Planning guide / motor-starting
Introduction
The motor-starting calculator estimates locked-rotor apparent power, a starter-adjusted starting demand, three-phase current, and an optional comparison against a generator rating. Use it during an early load review when a motor is one of the large step loads on a standby source, or when comparing starter assumptions before a detailed transient study is available. It makes the code midpoint, starter factor, voltage, and optional generator size visible so the screening estimate can be followed.
Enter motor horsepower, the selected NEMA locked-rotor code, the starter type, and line-to-line voltage. If you want the optional step-load percentage and screening band, enter the generator nameplate kVA in the last field. A blank generator field is valid: the calculator still returns the motor kVA and current without trying to classify the load against a source.
Treat the selected starter factor as a planning input supplied by this calculator, not a manufacturer test result. Actual starting current and voltage dip depend on the motor, starter settings, driven load, source impedance, concurrent loads, alternator response, and the starting sequence. Use the estimate to identify a scenario for further review, not to approve a generator or motor installation.
Method / Calculation
How it works
The first step converts motor horsepower to a locked-rotor kVA estimate. The calculator looks up the selected NEMA MG 1 code midpoint in kVA per horsepower and multiplies it by the entered horsepower. The code midpoint is an input to this simplified estimate; it is not a substitute for the motor nameplate or locked-rotor data from the equipment documentation.
Next, the tool multiplies locked-rotor kVA by the planning factor associated with the chosen starter. Across-the-line uses 1.00; wye-delta uses 0.33; a soft starter uses 0.40; and a VFD uses 0.15. The resulting effective starting kVA is then used for current and, when supplied, the generator comparison. These factors are deliberately visible because changing them changes both outputs.
For a balanced three-phase load, current follows apparent power divided by the line-to-line voltage and the square root of three. The calculator converts kVA to VA by multiplying by 1,000 before dividing. The optional step-load percentage divides the effective starting kVA by the entered generator kVA and multiplies by 100. The displayed risk bands are the calculator’s screening labels only.
The math does not solve a motor circuit or transient response. It uses a single entered voltage and a single motor start, and it does not subtract the motor’s running demand from an existing generator load. Keep those boundaries in mind when comparing this isolated step-load percentage with a facility load schedule.
Locked-rotor kVA = motor HP × selected NEMA MG 1 midpoint (kVA/HP)
Effective starting kVA = locked-rotor kVA × starter planning factor
Balanced three-phase starting amps = effective starting kVA × 1,000 ÷ (√3 × line-to-line volts)
Optional step load = effective starting kVA ÷ generator kVA × 100
- Record the motor horsepower from the motor schedule or nameplate and select the listed NEMA code being evaluated. Keep the source for that code visible in the calculation notes.
- Choose the starter type that matches the scenario. If settings or a manufacturer starting-current curve are available, use them in the final engineering review rather than treating the preset factor as measured behavior.
- Enter the system line-to-line voltage. The current formula is three-phase and assumes a balanced load; it is not the correct conversion for a single-phase motor.
- Multiply horsepower by the selected kVA-per-horsepower midpoint to obtain the calculator’s locked-rotor kVA estimate.
- Apply the starter factor, then convert effective kVA to amps with √3 and voltage. Preserve the intermediate locked-rotor and effective kVA values when comparing options.
- If the generator field is filled, divide effective starting kVA by generator kVA and read the percentage as an isolated ratio. Review any result with the full operating load and transient requirements.
The page cites NEMA MG 1 as the basis for the displayed locked-rotor code midpoint lookup. The starter factors and risk-band cutoffs are application planning assumptions. Confirm the motor data, starter behavior, generator ratings, allowable dip, and sequence with the equipment manufacturers and responsible engineer.
A useful record identifies the motor, the code midpoint, starter type and factor, line voltage, and generator rating if entered. Re-run the estimate when any of those scenario details changes, and do not combine results from different motors as if they represented one simultaneous start.
When several motors are present, evaluate the actual starting order rather than summing every locked-rotor value by default. A motor that starts after another load has stabilized presents a different operating condition from simultaneous starts, while the already-running equipment still contributes to the generator load. Keep the running load and the new starting increment separate in the scenario notes. The calculator reports one motor’s selected step and does not model acceleration, transfer timing, protective-device operation, or alternator recovery. Use it to describe the question for a more complete review, then apply the actual one-line diagram, load schedule, and equipment data.
