Star-Delta Starter Calculator
Calculate Smarter. Work Faster.
Size the main, star and delta contactors, the overload relay setting, the fuse and the MCCB from motor kW, HP or nameplate current — with full power and control circuit diagrams below.
Motor & Starter Data
Enter the motor rating or its nameplate current, and get every device rating the starter needs.
Check the motor first. Star-delta only works if the motor is rated for delta connection at this supply voltage and has all six winding ends brought out to the terminal box. Efficiency and power factor above are illustrative defaults — replace them with the actual nameplate figures.
Enter motor data and hit calculate
Enter values above to see a step-by-step breakdown.
How a Star-Delta Starter Is Sized
A star-delta starter cuts the starting current of a three-phase induction motor to roughly a third of its direct-on-line value by first connecting the windings in star, then switching them to delta once the motor is near full speed. Every device rating in the panel follows from one number — the motor's full load current.
Step 1 — full load current. Take it from the nameplate where you can. Where you can't, calculate it from the rated output — the Motor Current Calculator does this on its own:
with P the rated mechanical output in watts, V the line-to-line voltage, η the efficiency and cos φ the power factor. A 7.5 kW motor at 415 V with 98% efficiency and 0.8 power factor draws 7500 / (1.732 × 415 × 0.98 × 0.8) = 13.3 A.
Step 2 — why star gives one third. In star, each winding sees only V/√3 instead of the full line voltage, so winding current falls by √3. On top of that, the line current in star equals the winding current, whereas in delta it is √3 times the winding current. The two factors multiply:
You can compare the two starting currents side by side in the Motor Starting Current Calculator. Starting torque follows the square of the winding voltage, so in theory it drops to about a third as well — published figures of roughly 25–33% are quoted in practice, since the actual value depends on the motor design, the supply voltage and the load. That trade-off — roughly a third of the current for roughly a third of the torque — decides whether star-delta suits your load at all.
Step 3 — the three contactors. This is where most sizing errors happen. Look at the power diagram below: in delta running, the main and delta contactors each sit in series with a motor winding, so each carries winding current, not line current:
The star contactor carries only the star-connection starting current for a few seconds and is conventionally rated at 0.33 × IFLC. So for our 13.3 A motor: main and delta contactors 7.7 A each, star contactor 4.4 A — all selected against the AC-3 rating at the actual supply voltage, never an AC-1 resistive rating.
Step 4 — overload relay. In the standard arrangement the thermal relay sits between the main contactor and the motor terminals, in the winding path, so it is set to 0.58 × IFLC as well. Fit it in the incoming line instead and it sees full line current, so it is set to 100% of IFLC. Getting this backwards is a common star-delta commissioning error, and it leaves the motor with no useful overload protection.
gG and aM fuses are not sized the same way. A gG fuse is general purpose, covering overload and short circuit, and must carry the starting current without nuisance operation. An aM fuse is motor-rated back-up protection: it deliberately does not protect against overload and is only valid in combination with an overload relay, so its rating comes from the manufacturer's coordination table rather than a percentage of full load current. Applying one percentage to both, as the quick reference does, is a screening step only.
What the breaker figure does not decide. A percentage of full load current sets a ceiling and nothing more. The actual MCCB or MCB still has to be selected for its frame size, trip unit type and setting, an instantaneous setting clear of the star starting current, breaking capacity against the prospective fault level at that point, and coordination with the cable ampacity and the devices upstream.
Step 5 — short-circuit protection. The 175% and 250% figures used by the calculator come from NEC-style motor branch-circuit sizing — they are not universal IEC values, and IEC 60947-4-1, which covers contactors and starters, does not prescribe them. Treat them as maximum calculated references: round to a standard rating without exceeding the ceiling, noting that some codes permit the next higher standard rating where the calculated value falls between sizes. For an IEC installation, switch the calculator's protection basis to IEC / manufacturer coordination and take the device from your local code and the maker's coordination tables instead. These devices provide short-circuit and earth-fault protection as part of the motor branch circuit; they are not a substitute for the overload relay.
The calculator above applies all five steps at once and rounds each contactor result up to the next commonly available AC-3 rating used by the major manufacturers — a starting point for selection, not a substitute for the maker's own star-delta combination table.
Star-Delta Starter Power Circuit Diagram
Reading the power circuit. The incoming three phases pass through the MCCB and split into two paths. The first goes through the main contactor KM1, then the thermal overload relay, and lands on winding ends U1, V1 and W1. The second goes through the delta contactor KM3 and lands on the other winding ends, cross-connected: L1 to W2, L2 to U2, L3 to V2. That cross-connection is what forms the delta; wiring it straight across (L1 to U2, L2 to V2, L3 to W2) reverses the rotation and is a classic panel-wiring error.
