Motor Locked Rotor Current & Inrush Calculator

Q: What do NEMA Locked Rotor Code Letters mean?

NEMA Code Letters (A through V) stamped on motor nameplates define the starting kVA per horsepower drawn when the rotor is at a dead stop (0 RPM). For example, Code G corresponds to 5.60 to 6.29 kVA/HP, meaning a 100 HP Code G motor will demand between 560 and 629 kVA during starting.

Q: Why does reduced-voltage starting cut inrush current?

Motor torque and inrush current both vary with applied voltage (current ∝ V, torque ∝ V²). Starting with an autotransformer or soft-starter at 65% voltage cuts starting current to 42% of across-the-line inrush, softening utility bus flicker.

Q: How large must an emergency standby generator be to start an electric motor?

For standard across-the-line starting, a generator must be sized 2.5x to 3x the motor running kW to supply the huge reactive kVA inrush without stalling the diesel engine or collapsing generator exciter voltage.

Compute motor starting inrush current (LRA), starting kVA, and line voltage drop from NEMA Code Letters A through V per NEMA MG-1 and NEC 430.7(B).

Motor Nameplate Ratings

Motor Horsepower (HP):

Voltage (V):

NEMA Code Letter (kVA / HP):

Starting Method & Source

Starting Method:

Source Transformer (kVA):

Locked Rotor Current Results

Starting Inrush Current (LRA)

-- Amps

--x times Full Load Amps

Starting kVA Demand

-- kVA

Instantaneous Inrush kVA

Estimated Voltage Dip

-- %

Transformer Bus Sag

Full Load Current (Running FLA): -- A

Generator Sizing Recommendation: -- kW standby

Voltage Dip Severity: Acceptable (< 15%)

NEMA MG-1 locked rotor codes define the maximum starting kVA envelope to evaluate upstream transformer voltage drop.

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Frequently Asked Questions

What do NEMA Locked Rotor Code Letters mean?

NEMA Code Letters (A through V) stamped on motor nameplates define the starting kVA per horsepower drawn when the rotor is at a dead stop (0 RPM). For example, Code G corresponds to 5.60 to 6.29 kVA/HP, meaning a 100 HP Code G motor will demand between 560 and 629 kVA during starting.

Why does reduced-voltage starting cut inrush current?

Motor torque and inrush current both vary with applied voltage (current ∝ V, torque ∝ V²). Starting with an autotransformer or soft-starter at 65% voltage cuts starting current to 42% of across-the-line inrush, softening utility bus flicker.

How large must an emergency standby generator be to start an electric motor?

For standard across-the-line starting, a generator must be sized 2.5x to 3x the motor running kW to supply the huge reactive kVA inrush without stalling the diesel engine or collapsing generator exciter voltage.