Motor Starter Sizing Calculator

What the calculator does

Every motor circuit needs four things chosen to suit each other: a switching device that can make and break the motor current thousands of times (the contactor), a device that trips on a sustained overcurrent before the winding overheats (the overload relay), a device that clears a short circuit (an MPCB, a motor-rated MCB or an MCCB), and a cable that carries the full-load current. All four hang off one number, the full-load current, and the second number that matters, the starting current, depends on how you start the motor.

The calculator works out the full-load current from the nameplate kW, efficiency and power factor, then the starting current and the starting torque for direct-on-line, star-delta, soft starter and variable-frequency drive from the locked-rotor multiple on the nameplate. For the method you choose it gives the AC-3 contactor rating from the standard sizes, the overload relay setting, the short-circuit device rating and the instantaneous trip it must ride through, and the design current to take into the cable sizing calculator. The comparison table under the results shows all four methods at once, so the choice between them is visible in amps and percent rather than in a brochure.

Formula

Three-phase FLC = P ÷ (√3 × V × η × pf) Single-phase FLC = P ÷ (V × η × pf)

Starting current: DOL = k × FLC star-delta = k × FLC ÷ 3 soft starter = limit × FLC drive = limit × FLC
Starting torque, relative to the DOL locked-rotor torque: DOL 100 % star-delta 33 % soft starter (limit ÷ k)² × 100 %

Contactor (AC-3): DOL, soft starter, drive ≥ FLC star-delta main and delta ≥ FLC ÷ √3, star ≥ FLC ÷ 3
Overload relay: FLC in the line 0.58 × FLC in the winding leads (star-delta)
Short-circuit device ≥ 1.25 × FLC, next standard rating; its instantaneous trip > 1.2 × starting current

where P is the rated output in watts, V the line voltage (400 V line-to-line for three-phase, 230 V for single-phase), η the efficiency and pf the power factor at full load, and k the locked-rotor current as a multiple of the full-load current, which IEC 60034-1 requires on the nameplate as I_A/I_N. A soft starter holds the current at its limit by reducing the voltage, and torque falls with the square of the voltage, so the torque at a current limit of limit × FLC is (limit ÷ k)² of the locked-rotor torque. A drive supplies rated frequency-proportional voltage from zero speed, so the motor develops rated torque at about rated current from standstill. The contactor ratings are the next size up from the AC-3 sizes commonly sold (9, 12, 18, 25, 32, 38, 40, 50, 65, 80, 95, 115, 150, 185, 225, 265, 330 and 400 A); the short-circuit device from 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200 and 250 A.

Worked example

A 15 kW four-pole motor driving a chilled-water pump in a Karachi office building: 400 V three-phase, nameplate efficiency 91 %, power factor 0.85, I_A/I_N = 7, started direct-on-line.

Full-load current = 15 000 ÷ (√3 × 400 × 0.91 × 0.85) = 15 000 ÷ 535.9 = 28.0 A.
Starting current direct-on-line = 7 × 28.0 = 196 A. The same motor would draw 65 A in star (2.3 × FLC), 84 A on a soft starter limited to 3 × FLC, and 42 A on a drive limited to 1.5 × FLC.
Starting torque: DOL gives the full locked-rotor torque; star-delta gives 33 % of it; the soft starter at 3 × FLC gives (3 ÷ 7)² = 18 %; the drive gives rated torque from standstill.
Contactor: AC-3 rating at or above 28.0 A, so 32 A. Overload relay set to 28.0 A.
Short-circuit device: 1.25 × 28.0 = 35.0 A, next standard rating 40 A. Its instantaneous trip must exceed 1.2 × 196 = 235 A, which is 5.9 × its rating: a type C MCB (trips somewhere between 5 and 10 × I_n) may open on the inrush, so use a type D MCB or a motor protection circuit breaker whose magnetic trip is around 12 × I_n.
Cable design current 28.0 A, to take into the cable sizing calculator with the route length. In a motor circuit the overload relay, set to 28.0 A, is the cable's overload protection, so the 40 A short-circuit device does not have to be at or below the cable's current-carrying capacity; its let-through energy must be within what the cable withstands.

