Calculate motor starting current (inrush amps) from full-load amps or motor kW/hp, using typical multipliers for direct-on-line, star-delta or VFD starts.

Apply a stated starting-current multiplier to nameplate full-load current.

Typical multipliers, not manufacturer ratings. Use the motor’s locked-rotor amps (LRA) if you have it.

Optional settings

Motor Startup Current Formula

Estimate starting current as Isu = multiplier × FLA. The formula below is the illustrative 6× case for balanced three-phase AC. Use manufacturer motor LRA and starter/drive data when available; the presets are not guaranteed limits.

Isu = 6 × (RP / (SQRT(3) × V × PF × E))
  • Where Isu is the Motor Startup Current (amps)
  • RP is rated mechanical output power (watts) 
  • V is RMS line-to-line voltage (volts) for three-phase AC 
  • PF is the power factor 
  • E is full-load efficiency as a fraction (the calculator input uses percent)

In power mode, divide rated mechanical output watts by phase factor × RMS voltage × power factor × efficiency fraction, then multiply by the selected starting-current assumption. The phase factor is √3 for balanced three-phase and 1 for single-phase. Alternatively enter nameplate FLA directly, or select the inverse mode.

How to Calculate Motor Startup Current?

The following example problems outline how to calculate Motor Startup Current.

Example Problem #1

  1. First, determine the rated power (watts). In this example, the rated power (watts) is determined to be 415.
  2. Next, determine the voltage (volts). For this problem, the voltage (volts) is measured to be 40.
  3. Next, determine the power factor. In this case, the power factor is found to be 0.75.
  4. Next, determine the efficiency. For this problem, this is .75 (75%).
  5. Finally, calculate the Motor Startup Current using the formula above: 

Isu = 6 * ( RP / (SQRT(3)*V*PF*E) )

Inserting the values from above and solving yields: 

Isu = 6 * ( 415 / (SQRT(3)*40*.75*.75) ) ≈ 63.893 (amps)

Example Problem #2

Using the same method as above, determine the variables required by the formula. For this example problem, these are:

rated power (watts) = 52

voltage (volts) = 23

power factor = 0.5

efficiency = .60

Entering these given values into the calculator or the formula above yields: 

Isu = 6 * ( 52 / (SQRT(3)*23*.5*.60) ) ≈ 26.106 (amps)