DC and Simple Single-Phase Estimate
Vdrop = 2 × L × I × R
The factor of two includes outgoing and return paths when L is one-way length. Units must be consistent.
Balanced Three-Phase Estimate
Vdrop = √3 × L × I × R
This resistance-only form is useful for estimates but does not include phase angle or conductor reactance.
AC Impedance
At AC, conductor impedance includes resistance and reactance. A more detailed calculation uses load power factor and the resistive and reactive components of voltage drop. Raceway material, conductor spacing, parallel conductors, and frequency affect reactance.
Low-Voltage DC Sensitivity
A fixed voltage loss is a larger percentage of a low-voltage source. A 2 V drop is only 1.67% of 120 V but 8.33% of 24 V. High-current battery and solar circuits can therefore require large conductors even over modest distances.
Voltage-drop checks do not replace ampacity, protection, temperature, terminal, and installation-rule checks.