Quick Answer
Quick answer: Use this page as a motor starting-current workflow: collect motor FLA, HP or kW, line voltage, starting method, locked-rotor basis, soft-starter limit or VFD overload setting, and source data, then run the Motor Starting Current Calculator before using the formulas and reference tables below. Use the separate voltage-drop check after the starting-current result.
The motor starting-current calculator workflow begins with the motor nameplate and the starting method. DOL, star-delta, autotransformer, soft starter, and VFD starts do not share the same current, torque, or voltage-sag behavior, so the correct answer depends on the actual input set.
Use the page in this order:
- Enter the motor nameplate FLA and record the HP/kW and voltage context.
- Select the starting method and any current-limit or tap setting.
- If the motor provides a locked-rotor code letter, use the formula below to derive a current basis, then enter the resulting locked-rotor multiple when the calculator input is available; the calculator does not accept a code letter directly.
- Compare the calculated starting current with starter rating, a separate conductor voltage-drop check, and upstream equipment limits.
Motor Starting-Current Calculator Workflow Before Formulas
Inputs to collect
| Input | Why it matters |
|---|---|
| Motor FLA | Required current basis for the calculator's starting screen |
| HP or kW | Needed when locked-rotor kVA/HP is used instead of nameplate FLA |
| Line voltage | Sets the motor voltage context and starting-kVA result |
| Locked-rotor multiple or code-letter data | Use the multiple directly in the calculator, or use the code-letter formula below to derive a separate basis |
| Starting method | DOL, star-delta, autotransformer, soft starter, and VFD use different relationships |
| Load torque and inertia | Determines whether a reduced-current method can still accelerate the load |
| Available fault level at the motor bus | Optional source-stiffness comparison in the calculator |
Calculator-led starting method checks
After entering the motor data, use the result as a screening value rather than a standalone design answer:
| Starting method | Calculator check |
|---|---|
| DOL | Compare locked-rotor or multiplier current with source voltage sag |
| Star-delta | Confirm reduced torque, six-lead motor wiring, and transition requirements |
| Autotransformer | Check tap setting, current reduction, torque reduction, and starter duty |
| Soft starter | Check current limit, ramp time, bypass contactor, and load acceleration |
| VFD | Check drive overload rating, current limit, acceleration profile, and harmonic/EMI requirements |
For running-current comparison, use the Motor Current Calculator. For NEC table full-load current lookup, use the Full Load Current Calculator.
Core Starting Current Formulas
Formula 1: Starting Current From FLA
I_start = I_FLA x Starting Current Ratio
| Variable | Description |
|---|---|
| I_start | Starting current in amperes |
| I_FLA | Motor full-load amps from nameplate or selected basis |
| Starting current ratio | Method-specific multiplier or current-limit setting |
Use this relationship when the motor nameplate FLA is the governing input and the starter method is modeled as a current multiple.
Formula 2: Locked-Rotor Current From NEMA Code Letter
For three-phase motors:
I_LR = (kVA/HP x HP x 1000) / (sqrt(3) x V_line)
| Variable | Description |
|---|---|
| I_LR | Locked-rotor current in amperes |
| kVA/HP | Locked-rotor kVA per horsepower from the NEMA code letter |
| HP | Motor horsepower |
| V_line | Line-to-line voltage |
Use this method when the nameplate code letter is available. NEMA MG 1 defines the code letter as a nameplate marker for locked-rotor kVA per horsepower, so the code-letter input should come from the actual motor.
Formula 3: Starting Current by Method
| Method | Formula basis | Main caution |
|---|---|---|
| DOL | I_start = I_LR or selected FLA multiplier | Highest source voltage sag |
| Star-delta | I_start is derived from the DOL reference | Starting torque is also reduced |
| Autotransformer | I_start changes with tap setting | Tap affects both current and torque |
| Soft starter | I_start follows the programmed current limit | Too-low current can stall the load |
| VFD | I_start follows drive current limit and torque demand | Check drive overload and ramp settings |
NEMA Code Letter Reference After Calculator Inputs
Use this table only after you know that the motor nameplate code-letter method applies.
| Code Letter | Locked-Rotor kVA/HP Range |
|---|---|
| A | 0.0-3.15 |
| B | 3.15-3.55 |
| C | 3.55-4.0 |
| D | 4.0-4.5 |
| E | 4.5-5.0 |
| F | 5.0-5.6 |
| G | 5.6-6.3 |
| H | 6.3-7.1 |
| J | 7.1-8.0 |
| K | 8.0-9.0 |
| L | 9.0-10.0 |
| M | 10.0-11.2 |
| N | 11.2-12.5 |
| P | 12.5-14.0 |
| R | 14.0-16.0 |
| S | 16.0-18.0 |
| T | 18.0-20.0 |
| U | 20.0-22.4 |
| V | 22.4 and up |
Do not confuse the locked-rotor code letter with the NEMA design letter. The code letter is for locked-rotor kVA/HP; design letters describe torque and speed characteristics.
