Motor StartingVoltage Drop Calculator
Estimate running and starting voltage drop on single-phase and three-phase motor circuits using NEC Table 9 style AC impedance assumptions.
This tool is a planning check for motor feeders and branch circuits. It compares normal-load and start-up voltage drop using line voltage, full-load current, conductor impedance, and a locked-rotor current multiplier. It does not replace motor nameplate data, utility source studies, short-circuit analysis, or manufacturer starting limitations.
Motor Circuit Inputs
running current, starting multiplier, and conductor impedance
Typical across-the-line starts often land around 5x to 7x full-load current with a lower power factor than the running condition. Soft starters, VFDs, reduced-voltage starters, and motor-specific curves can materially reduce this result.
Voltage-Drop Results
running versus motor-starting condition
The running result uses the familiar AC drop relationship with effective impedance derived from resistance, reactance, and load power factor.
The starting result multiplies full-load current by the selected inrush factor and applies a lower starting power factor. That is useful for screening nuisance low-voltage starts, dimming complaints, and weak-feeder layouts.
Practical targets vary by motor and process. Many designers try to keep normal operation near 3% and starting events comfortably below the point where contactors chatter or acceleration becomes unacceptable.
AC Voltage Drop Calculator
Run a broader feeder or branch-circuit study with AC impedance and power-factor assumptions.
DC Voltage Drop Calculator
Use the DC workflow for batteries, controls, and low-voltage direct-current circuits.
Wire Size Calculator
Move from a motor planning check to a conductor recommendation based on load and drop target.
Wire Tables
Compare common copper and aluminum conductor sizes before you adjust the motor feeder design.
NEC Standards
Review the code context before finalizing motor branch-circuit or feeder conductor decisions.