Enter the rated power (watts), the voltage (volts), efficiency, and the power factor into the calculator to determine the Motor Start-Up Current. 

Motor Start Up Current Formula

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

Isu = 4 * ( 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

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)