Enter Values
Formula
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.
Example
A 120 V single-phase circuit carrying 20 A for 100 ft with 0.5 ohms/1,000 ft resistance drops 2.00 V (1.67%).
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.