The star contactor KM2 does something quite different — it is tapped from those same three motor leads, on the motor side of KM3, and shorts W2, U2 and V2 together to form the star point. That position matters: KM3 is open during starting, so KM2 has to connect to the winding ends themselves and not through the delta contactor. During starting, KM1 and KM2 are closed and the windings form a star fed at line voltage. After the timer runs out, KM2 opens and KM3 closes, and the same windings become a delta.
Notice where the overload relay sits: after KM1, in the winding path. That position is exactly why it is set to 58% of the nameplate current rather than 100%.
The 0.58 basis applies to the conventional star-delta arrangement drawn here, where the main and delta contactors both sit in the winding path. A different starter topology, or a manufacturer's tested combination, may size these devices differently.
Star-Delta Control Circuit Diagram (with Timer)
Reading the control circuit. Control supply enters through its own MCB and passes through the STOP button (normally closed), the START button (normally open, latched by KM1's own 13-14 auxiliary contact) and the overload relay's 95-96 contact. Break any of those three and everything drops out — which is precisely what you want from a stop button, a power loss or an overload trip.
Below the distribution rail sit four parallel branches. KM1 and the timer KT energise immediately and stay energised for the whole run. The star branch is fed through the timer's NC timer contact, so KM2 pulls in at once; the delta branch is fed through its NO timer contact, so KM3 waits. When the set time elapses the two timer contacts swap state, KM2 drops out and KM3 pulls in.
The two NC auxiliary contacts — KM3 21-22 in the star branch and KM2 21-22 in the delta branch — are the electrical interlock. Fit the mechanical interlock supplied with the contactor pair as well: a welded contact ignores wiring logic, and star and delta closed together is a phase-to-phase short across the supply.
Starting Sequence Step by Step
| Stage | KM1 main | KM2 star | KM3 delta | What the motor sees |
|---|---|---|---|---|
| Rest | Open | Open | Open | De-energised |
| START pressed | Closed | Closed | Open | Windings in star — theoretically ≈ 1/3 of DOL current and torque |
| Accelerating | Closed | Closed | Open | Speeds up to roughly 80–90% of full speed |
| Timer times out | Closed | Opens | Open | Open transition — motor momentarily disconnected |
| Changeover | Closed | Open | Closes | Windings in delta, full line voltage, brief inrush |
| Running | Closed | Open | Closed | Normal delta running, protected by the O/L relay |
The one-third figures are theoretical ratios. The current actually drawn during acceleration and at changeover, and the torque actually developed, depend on the motor characteristic, the supply and the load.
Open transition is the weak point. Because KM2 must open before KM3 closes, the motor spends a few tens of milliseconds disconnected. It keeps spinning and behaves briefly as a generator, and its residual voltage drifts out of phase with the supply. When delta closes, the phase difference can produce an inrush higher than a direct-on-line start — the very thing the starter was installed to avoid. Keeping the dead time short helps; where the surge genuinely matters, a closed-transition starter or a soft starter is the better answer.
Overload Relay, Fuse and MCCB Selection
Three different devices protect three different things, and mixing up their jobs is how motors burn out in panels that looked fully protected on paper.
Thermal overload relay — protects the motor. It models the heating of the windings and protects the motor against sustained overload according to its trip class, its setting and the manufacturer's tripping characteristic. In the standard star-delta layout it sits between the main contactor and the motor, carrying winding current, so it is set to:
Choose a relay whose adjustment range brackets that figure comfortably, ideally near the middle of its range rather than at either end. A relay in the winding but set to full line current is roughly 1.7 times too high — it will let the motor cook.
Where the relay sits is part of the starter's coordination type. Manufacturers publish tested combinations of short-circuit device, contactors and relay, and the tested arrangement decides both the relay position and how much damage is acceptable after a fault — type 1 versus type 2 coordination. Follow the maker's tested combination rather than mixing devices from different tables.
Fuses — protect against short circuit. Use time-delay (motor-rated) fuses so that starting inrush does not blow them, and keep the rating at or below 175% of full load current:
Circuit breaker — the alternative to fuses. An inverse-time MCCB or a motor-rated MCB is normally limited to 250% of full load current:
Both figures are ceilings. Round down to the nearest standard rating, then confirm the device's magnetic trip clears the locked-rotor current without nuisance tripping on the star start.
Making and breaking capacity. Utilisation category AC-3 defines the making and breaking duty for motor starting as multiples of the contactor's own rated operational current — not of the motor's full load current — and published data is commonly of the order of ten times that rating for making and several times it for breaking. The figure below is a reference check on the order of magnitude only — the real duty comes from the manufacturer's AC-3 data for the frame you choose, at your supply voltage. An AC-1 (resistive) rating of the same numerical value is not equivalent and must not be used for motor duty.
Finally, remember what is not protected by any of this: single phasing, locked rotor on a jammed load, repeated starting and insulation failure all need their own answers — a phase-failure relay, a properly set thermal relay, a duty-cycle limit, and periodic insulation resistance testing.
