A three-phase squirrel-cage induction motor started directly across the line typically draws 6–8 times its rated full-load current during starting, according to ABB’s Softstarter Handbook. That is a current multiplier—not a starting-watts multiplier—and some motors draw substantially more. Use the table below for preliminary estimates when motor-specific starting data is unavailable.
Motor Starting Surge Multiplier Table
Start with the motor’s rated full-load current in amperes, its supply voltage, and its starting method. The table returns a preliminary starting-current ratio or identifies where equipment-specific data is needed. The numerical ranges apply to three-phase squirrel-cage induction motors; they are not universal factors for household appliances or single-phase compressor motors.
| Motor and starting method | Starting line current ÷ rated full-load current | Conditions and limitations | Source edition or date |
|---|---|---|---|
| Three-phase squirrel-cage motor: direct-on-line, also called across-the-line starting | Typically 6–8× | Assumes full-voltage starting. ABB reports that some motors exceed 10×; the typical range is not an upper limit or an instantaneous peak-current rating. | ABB, Softstarter Handbook, document 1SFC132060M0201, November 2010, pp. 8 and 13. |
| Three-phase squirrel-cage motor: star-delta, also called wye-delta starting | Approximately one-third of that motor’s delta-connected DOL starting current; about 2.0–2.7× if its DOL ratio is 6–8× | Applies only during the star-connected stage, at the same supply line voltage, with a motor rated to run in delta at that voltage. The 2.0–2.7× range is calculated from ABB’s ratios, not a separately published equipment rating. Transition to delta can produce a much higher current peak. | ABB, Softstarter Handbook, document 1SFC132060M0201, November 2010, pp. 8 and 14–15. |
| Three-phase squirrel-cage motor: electronic softstarter | No fixed multiplier | Starting current depends on the selected current limit, voltage ramp, motor characteristics, and driven load. Use the actual starter settings and application data rather than an assumed percentage reduction. | Kohler, Sizing Generators for Motor Starting, Power Systems Topics 103, G26-7 KPS 103, January 2018, p. 4. |
| Three-phase motor supplied through a variable-frequency drive, or VFD | No fixed generator-side multiplier | The drive controls motor starting, but its motor-output current is not the same as its generator-input current. Evaluate the drive’s input requirements and nonlinear-load effects rather than applying a DOL motor factor. | ABB, Softstarter Handbook, document 1SFC132060M0201, November 2010, pp. 16–17; Kohler, Power Systems Topics 103, January 2018, p. 5. |
These are manufacturer reference values and relationships, not code-required sizing factors. The cited editions identify where the values came from; they do not establish the starting performance of a particular motor.
What the Multiplier Means
For this table, the multiplier is:
[
M=\frac{I_{\text{start}}}{I_{\text{rated}}}
]
Therefore:
[
I_{\text{start}}=M I_{\text{rated}}
]
Where:
- (M) is the dimensionless starting-current multiplier.
- (I_{\text{start}}) is the estimated starting line current, in amperes.
- (I_{\text{rated}}) is the motor’s rated full-load line current, in amperes, at the specified voltage and connection.
Use rated full-load current—not a lightly loaded running-current measurement—as the denominator for the table’s ratios. ABB distinguishes nameplate rated current from the lower current a motor may draw when unloaded.
At unchanged voltage and phase configuration, the current ratio also gives the apparent-power ratio in kVA. It does not give the real-power ratio in kW, because motor power factor changes during acceleration. Shaft-output horsepower or kW is also not the same quantity as electrical input power.
How to Use the Table
Worked Example: A 400 V Three-Phase Motor
Assume a hypothetical motor has:
- Rated supply: 400 V line-to-line, three-phase.
- Rated full-load line current: 20 A.
- Starting method: direct-on-line.
- No manufacturer starting-current figure available.
Using the typical 6–8× DOL range:
[
I_{\text{start}}=20\times(6\text{ to }8)=120\text{–}160\text{ A}
]
For a balanced three-phase load, apparent power is:
[
S_{\text{start}}=\frac{\sqrt{3},V_{\text{LL}}I_{\text{start}}}{1000}
]
Where (S_{\text{start}}) is starting apparent power in kVA and (V_{\text{LL}}) is line-to-line voltage in volts.
At an assumed 400 V throughout the calculation:
[
S_{\text{start}}=
\frac{\sqrt{3}\times400\times(120\text{ to }160)}{1000}
\approx83.1\text{–}110.9\text{ kVA}
]
The motor’s full-load apparent power at those same rated conditions is approximately 13.9 kVA. The 83.1–110.9 kVA result is a preliminary full-voltage starting estimate, not a required generator nameplate rating. Actual generator voltage can dip during starting, changing both current and the torque available to accelerate the load.
Do not convert this result to starting kW using the motor’s running power factor. Starting kW requires the power factor applicable during the starting event.
Replace the Estimate With Motor-Specific Data
Before using a generic multiplier for equipment selection, obtain the motor or equipment manufacturer’s starting data at the intended voltage and frequency.
Relevant information includes:
- Rated full-load current and the correct winding connection.
- Locked-rotor current, often marked LRA, or manufacturer starting-current data.
- A locked-rotor kVA-per-horsepower code letter, where provided.
- Starter type, settings, and transition arrangement.
- Driven-load torque, inertia, and expected acceleration time.
A locked-rotor code letter expresses a range of kVA per horsepower; it is not itself a current multiplier. Use the applicable code-letter data with the motor’s horsepower rating rather than treating the letter as a universal surge factor.
For a single-phase pump, refrigerator, air conditioner, or other packaged appliance, obtain starting information for the actual equipment. Do not assign the three-phase table’s range solely because the appliance contains a motor.
Applying the Estimate to a Generator Load List
Evaluate each starting event with the loads already running on the generator. If motors start sequentially, examine the relevant steps separately. If controls can cause several motors to restart together after an outage, evaluate that combined event rather than assuming sequential operation. Existing generator load and driven-load characteristics affect starting performance.
Do not simply add every motor’s starting demand unless simultaneous starting is the intended or credible operating condition. Conversely, considering only the largest motor can miss a more demanding step that occurs after other loads are already connected.
For a specific generator, ask the supplier to evaluate the proposed engine, alternator, excitation system, and controls against the actual load sequence. The relevant checks include voltage dip, voltage recovery, frequency response, and the motor’s ability to accelerate its load. A current multiplier or alternator-only starting-kVA figure cannot establish all of those outcomes.
The table does not size conductors, protective devices, or motor controllers. Those selections still require the equipment instructions and the electrical-code edition adopted in the installation’s jurisdiction, applied by the responsible qualified professional.