Estimate motor startup current from nameplate FLA or AC motor power, voltage, power factor, efficiency, and starter type, with reverse FLA calculation.

⚡Motor Inrush Estimate
× FLA
Required: motor nameplate full-load current and starting multiplier.
A
V
%
A
Estimated starting current
—
Full-load current—
Starting multiplier—
Starting current—
Percent of FLA—
—
This is a planning estimate for AC motors. Actual inrush depends on motor design, locked-rotor data, load torque, acceleration time, supply impedance, and starter settings. Use nameplate FLA and manufacturer LRA data when available. Do not size conductors, breakers, overloads, contactors, or generators from this estimate alone.

Motor Start Up Current Formula

The following formula is used to calculate the Motor Start Up Current.

Isu = 6 * (RP / (SQRT(3)*V*PF*E))
  • Where Isu is the Motor Start Up Current (amps)
  • RP is the rated power (watts) 
  • V is the voltage (volts) 
  • PF is the power factor 
  • E is the efficiency

To calculate the motor start up current, divide the rated power by the result of the square root of 3 times, voltage times power factor, times efficiency, then finally multiply that result by 6.

How to Calculate Motor Start Up Current?

The following example problems outline how to calculate Motor Start Up 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 15.
  4. Next, determine the efficiency. For this problem, this is .75 (75%).
  5. Finally, calculate the Motor Start Up Current using the formula above: 

Isu = 4 * ( RP / (SQRT(3)*V*PF*E) )

Inserting the values from above and solving yields: 

Isu = 4 * ( 415 / (SQRT(3)*40*15*.75) ) = 2.129 (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 = 5

efficiency = .60

Enter these given values into the calculator or above yields: 

Isu = 4 * ( 52 / (SQRT(3)*23*5*.60) ) = 1.74 (amps)