % |
|---|---|---|---|
| 14 | 4110 | 9.7 | 4.0% |
| 12 | 6530 | 6.1 | 2.5% |
| 10 | 10380 | 3.8 | 1.6% |
| 8 | 16510 | 2.4 | 1.0% |
Material & Phase Impact
| Factor | Copper | Aluminum | 3-Phase |
|---|---|---|---|
| Resistivity | 10.4 Ω-cmil/ft | 17.0 Ω-cmil/ft | √3 factor |
| Drop | Lower | +63% | -13% |
Key Factors
- Length: Double = double drop.
- Current: Higher load = higher drop.
- Temperature: Derate 4% per 10°C over 75°C.
- Conduit: PVC vs EMT.
Practical Applications
Use for:
- Subpanel: 100 ft feeder.
- EV Charger: 50A, 50 ft.
- Lighting: Long runs.
- Motors: 80% voltage min.
Tips to Reduce Voltage Drop
- Larger Wire: #10 vs #12.
- Shorter Run: Move panel.
- Higher Voltage: 240V vs 120V.
- Aluminum: Larger size.
Common Mistakes
- Round Trip: Forget ×2.
- Phase Factor: Ignore √3.
- Max Load: Use peak, not average.
- No Derating: Hot ambient.
Advanced Considerations
- Reactance: Long AC runs.
- PF Correction: Reduce I.
- Voltage Regulator: Critical loads.
- NEC Table 8: Exact ρ values.
Conclusion
Accurate voltage drop calculation ensures efficient, safe electrical systems. Our Voltage Drop Calculator delivers instant results with NEC compliance. Combine with Wire Size and Breaker Size tools for complete design. Power smart with 1000 Calculators.