What the Motor Starting Current Calculator does
This calculator compares motor starting current and torque across seven starting methods: direct on line, star-delta, three autotransformer taps, a soft starter and a variable-speed drive. For each one it gives the starting current, the starting kVA, the torque retained as a percentage of direct-on-line, and an estimate of the voltage dip the supply will see.
The number the page keeps in front of you is the torque column. Every method that reduces starting current reduces starting torque by the same proportion, because torque follows the square of the winding voltage and current follows it linearly. A method that cuts the current to a third also cuts the torque to a third - and if the load needs more than that, the motor will not accelerate at all and will sit at locked rotor drawing the full starting current until something trips.
How to use it
- Enter the motor's full-load current, from the nameplate or from the full-load current calculator.
- Enter the locked-rotor multiple from the data sheet. If you only have a NEMA code letter, pick it from the list and enter the horsepower - the calculator uses the midpoint of the code band and says so.
- Enter the supply fault level in MVA for a mains supply, or the generator rating and transient reactance for a generating set. The two produce dips by quite different mechanisms.
- Set the soft starter and drive current limits if you are considering them; both are settings rather than fixed properties.
- Read the table across, not down. A method is only viable if the torque it leaves is comfortably above what the load needs at standstill.
Reading the results
The starting multiple is a property of the motor, set by its impedance at standstill. It is typically 5.5 to 8 times full-load current for a modern cage induction motor, and it does not depend on the load.
Starting kVA is what the supply is asked for during the start. It is the figure that produces the voltage dip and the figure a generator has to be able to accept.
Torque as a percentage of direct-on-line is the viability test. A centrifugal pump or fan needs very little torque at standstill and starts happily in star-delta; a loaded conveyor or a screw compressor does not.
The voltage dip estimate is indicative. Below about 10% most installations are untroubled; above 15% lighting flickers visibly, contactors start dropping out around 20%, and the motor itself may stall.
Worked example: a 30 kW pump motor, 54.14 A full load, on a 10 MVA supply
At a locked-rotor multiple of 6, direct-on-line starting current is 324.8 A and the starting kVA is root-three x 400 x 324.8 / 1000 = 225.1 kVA.
The dip on a 10 MVA supply is roughly 225.1 kVA / (10,000 + 225.1) kVA = 2.2% - barely noticeable. Direct on line is fine here.
Star-delta would bring the current to 108.3 A and the dip to 0.75%, at the cost of a third of the torque. For a centrifugal pump, whose torque demand at standstill is very low, that is a good trade if anything else on the supply is sensitive.
A 65% autotransformer tap gives 0.4225 of both: 137.2 A and 42% of the torque. The current reduction follows the square of the tap ratio because the autotransformer transforms the current as well as the voltage, which is what makes it more efficient per unit of torque than star-delta.
Now put the same motor on a 250 kVA standby generator instead. The 225 kVA start is nearly the whole generator rating, and the dip - driven by the generator's 20% transient reactance rather than by a fault level - comes out around 15%. That is deep enough to disturb everything else on the set, which is why standby installations so often specify a soft starter or a drive even where the mains supply would have taken direct on line without complaint.
Formulas and scoring rules
- Direct-on-line current
I_start = multiple x I_full-loadThe multiple is the motor's locked-rotor ratio, from the data sheet or the code letter.- Starting kVA
S = root-three x V x I_start / 1000Three phase; drop the root-three for single phase.- From a NEMA code letter
locked-rotor kVA = kVA/hp band midpoint x horsepowerThe code letter gives a band, not a number; the midpoint is used and the band is shown.- Star-delta
current and torque both 1/3 of direct on lineThe windings see 1/root-three of the line voltage, so current falls by root-three within the winding and by 3 in the line.- Autotransformer at tap x
current and torque both x^2 of direct on line65% tap gives 0.4225; 80% gives 0.64.- Soft starter
current is the limit you set; torque is the square of the current ratio- Voltage dip, mains
dip = S_start / (S_fault + S_start)- Voltage dip, generator
driven by the transient reactance Xd' rather than a fault levelA generator is a far weaker source than its kVA rating alone suggests.
Why torque falls with the square of the voltage
An induction motor's torque is proportional to the square of the voltage applied to its windings. Halve the voltage and you get a quarter of the torque, while the current only halves. That relationship is not a design choice; it comes straight from the machine's equivalent circuit, and it is the fundamental constraint on every reduced-voltage starting method.