Switching the method to star-delta in the calculator gives main and delta contactors of 18 A (for 16.2 A each), a star contactor of 12 A (for 9.3 A), an overload relay set to 16.2 A in the winding leads and six motor leads each sized for 16.2 A, at the price of a third of the starting torque.

The four starting methods compared

Method	Starting current	Starting torque	Cost and parts	Shock and wear	When to use
Direct-on-line	6–8 × FLC (the nameplate figure)	100 % of locked-rotor torque	Lowest: one contactor, one overload relay	Full current and full torque in one step; hard on couplings, belts and the supply	Most motors up to about 7.5–11 kW on a stiff supply, and larger where the supply and the machine tolerate it
Star-delta	About a third: 2–2.7 × FLC while in star	33 %	Low: three contactors, a timer, six motor leads	A current and torque peak at the open transition, sometimes near the DOL value	Fans, centrifugal pumps against a closed valve, machine tools starting unloaded; the motor must be delta-connected at the supply voltage
Soft starter	Set by the user, 2–4 × FLC	(limit ÷ k)²: 18 % at 3 × FLC with k = 7	Medium: the starter plus a bypass contactor	Smooth voltage ramp; no transient; thyristors dissipate heat unless bypassed	Pumps (water hammer), conveyors, fans, belt drives; many starts per hour
Variable-frequency drive	1–1.5 × FLC	Rated torque from standstill	Highest: the drive, filters, screened motor cable	None: the motor accelerates under control	Any load that needs speed control; fans and pumps on variable duty; very frequent starting

Locked-rotor torque is typically 1.5–2.5 times rated torque for design N motors (IEC 60034-12), so 33 % of it may still be below the rated torque, which is why star-delta cannot start a loaded machine.

Standard AC-3 contactor ratings and the motors they suit at 400 V

AC-3 rating	FLC equivalent at 400 V, η 0.9, pf 0.85	Largest standard motor rating
9 A	4.8 kW	4 kW
12 A	6.4 kW	5.5 kW
18 A	9.5 kW	7.5 kW
25 A	13.3 kW	11 kW
32 A	17.0 kW	15 kW
38 A	20.1 kW	18.5 kW
40 A	21.2 kW	18.5 kW
50 A	26.5 kW	22 kW
65 A	34.5 kW	30 kW
80 A	42.4 kW	37 kW
95 A	50.4 kW	45 kW

The middle column is √3 × 400 V × I × 0.9 × 0.85, that is the shaft power a motor of that full-load current would have at typical efficiency and power factor; the right column is the largest standard IEC rating whose full-load current fits. Makers assign kW to their contactors from their own motor data and the figures differ by one step here and there; the catalogue governs.

Assumptions and limitations

Nameplate data governs. The default efficiency, power factor and locked-rotor multiple are typical of a 15 kW four-pole IE3 motor; a real motor's full-load current is on its nameplate and should be used directly. The locked-rotor multiple varies from about 5 to 9 between motor designs; IEC 60034-12 design N motors are generally 6–8 × FLC, and high-efficiency motors sit at the high end because of their lower rotor resistance.
AC-3, not AC-4. The contactor is chosen for utilisation category AC-3 (starting a squirrel-cage motor and switching it off while running). Inching, plugging and reversing under load are AC-4, for which the same contactor is rated far lower; use the maker's AC-4 table.
Starts per hour and thermal limits are not modelled. Each start heats the rotor; the motor maker states how many starts an hour it tolerates from cold and from hot, and the overload relay's trip class (10, 20 or 30) must allow the run-up time. Long, heavy starts need a class 20 or 30 relay or a motor built for them.
Coordination is a catalogue job. IEC 60947-4-1 defines type 1 coordination (the starter may be damaged by a short circuit but nothing outside it is) and type 2 (the contactor and relay survive and can be reused after a short circuit, with contact welding allowed). Which combination of breaker, contactor and relay achieves type 2 at a given prospective short-circuit current is tested by the maker and published in a table; the ratings here get you to the right rows of that table, not past it.
No voltage-dip calculation. The starting current pulls the supply down in proportion to the source impedance. On a utility supply with a nearby transformer the dip is usually small; on a long rural feeder or a generator it may be large enough to fail the start or trip other loads, and it must be worked out separately.
Star-delta needs six leads and a delta-connected motor. The motor must be wound so that its delta connection suits the supply voltage: a 400/690 V nameplate for a 400 V supply, with all six winding ends brought out. A 230/400 V motor cannot be star-delta started on 400 V.
Single-phase motors start their own way. Capacitor-start and split-phase motors have a starting winding and a centrifugal switch; the DOL figures apply loosely and star-delta does not apply at all. The single-phase option is for motors up to about 3 kW; above that the calculator warns.
Cable design current only. The cable's size also depends on the route, the installation method, grouping and ambient temperature, and on the voltage drop, which for a motor circuit should be checked at the starting current as well as at full load. Take the design current into the cable sizing calculator.

