Tool
Voltage drop calculator
Built for engineers who need a fast design check, a clearer conductor-sizing decision, and a result they can review without hunting through separate tables.
Built for practice
Design basis
Set the circuit objective and electrical conditions.
Start with the segment you are checking, then enter the known load, system voltage, and one-way conductor length.
Objective -
Default target -
Formula basis -
Conductor model
Choose the conductor for the run.
Preset sizes cover common building-wire checks for copper and aluminum conductors.
The calculator uses the selected insulation temperature rating for resistance and base ampacity checks.
Base ampacity -
Resistance @ rating -
Parallel path -
Method notes
Calculation notes.
NEC design guidance
Use 3% as a typical branch or feeder target and 5% for the combined path to the farthest outlet. Confirm the final requirement with the project code edition and AHJ.
Voltage-drop method
The result uses conductor resistance, one-way length, load current, material, insulation rating, and system type.
Base ampacity only
Preset ampacity values are base references before ambient correction, conductor-count adjustment, rooftop effects, and terminal limits.
Engineer review triggers
Recheck long motor feeders, nonlinear loads, and parallel runs when field conditions require manufacturer data or a full engineering review.
Reference basis
- NFPA 70 voltage-drop informational guidance for feeders and branch circuits
- Southwire Power Cable Manual conductor property and AC/DC voltage-drop equations
- Common NEC ampacity table values used for base conductor sizing checks