Voltage Drop Calculator
Calculate voltage drop over a wire run to determine if your circuit meets NEC recommendations.
Results
Visualization
How It Works
Voltage drop is the reduction in voltage as electrical current flows through a conductor. Excessive drop causes equipment to malfunction, lights to dim, and motors to overheat.
The Formula
Vdrop = (2 × K × I × D) / cmil
Drop % = Vdrop / Vsource × 100
Drop % = Vdrop / Vsource × 100
Variables
- K — Resistivity constant (12.9 copper, 21.2 aluminum)
- I — Current in amperes
- D — One-way distance in feet
- cmil — Circular mil area of the wire gauge
Example
A 20A load on 12 AWG copper wire at 120V over 100 ft: V_drop = (2 x 12.9 x 20 x 100) / 6530 = 7.9V. That is 6.6% drop, which exceeds the 3% recommendation.
Tips
- NEC recommends max 3% drop on branch circuits and 5% total (feeder + branch).
- Longer runs require larger wire to keep voltage drop within limits.
- Voltage drop affects both the hot and neutral conductors (hence the factor of 2).
- Higher voltage circuits (240V, 480V) have lower percentage drop for the same wire and distance.
- Voltage drop is particularly critical for motor circuits and sensitive electronics.