Motor Starting Voltage Drop Calculator
Enter the source, transformer, feeder, and locked-rotor motor data to estimate the starting voltage at the motor terminals. The calculator also reverse-solves the one-way feeder length that reaches your selected minimum starting voltage.
Motor terminal voltage434.77
- Retained motor voltage
- 94.52%
- Initial motor voltage drop
- 5.48%
- Transformer secondary bus voltage
- 447.74
- Actual starting current
- 567.09
- Starting apparent power
- 427.04
- Available locked-rotor torque
- 89.33%
- Maximum feeder length
- 1,387.3
- Starting-voltage check
- Pass: motor terminal voltage is at or above the selected minimum.
Uses an initial locked-rotor equivalent circuit for a balanced three-phase start.
Feeder length is one way; conductor resistance and reactance are entered per 1,000 ft per phase and divided by parallel sets.
How to use this calculator
- Enter the nominal three-phase voltage and the utility short-circuit capacity, using zero MVA for an infinite source.
- Enter transformer kVA, impedance, and X/R ratio, or set transformer impedance to zero when no transformer is included.
- Enter the motor full-load amps, locked-rotor current multiplier, and locked-rotor power factor.
- Enter one-way feeder length, conductor impedance per 1,000 ft, and parallel conductor sets.
- Set the minimum acceptable starting voltage and compare it with the calculated retained voltage and maximum feeder length.
How the motor-start voltage is calculated
This calculator uses the initial locked-rotor equivalent circuit used for preliminary motor-starting studies. The utility source, transformer, feeder, and locked-rotor motor are represented as series impedances on the motor-voltage base. The result is the first-cycle steady equivalent estimate before the motor accelerates.
Let V be the nominal line-to-line voltage and E = V / sqrt(3) be the phase voltage. If utility short-circuit MVA is greater than zero, the source impedance magnitude is |Zs| = V^2 / Ssc. The entered X/R ratio splits that magnitude into Rs = |Zs| / sqrt(1 + q^2) and Xs = q x Rs. A zero short-circuit MVA entry is treated as an infinite source with zero impedance.
The transformer magnitude is |Zt| = zt x V^2 / St, where zt is transformer per-unit impedance and St is transformer volt-amperes. Feeder impedance is Zf = L x (r + jx) / (1000 x n), using the one-way length and the number of parallel conductor sets per phase.
The rated-voltage locked-rotor current is I0 = FLA x multiplier. Motor locked-rotor impedance magnitude is |Zm| = E / I0, and locked-rotor power factor p splits it into Zm = |Zm| x (p + j sqrt(1 - p^2)). At 100% power factor, the motor reactance term is zero.
The starting current is I = E / (Zs + Zt + Zf + Zm). Motor line voltage is Vm = sqrt(3) x |I x Zm|, retained voltage is Vm / V, and voltage drop is 1 - Vm / V. Starting kVA is sqrt(3) x Vm x |I| / 1000. Available locked-rotor torque is estimated as 100 x (Vm / V)^2 because induction-motor torque is approximately proportional to voltage squared near locked rotor.
What moves the answer most
The biggest movers are the locked-rotor current multiplier, transformer impedance, utility fault strength, and feeder impedance. A weak source or high-impedance transformer consumes voltage before the feeder is considered. A long feeder adds both resistance and reactance, so increasing conductor size or adding identical parallel sets can improve the retained voltage. A lower actual motor terminal voltage also lowers actual starting current, which is why measured inrush can be below the rated-voltage locked-rotor current.
What this calculator leaves out
The model is a balanced three-phase initial estimate. It does not simulate acceleration time, changing slip, driven-load torque, generator voltage recovery, contactor dropout, utility flicker limits, or the changing current profile of reduced-voltage starters. It also does not model soft starters, autotransformer taps, series reactors, VFDs, capacitor-assisted starts, or multiple motors starting together. Use manufacturer data and a full motor-starting study when the result is close to a facility or utility limit.
Worked example
For a 460 V motor on a 500 MVA utility source, a 1,000 kVA transformer at 5.75%, a 200 ft feeder with 0.08 ohm resistance and 0.05 ohm reactance per 1,000 ft, and a 100 A motor drawing 6 times FLA at 20% locked-rotor power factor, the calculator gives about 434.8 V at the motor terminals.
That is 94.5% retained voltage, or a 5.5% initial drop. Actual starting current is about 567 A, starting apparent power is about 427 kVA, and the available locked-rotor torque is about 89.3% of rated-voltage locked-rotor torque. With an 80% minimum starting-voltage criterion, the reverse solve gives a maximum one-way feeder length of about 1,387 ft for the entered conductor data.
Common questions
How much voltage drop is acceptable while a motor starts?
There is no single acceptable value for every installation. Many motors can produce useful starting torque at reduced voltage, but the controller, contactor, driven load, utility flicker limit, and facility process may impose stricter limits. Use the minimum starting-voltage field for the criterion that applies to the actual equipment.
Where can I find locked-rotor current and locked-rotor power factor?
Locked-rotor current may be on the motor nameplate as LRA, in manufacturer data, or estimated from a NEMA locked-rotor code when detailed data is not available. Locked-rotor power factor usually requires manufacturer data or a study assumption. A value near 20% is common for preliminary induction-motor estimates, but it should be replaced when project data exists.
Why is starting torque proportional to voltage squared?
Near locked rotor, induction-motor torque is approximately proportional to the square of applied terminal voltage. If retained voltage is 80%, available locked-rotor torque is roughly 0.80 squared, or 64% of rated-voltage locked-rotor torque. That is why a voltage sag that looks modest can matter to a high-inertia or high-breakaway load.
Should feeder length be entered one way or round trip?
Enter one-way length from the source or transformer bus to the motor. The model is a three-phase line-to-line equivalent and uses the per-phase conductor resistance and reactance per 1,000 ft. Do not double the length unless your conductor impedance data is already stated for a complete loop and you have converted it to match the field label.
How do parallel conductors change starting voltage drop?
Identical parallel sets divide the per-phase feeder impedance in this model. Two identical sets cut the entered conductor resistance and reactance in half; four sets cut them to one quarter. The sets must be the same length and construction for that assumption to be reasonable.
Can this model a soft starter, autotransformer starter, or VFD?
No. This is an across-the-line locked-rotor equivalent circuit. Soft starters, autotransformer starters, series reactors, and VFDs change voltage, current, harmonics, or control behavior over time, so they require equipment-specific modeling.