Motor Voltage Drop Estimator
Estimate motor terminal voltage drop during running and starting conditions from project-specific conductor impedance and motor data.
Circuit and conductor data
Use conductor or cable data for the operating temperature and construction.
Use manufacturer or engineering data for the actual conductor arrangement. Enter zero only when the chosen method treats reactance as negligible.
Running and starting values
Use motor or manufacturer starting data for the selected starting method.
Motor terminal voltage estimate
About this estimator
Enter one-way circuit length, source voltage, conductor resistance and reactance, and motor running and starting current and power factor. The estimator calculates voltage drop and estimated motor terminal voltage for each condition.
Use conductor impedance and motor data for the actual installation. The default values are examples only; replace them with project data.
Method and assumptions
The resistance and reactance components are combined using the entered power factor: R × PF + X × √(1 − PF²). The estimator multiplies this value by current, one-way conductor length, and a phase factor of 2 for single-phase or √3 for balanced three-phase circuits.
It treats source voltage, current, power factor, and conductor impedance as fixed for each condition. The calculation does not include upstream transformer or generator impedance or represent a time-varying motor-start event.
Interpret the result
Compare running and starting estimates with motor and equipment voltage requirements and the project design criteria. A calculated starting voltage does not establish that the motor will accelerate or that the supply will recover without affecting other loads.
FAQs
Where should conductor resistance and reactance come from?
Use manufacturer or engineering data for the installed conductor construction, arrangement, and operating temperature. Resistance and reactance values must use ohms per 1,000 feet.
What starting current should I enter?
Use motor or manufacturer data for the actual motor and starting method. Do not infer starting current from horsepower alone.
Does starting load voltage predict whether the motor will start?
No. This is a steady-state impedance estimate at the entered current and power factor. It does not model acceleration, changing motor current, source impedance, or voltage recovery.
Does this choose conductor size or approve a circuit?
No. It does not check ampacity, overcurrent protection, wiring method, motor starting requirements, equipment limits, or locally adopted electrical rules.
Sources
- Use motor manufacturer data for running and starting current, power factor, and starting method.
- Use cable manufacturer or engineering data for AC resistance and reactance at the installed conductor arrangement and operating temperature.
- Verify motor voltage requirements, source impedance, starting performance, and local electrical requirements separately.