Quick Answer
Quick answer: Use this page as a motor starting-current workflow: collect motor FLA, HP or kW, line voltage, phase, NEMA code letter, starting method, soft-starter limit or VFD overload setting, and the available voltage-drop target, then run the Motor Starting Current Calculator before using the formulas and reference tables below.
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 or choose the HP/kW, voltage, and phase basis.
- Select the starting method and any current-limit or tap setting.
- If using locked-rotor code, enter the NEMA code letter from the nameplate.
- Compare the calculated starting current with starter rating, conductor voltage drop, and upstream equipment limits.
Motor Starting-Current Calculator Workflow Before Formulas
Inputs to collect
| Input | Why it matters |
|---|---|
| Motor FLA or table FLC | Sets the running-current reference used by many starter methods |
| HP or kW | Needed when locked-rotor kVA/HP is used instead of nameplate FLA |
| Line voltage and phase | Changes the current produced by a kVA/HP calculation |
| NEMA code letter | Converts locked-rotor kVA per horsepower into current |
| 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 |
| Allowed voltage drop | Connects the starting-current result to the feeder and source check |
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 motor HP or nameplate FLA.
- Enter line voltage and phase.
- Select DOL or locked-rotor code-letter mode.
- Enter the actual NEMA code letter when using the code-letter method.
- Send the result to the 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 current-limit setting and acceleration time.
- Identify the load type and torque requirement.
- Compare the result with DOL and with the minimum current needed to accelerate the load.
VFD Check
Use this path when the drive controls frequency and voltage:
- Enter motor FLA and drive overload rating.
- Enter acceleration time and current-limit setting.
- Confirm that the VFD is rated for the motor voltage, duty, enclosure, and ambient condition.
- 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, HP, and code letter |
| 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, HP, voltage, NEMA code letter, current limit, and load profile entered for the installation.