Frequently asked questions

Why is the starting current so high?

At standstill an induction motor is a transformer with its secondary, the rotor, short-circuited. Nothing limits the current but the leakage reactance and the winding resistances, which are small by design so that the motor runs efficiently, and the rotor is not turning, so it generates no back-EMF to oppose the supply. As the rotor accelerates the slip falls, the rotor frequency and its induced EMF fall, and the current with them; only near full speed does it drop to the full-load value. The multiple, 6–8 for most modern motors, is a property of the motor and does not depend on the load: a motor started with nothing on its shaft draws the same inrush, just for a shorter time.

Why can star-delta only start light loads?

In star each winding sees the line voltage divided by √3, so the winding current is 1/√3 of its delta value, and the line current, which in delta is √3 times the winding current, ends up one third. Torque goes with the square of the voltage, so it is also one third. Locked-rotor torque for a typical motor is perhaps twice the rated torque, which leaves about two thirds of rated torque in star: enough to spin up a fan or a pump with a closed valve, not enough for a loaded compressor or a full conveyor belt. If the motor does not reach near full speed before the timer switches to delta, the changeover current is close to the DOL inrush and the starter has achieved nothing.

Does a soft starter save energy?

No. Once the motor is up to speed the starter is either bypassed by a contactor or dropping a volt or so across its thyristors, and the motor draws what the load demands. What a soft starter saves is the mechanical and electrical shock: the torque step that snatches belts and couplings, the water hammer when a pump starts against a full column, and the voltage dip that a 7 × inrush puts on the supply. The energy-saving modes some starters offer reduce the voltage on a lightly loaded motor to trim its magnetising losses, and the gain is small and only exists at light load.

When does a drive pay for itself?

When the motor should not always run at full speed. A fan or a centrifugal pump obeys the affinity laws, so 80 % speed needs about half the power; if the process is throttled or dampered for much of the day, a drive removes the throttling loss and the saving is usually large. A drive also pays where a machine starts many times an hour (its start is gentle and current-limited, so contactor and motor wear disappear), or where the process needs a controlled speed or a controlled ramp. It rarely pays purely as a starter for a motor that then runs flat out: a soft starter does that job for less, with fewer harmonics and no screened cable.

What about single-phase motors?

Choose the 230 V single-phase supply and the calculator gives the full-load current from P ÷ (V × η × pf) and sizes the contactor, overload and breaker in the same way. Single-phase motors have their own starting arrangement, a capacitor or a split-phase winding with a centrifugal switch, so the starting current is whatever the nameplate says rather than a clean multiple; enter the nameplate ratio and treat the result as direct-on-line. Above about 3 kW a single-phase motor becomes impractical, and a drive with single-phase input and three-phase output serves small three-phase motors where only single-phase supply exists.

References

IEC 60947-4-1, Low-voltage switchgear and controlgear — Part 4-1: Contactors and motor-starters — Electromechanical contactors and motor-starters — utilisation categories AC-3 and AC-4, overload relay trip classes, type 1 and type 2 coordination
IEC 60034-12, Rotating electrical machines — Part 12: Starting performance of single-speed three-phase cage induction motors — design N and H limits on locked-rotor apparent power and torque
IEC 60034-1, Rotating electrical machines — Part 1: Rating and performance — nameplate content including the locked-rotor current ratio
IEC 60364-4-43, Low-voltage electrical installations — Part 4-43: Protection for safety — Protection against overcurrent — overload and short-circuit protection of motor circuits
Manufacturer catalogue data for IEC contactors and motor protection circuit breakers — AC-3 ratings, kW assignments and type 2 coordination tables are published per product range and govern the final selection

Last reviewed 2026-09-20.

Starter sizing