Calculation trace / Illustrative inputs
Fully worked example
Illustrative example using the calculator’s starting values: a 50 HP motor, NEMA code G, across-the-line starter, 480 V three-phase line voltage, and an optional 500 kVA generator comparison. These are sample inputs, not site equipment data.
- The page’s code G lookup is 6.68 kVA/HP. Locked-rotor estimate = 50 HP × 6.68 kVA/HP = 334 kVA.
- Across-the-line has a planning factor of 1.00. Effective starting demand = 334 kVA × 1.00 = 334 kVA.
- Starting current = 334 kVA × 1,000 ÷ (√3 × 480 V) = about 401.7 A, shown as approximately 401.7 A before display rounding.
- With 500 kVA entered, step load = 334 kVA ÷ 500 kVA × 100 = 66.8%. The tool places a value above 50% in its high-risk screening band; that label is not a transient-performance conclusion.
Illustrative calculator result: 334 kVA locked-rotor demand, 334 kVA effective starting demand, about 401.7 A, and a 66.8% optional step-load ratio.
These values follow the on-page code midpoint and starter factor. They omit concurrent load, motor running demand, source impedance, acceleration time, voltage and frequency recovery, and protection behavior. Planning estimate only; obtain manufacturer data and licensed engineering review.
Review / Input checks
Common mistakes
- Entering a different motor rating basis. Use horsepower for the motor being evaluated, not a generator rating, brake horsepower from an unrelated operating point, or a facility total. If the motor schedule uses another unit or has changed, reconcile it before comparing scenarios.
- Treating the NEMA code midpoint as a motor-specific measurement. The selected code maps to a lookup value in the calculator. It does not replace actual nameplate, manufacturer, or test information. Record the selected code and verify the appropriate data for the actual motor.
- Applying a starter factor as a guarantee. The preset factor is a planning multiplier, not a promise about current reduction. Starter configuration, settings, motor behavior, and load torque matter. Use supplier curves or engineering data when the starting event must be evaluated in detail.
- Using phase-to-neutral voltage. The current conversion uses balanced three-phase line-to-line voltage. Enter the system voltage between phases. A phase voltage or single-phase circuit value changes the denominator and makes the estimate inconsistent.
- Reading step-load bands as generator approval. The percentage compares one effective starting kVA value with generator kVA. It does not include the running facility load, alternator transient capability, allowed dip, sequence, or protection coordination. Review the full scenario.
Questions / Planning use
Frequently asked questions
What does locked-rotor kVA represent here?
It is a planning estimate calculated from motor horsepower and the selected NEMA MG 1 code midpoint in kVA per horsepower. It is not a measured motor result. Use actual equipment data for a design or detailed generator study.
Why does the starter choice change the effective starting kVA?
The calculator applies a fixed planning factor to the locked-rotor estimate for the selected starter type. The factor is an explicit application assumption and does not model settings, acceleration, motor design, or manufacturer performance.
Can I use the current result for a single-phase motor?
No. The displayed current formula assumes a balanced three-phase load and uses line-to-line voltage with √3. A single-phase motor needs a different calculation and appropriate motor data.
What does the generator step-load percentage tell me?
It compares the selected motor’s effective starting kVA with the generator kVA entered in the optional field. The percent and band are screening outputs, not a prediction of voltage dip or a statement that the generator can start the motor.
Which inputs should I verify before using this estimate?
Verify motor horsepower and locked-rotor information, the NEMA code basis, actual starter characteristics, line voltage, generator rating, simultaneous loads, and starting sequence. Use manufacturer data and qualified engineering review for equipment decisions.
Continue / Related tools
Related calculators
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Compare a running-load estimate and a separate motor-start allowance when framing generator capacity scenarios.
- Generator fuel burn and runtime calculator
Estimate fuel use after a generator operating load and target runtime have been established.
- kW, kVA, and amps calculator
Convert steady-state electrical quantities when checking the motor’s operating load data.