Motor Starting Current Calculator
Estimate motor inrush current and the voltage dip it causes on starting.
About Motor Starting Current Calculator
Direct-on-line starting draws six to eight times full-load current, which causes a voltage dip proportional to how weak the supply is relative to the motor. Softer starting methods trade starting torque for reduced inrush. Voltage dips above about 5 per cent cause visible light flicker and can drop contactors elsewhere in the installation.
Formula
Motor Starting Current
Starting current = full-load current × method multiple; dip ≈ starting kVA ÷ (fault level + starting kVA)
- DOL
- Direct on line, drawing six to eight times full-load current
- star-delta
- Reduces starting current to about a third of DOL
Worked example
With the default values below, Motor Starting Current Calculator returns Starting Current of 175.50. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Full-load currentminimum 0 A
- 27 A
- Starting methodone of Direct on line, Star-delta, Soft starter, Variable speed drive
- Direct on line
- Supply fault levelminimum 1 kVA
- 500 kVA
- Motor apparent powerminimum 0.1 kVA
- 19 kVA
Results
- Starting Current
- 175.50 A
- Multiple of Full-Load Current
- 6.50 x
- Estimated Voltage Dip
- 100.00%
- Starting Apparent Power
- 123.50 kVA
- Verdict
- Excessive dip — use a softer starting method
Step by step
- 1Values used
- 2Motor Starting Current
- 3Starting Current= 175.50 A
- 4Multiple of Full-Load Current= 6.50 x
- 5Estimated Voltage Dip= 100.00
- 6Starting Apparent Power= 123.50 kVA
- 7Verdict= Excessive dip — use a softer starting method
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Motor Starting Current calculated?
Starting current = full-load current × method multiple; dip ≈ starting kVA ÷ (fault level + starting kVA) Direct-on-line starting draws six to eight times full-load current, which causes a voltage dip proportional to how weak the supply is relative to the motor. Softer starting methods trade starting torque for reduced inrush.
Why does Motor Starting Current matter?
Voltage dips above about 5 per cent cause visible light flicker and can drop contactors elsewhere in the installation.
What values do I need to enter?
This calculator takes 4 inputs: Full-load current, Starting method, Supply fault level, Motor apparent power. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.