It gives the whole field its shape. Star-delta is popular because it is cheap and because 33% torque is plenty for a fan or a centrifugal pump. Autotransformer starting exists because it delivers more torque per unit of line current than star-delta does. And variable-speed drives are the only method that escapes the trade-off entirely: by lowering the frequency as well as the voltage, a drive keeps the flux constant and can produce full torque at little more than full-load current.
Generators are weaker than they look
A 250 kVA generator will not start a motor that asks for 225 kVA the way a 250 kVA transformer would. A transformer at 5% impedance is effectively a 5 MVA source; a generator's transient reactance of 15 to 25% makes it a far softer source, so the same starting kVA produces a much deeper dip.
The usual rule of thumb is that a generator can start a motor of perhaps a third to a half of its own kVA rating direct on line, depending on the acceptable dip and on the generator's own design. Where more is needed, the answers are a soft starter, a drive, an oversized generator, or starting the motors in sequence rather than together - and the generator supplier will usually run a proper transient study on request.
Limitations: what the result does not prove
- The voltage dip is a first estimate. It ignores the impedance angle, the motor's power factor at start - typically 0.2 to 0.4 - and the cable between the board and the motor, all of which affect it.
- The starting current multiple should come from the motor data sheet. A NEMA code letter gives a band rather than a value, and the midpoint is used.
- The acceleration time and the thermal withstand of the motor during starting are not calculated. A high-inertia load can take long enough to start that the rotor overheats before it reaches speed.
- Soft starter and drive behaviour depends heavily on the specific product and its settings. The figures here are what those settings imply, not what a specific unit does.
- Nothing here is a compliance certificate. A qualified engineer must verify the design against the applicable standard and the local regulations.
Privacy: where your data goes
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Standards and sources
- IEC 60034-12 - Starting performance of cage induction motors - checked 19 Sep 2026
- IEC 60034-1 - Rotating electrical machines: rating and performance
- NEMA MG 1 - Motors and Generators (locked-rotor kVA code letters)
Frequently asked questions
How much current does a motor draw when starting?
Typically six to eight times its full-load current for a direct-on-line start, held for the few seconds it takes to reach speed. The exact multiple is a property of the motor's impedance at standstill and is on the data sheet or implied by the NEMA code letter; 6 is the conventional figure when nothing better is available.
Does star-delta reduce the starting current?
To a third of the direct-on-line value, yes - and it reduces the torque to a third as well. It only works on a motor whose windings are rated for delta operation at the supply voltage, and only on loads that need very little torque at standstill, which in practice means fans, centrifugal pumps and unloaded compressors.
What is a NEMA code letter?
A letter on the nameplate giving the motor's locked-rotor kVA per horsepower as a band: F is 5.0 to 5.59, G is 5.6 to 6.29, and so on. Multiply the band by the horsepower to get locked-rotor kVA, and divide by root-three times the voltage for the current. Because it is a band, two motors with the same letter can differ by more than 10%.
How much voltage dip is acceptable when a motor starts?
Under 10% is generally comfortable. Above about 15% lighting flickers visibly and other equipment starts to notice; contactor coils typically drop out somewhere around 20%, which can trip the whole board. There is no single limit - the supply agreement, the sensitivity of the other loads and how often the motor starts all matter.
Can a generator start a large motor?
A generating set can usually start a motor of roughly a third to a half of its own kVA rating direct on line, far less than a transformer of the same rating could. The reason is the generator's transient reactance, which makes it a much softer source. A soft starter, a drive, sequential starting or a larger set are the ways round it.
Why does a soft starter not reduce current as much as star-delta?
It can - if you set it to. A soft starter's current limit is a setting, so setting it to 2 times full load gives you less current than star-delta's 3 times. The catch is the same one: torque follows the square of the current ratio, so a 2 times limit leaves only 11% of direct-on-line torque, which most loads cannot accelerate against.
Does a variable-speed drive avoid the starting current problem?
Essentially yes, and it is the only method that does. By reducing the frequency along with the voltage the drive keeps the motor's flux at its normal value, so full torque is available at little more than full-load current. The costs are the price of the drive, the harmonics it injects into the supply, and the need for suitable motor cable and sometimes an output filter.
How long does a motor draw starting current for?
Until it reaches speed, which is a few seconds for a fan or a pump and can be twenty seconds or more for a high-inertia load such as a large centrifuge or a chipper. The duration matters as much as the magnitude: the rotor has a thermal withstand time at locked rotor, and a start that takes longer than it will damage the motor even though the current itself is normal.
Last reviewed by the A2Z.Tools team against the sources listed above.
