How to Calculate Motor Inrush Current

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A motor, with the power turned off, presents little resistance to electricity because the motor windings act almost like a short until they become energised. When power is turned on, the current flow may be many times what the current flow is when the motor is operating at rated speed under a load.

This inrush of current lasts only a fraction of a second. No calculation for the exact inrush current is possible, but a range may be determined if the manufacturer's documentation does not specify it.

Read the motor nameplate on the motor and find the voltage listed on it. The National Electric Code mandates all motors have a nameplate that provides information specific to the electrical operating characteristics of the motor.

Look for the "Locked Rotor Letter Code" or "Locked Rotor Code" on the motor nameplate. This code will be a letter from "A" to "V" but will not include "I," "O" or "Q." These letters are omitted to avoid confusion.

  • A motor, with the power turned off, presents little resistance to electricity because the motor windings act almost like a short until they become energised.
  • No calculation for the exact inrush current is possible, but a range may be determined if the manufacturer's documentation does not specify it.

Consult the NEMA Locked Rotor Code Table and find the letter code on it. Follow the letter code row to the right and find the range given. The range is given in thousands of Volt-Amps, or kilowatts.

Multiply each number in the range by 1,000. Divide each result by the motor voltage found on the nameplate. The resulting range is the inrush current range. For example: A 3.5-horsepower, three-phase motor nameplate lists the motor voltage as 230 volts and the locked rotor code is "K." The range given on the table is 8.0 to 8.99 kVA. Multiplying by 1,000, the range becomes 8,000 to 8,990 VA. Dividing by the motor voltage of 230 volts gives the inrush current range as 34.8 amps to 39.1 amps.

  • Consult the NEMA Locked Rotor Code Table and find the letter code on it.
  • For example: A 3.5-horsepower, three-phase motor nameplate lists the motor voltage as 230 volts and the locked rotor code is "K." The range given on the table is 8.0 to 8.99 kVA.
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