Starting Method Reference After the Calculator
| Method | Typical current basis | Practical use |
|---|---|---|
| DOL | Highest inrush basis | Strong source, simple starter, load can tolerate voltage sag |
| Star-delta | Reduced-current start from DOL reference | Six-lead motors where reduced torque is acceptable |
| Autotransformer | Reduced-voltage tap calculation | Larger motors needing adjustable current and torque reduction |
| Soft starter | Current limit and ramp profile | Mechanical stress reduction and moderate current control |
| VFD | Drive current limit and speed ramp | Lowest supply disturbance and speed-control applications |
The Motor Starting Current Chart is useful as a reference sheet after the calculator identifies the method and inputs.
Workflow Examples Without Static Answers
DOL or Locked-Rotor Check
Use this calculator path when the motor starts across the line:
- Enter the motor rating context and nameplate FLA.
- Enter line voltage.
- Select DOL and enter the actual locked-rotor multiple when it is known.
- If only a NEMA code letter is available, use the formula section to derive the locked-rotor basis before entering the calculator value.
- Send the result to the separate starting voltage-drop check.
Star-Delta Check
Use this path only after confirming that the motor and starter are suitable:
- Enter the same motor data used for the DOL reference.
- Select star-delta as the starting method.
- Confirm that reduced starting torque is acceptable for the driven load.
- Check transition current and voltage dip against the project limits.
Soft-Starter Check
Use this path when the starter imposes a current limit:
- Enter nameplate FLA.
- Enter the programmed current-limit setting.
- Compare the result with DOL, then review acceleration time, load type, and torque requirement separately.
VFD Check
Use this path when the drive controls frequency and voltage:
- Enter motor FLA and the drive start-current limit or overload basis.
- Confirm that the VFD is rated for the motor voltage, duty, enclosure, and ambient condition.
- Review acceleration time and load torque separately from the calculator result.
- Coordinate harmonic, grounding, cable, and manufacturer requirements separately from the starting-current result.
System Capacity and Voltage-Drop Review
Starting current is only one part of the decision. Before selecting a starter, check:
| Review item | Tool or document |
|---|---|
| Starting voltage drop | Voltage Drop Calculator |
| Running current | Motor Current Calculator |
| NEC table FLC | Full Load Current Calculator |
| Branch overcurrent screen | Breaker Sizing Calculator |
| Starter type and controls | Motor Starter Calculator |
For large motors or weak sources, coordinate the utility, generator, transformer, feeder impedance, voltage-sag tolerance, and manufacturer starting limits before approving a starting method.
Common Mistakes to Avoid
| Mistake | Consequence | Better workflow |
|---|---|---|
| Using FLA as DOL starting current | Understates source voltage sag | Use DOL multiplier or locked-rotor code mode |
| Reusing a sample motor result | Wrong for the actual nameplate | Enter the actual FLA, voltage, rating context, and locked-rotor basis |
| Ignoring reduced starting torque | Motor may fail to accelerate | Check load torque and inertia before star-delta or soft-start settings |
| Checking current but not voltage drop | Motor may start poorly even with a valid starter | Run the voltage-drop check with starting current |
| Treating VFD current as only a percentage | Drive overload, ramp, and load torque may control | Use VFD nameplate and manufacturer limits |
Related Calculators
| Calculator | Use When... |
|---|---|
| Motor Starting Current Calculator | Calculate starting current for a selected method |
| Motor Current Calculator | Compare formula running current |
| Full Load Current Calculator | Look up NEC table FLC |
| Breaker Sizing Calculator | Screen overcurrent protection |
| Voltage Drop Calculator | Check starting voltage drop |
Summary
Motor starting-current work should begin with the motor nameplate and the starting method, then move through the calculator, voltage-drop review, and starter coordination. The formulas on this page explain the method, but the result should come from the actual FLA, rating context, voltage, locked-rotor basis, current limit, and load profile reviewed for the installation.