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 or check a custom entry.
Preset sizes cover the most common building-wire checks. Use the custom option whenever your project needs a specific ampacity or conductor area.
The quick check treats the run as resistive at the selected conductor temperature. The advanced AC check adds the entered reactance and power factor for a closer feeder review.
Base ampacity -
Resistance @ temp -
Parallel path -
Method notes
What the calculator is doing in the background.
NEC design guidance
The page uses the common NEC design guidance of 3% maximum voltage drop on a feeder or branch-circuit segment and 5% maximum on the full feeder-plus-branch path to the farthest outlet. Treat these as design objectives, then verify the adopted code edition and AHJ interpretation for the project.
Two calculation paths
The quick check uses conductor resistance derived from circular-mil area and material resistivity at the entered temperature. The advanced AC check adds a reactance term and power factor for a closer approximation on long or more sensitive AC runs.
Base ampacity only
Preset ampacity values are intended as base conductor checks before ambient correction, conductor-count adjustment, rooftop effects, or terminal limits are finalized. Use the custom ampacity override when project conditions require a specific approved value.
Engineer review triggers
Recheck long motor feeders, harmonic-rich loads, nonlinear equipment, and parallel runs with manufacturer impedance data whenever exact field conditions matter more than a fast design-screening pass.
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