When to Use This Calculator
Use this calculator for a quick resistance-based voltage-drop estimate when current, one-way length, source voltage, and conductor resistance are known. It is useful for comparing circuit lengths, conductor sizes, and operating current assumptions.
Enter Values
Formula Summary
DC/single-phase drop = 2 × L × I × R ÷ 1000
Three-phase drop = √3 × L × I × R ÷ 1000
L is the one-way length in feet. R is conductor resistance in ohms per 1,000 feet. This simplified calculation does not include reactance.
Practical Examples
120 V branch circuit
120 V single-phase, 20 A, 100 ft, 0.5 Ω/1,000 ft
Drop = 2 × 100 × 20 × 0.5 ÷ 1000
Drop = 2.00 V (1.67%)
480 V three-phase feeder
480 V three-phase, 100 A, 150 ft, 0.08 Ω/1,000 ft
Drop = 1.732 × 150 × 100 × 0.08 ÷ 1000
Drop = 2.08 V (0.43%)
Finding Conductor Resistance
Use resistance in ohms per 1,000 feet for the conductor material, size, and temperature being evaluated. Resistance rises as conductor temperature rises. Manufacturer data or an applicable engineering table is preferable to a generic room-temperature value.
Enter the physical one-way route length. The calculator applies the return-path factor for DC and single-phase circuits.
Calculation Limits
This simplified method uses resistance only. AC feeders can also have inductive reactance, and the result can differ with conductor spacing, raceway type, power factor, harmonics, and parallel conductors.
Voltage-drop recommendations do not replace ampacity, overcurrent protection, terminal temperature, or code-compliance checks.
Common Mistakes
- Entering round-trip length instead of one-way length for this calculator.
- Using a room-temperature resistance value when conductor temperature is much higher.
- Ignoring reactance, power factor, harmonics, or conductor spacing on larger AC feeders.
- Treating voltage-drop guidance as a substitute for ampacity or overcurrent-protection checks.
Reference Notes
The calculation uses common resistance-based voltage-drop formulas for DC, single-phase, and balanced three-phase circuits. Use conductor resistance from manufacturer data or an applicable electrical table, and verify installation requirements with the adopted electrical code.