Motor Starting Current Calculator

What the motor starting current calculator shows

At the instant an induction motor starts, the rotor is still and the motor draws its locked rotor current, often six to eight times its running current. The nameplate shows this as a NEMA code letter that sets the locked rotor kVA per horsepower. The motor starting current calculator turns that letter into a current range, the starting kVA, and the line current you can expect with a reduced voltage starter.

Motor starting current formula

The code letter gives locked rotor kVA per horsepower. Multiply by the horsepower and convert to current:

A star-delta starter applies 1/√3 of the voltage to each winding and cuts the line current to one third. An autotransformer at tap x cuts the line current to about x squared, so a 65% tap gives about 42% of the locked rotor current.

Motor starting current calculator example

A 20 hp, 460 V, three phase motor with code letter G (5.6 to 6.3 kVA/hp):

The locked rotor current falls between 140.6 A and 158.1 A. With a star-delta starter the line current drops to 52.7 A, and with a 65% autotransformer tap it drops to 66.8 A.

How to use the motor starting current calculator

Enter the motor horsepower and the supply voltage, choose single or three phase, and pick the code letter from the motor nameplate. Then choose the starting method: across the line, star-delta, or an autotransformer tap. Press Calculate to read the locked rotor current range and the line current with your starter. Use the maximum figure when you size equipment, since it is the worst case for that code letter.

Reading the locked rotor current

The locked rotor current lasts only while the motor accelerates, usually a few seconds, but it causes voltage dip and sets the size of the source. A generator or small transformer that sees a large inrush can sag far enough to stall the motor or drop out contactors. Compare the starting kVA with the source: generator makers publish how much motor starting kVA each set can handle at a given voltage dip. The range reflects the code letter band; the maximum is the safe value for design.

When to use a locked rotor current calculator

• Sizing a generator to start a large motor
• Checking voltage dip on a transformer or long feeder
• Choosing between across the line and reduced voltage starting
• Setting instantaneous trips so breakers ride through inrush
• Checking whether a soft start or star-delta starter is worth adding

Common motor starting current calculator mistakes

• Using line to neutral voltage. Enter the line to line voltage for three phase motors.
• Using the minimum of the range. Design with the maximum value for the code letter.
• Confusing code letter with design letter. NEMA Design B is a torque class; the code letter sets inrush.
• Expecting full torque from a reduced voltage starter. Torque drops with the current.

Tips for managing motor inrush current

• Start the largest motor first when several share a generator.
• Use a soft starter or drive when voltage dip disturbs other loads.
• Check the nameplate code letter when you replace a motor.

Related motor and power tools

Starting current is one of three motor numbers you need. The motor FLA calculator gives the running current, the generator size calculator sizes the source, and the breaker size calculator picks the protection. This motor starting current calculator covers the inrush.

Locked rotor amps and the NEMA code letter

The code letter describes the motor, not the load. It groups motors by how much apparent power they draw with the rotor locked at full voltage, from letter A, below 3.15 kVA per hp, to V, at 22.4 and above. Most general purpose NEMA Design B motors carry letters F to H, which is why the familiar rule of six to seven times full load current works for them. Newer premium efficiency motors often carry J or K because lower winding resistance raises the inrush, a detail that catches people out when they replace an old motor and the breaker starts to trip on startup. Single phase motors behave the same way: a 10 hp, 230 V single phase motor with code H draws up to 308.7 A at start, which is why rural single phase services struggle with large pumps. Size the source and the instantaneous trip setting from the maximum value, and keep the running current for the overload relay.

Star delta starting current and other methods

Reduced voltage starting trades torque for current. Star-delta starting connects the windings in star so each sees 58% of line voltage; line current and torque both fall to one third, so it suits fans and pumps that start unloaded. Autotransformer starters offer taps, commonly 50%, 65%, and 80%; line current falls to roughly the square of the tap, 25%, 42%, or 64%, and torque falls by the same ratio. Soft starters ramp the voltage and usually hold current to three to four times full load. Variable frequency drives start a motor at or near rated current, which is the gentlest option for a weak supply. Pick the method that keeps voltage dip acceptable while still giving enough torque to break the load away.

NEMA code letter table

Locked rotor kVA per horsepower for each NEMA code letter, from NEC Table 430.7(B):

At 460 V three phase with code G, the maximum locked rotor current is 39.5 A for 5 hp, 79.1 A for 10 hp, 158.1 A for 20 hp, 395.4 A for 50 hp, and 790.7 A for 100 hp.

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