Setting the Star Timer
The timer decides when the motor leaves star and enters delta, and it is the one setting that has to be commissioned on the actual machine rather than copied from a catalogue.
Too short and the motor is still well below speed when delta closes, so the changeover draws nearly a full direct-on-line inrush — defeating the purpose of the starter and stressing the contacts. Too long and the motor sits in star past the point where it has stopped accelerating, drawing current, heating up and producing only a third of its torque.
How to set it in practice: start the motor with a clamp meter on one incoming phase and watch both the current and the motor's acceleration. In star the current rises well above full load current and should then fall as the motor speeds up, though how far and how quickly depends on the motor and the load. The right moment to change over is where that current has levelled out and the motor has stopped gaining speed — typically a few seconds for a fan or an unloaded pump, longer for a high-inertia load such as a large fan wheel or a centrifuge. Set the timer there, then confirm across two or three starts from cold.
Write the commissioned figure on the panel schedule along with the motor and load it was set for. If the motor is rewound, the impeller changed or the duty altered, the timer setting needs re-checking — it is not a fit-and-forget value.
Watch the starting frequency too. Star-delta starting still heats the rotor substantially. Most catalogue motors tolerate only a limited number of starts per hour, and a starter that cycles a pump every few minutes needs either a larger motor, a soft starter or a VFD rather than a shorter timer.
When Star-Delta Suits — and When It Doesn't
The whole method rests on one condition: the load must be able to accelerate on a third of the motor's starting torque. Everything else is detail.
Good candidates — centrifugal pump duties where the torque needed at start stays within the reduced torque available in star, fans and blowers, compressors fitted with unloaders, machine tools, and any drive that comes up to speed unloaded and takes up load afterwards.
Poor candidates — loaded conveyors, crushers, mixers with product in them, positive-displacement pumps, piston compressors without unloaders, and high-inertia loads that would sit in star for a long time. If the motor cannot reach roughly 80–90% of speed in star, the changeover surge makes the starter pointless.
The motor itself must also fit. All six winding ends have to be available at the terminal box, and the machine has to be rated for delta running at the supply voltage. A motor stamped 400 V delta / 690 V star is right for a 400 V star-delta starter; one stamped 400 V star is not, and connecting it star-delta would run it permanently under-voltage.
| Method | Starting current | Starting torque | Relative cost | Best for |
|---|---|---|---|---|
| Direct on line (DOL) | 6–8 × FLC | 100% | Lowest | Small motors, stiff supply, loads needing full torque |
| Star-delta | ≈ 2–2.7 × FLC | ≈ 25–33% (theoretical 33%) | Low | Light-start loads on motors with six leads |
| Soft starter | Adjustable, typically 2–4 × FLC | Adjustable ramp | Medium | Smooth starts, no transition surge, belt and pump protection |
| VFD | ≈ 1–1.5 × FLC | Full torque from zero speed | Highest | Speed control and energy saving on variable-flow duties |
Star-delta remains popular because it is cheap, repairable with parts from any wholesaler, and understood by every electrician on site. Where the duty is variable-flow, though, a VFD often repays its cost in energy alone, and where the load is awkward a soft starter avoids the open-transition surge entirely.
Star-Delta Sizing at a Glance (415 V, 3-Phase)
Illustrative sizing examples calculated from the formula — not manufacturer motor ratings. Real nameplate currents differ.
| Motor | IFLC (A) | Main & delta (0.58×) | Star (0.33×) | O/L setting | Fuse max (1.75×) | MCCB max (2.5×) |
|---|---|---|---|---|---|---|
| 7.5 kW (10 HP) | 13.3 | 7.7 A | 4.4 A | 7.7 A | 23 A | 33 A |
| 11 kW (15 HP) | 19.6 | 11.4 A | 6.5 A | 11.4 A | 34 A | 49 A |
| 18.5 kW (25 HP) | 32.9 | 19.1 A | 10.9 A | 19.1 A | 58 A | 82 A |
| 30 kW (40 HP) | 53.4 | 31.0 A | 17.6 A | 31.0 A | 93 A | 133 A |
| 55 kW (75 HP) | 97.8 | 56.8 A | 32.3 A | 56.8 A | 171 A | 245 A |
| 90 kW (120 HP) | 160.1 | 92.9 A | 52.8 A | 92.9 A | 280 A | 400 A |
Currents above assume 415 V, 92% efficiency and 0.85 power factor — good enough for a first pass, but always recalculate from the actual nameplate before ordering. Fuse and MCCB columns are ceilings, so round down to a standard rating; contactor columns are minimums, so round up to the next standard AC-3 frame.
Star-Delta Starter Design Examples
Example 1 — 7.5 kW, 415 V, η 98%, pf 0.8
IFLC = 7500 / (1.732 × 415 × 0.98 × 0.8) = 13.3 A. Main and delta contactors = 0.58 × 13.3 = 7.7 A each → order the next standard AC-3 frame, typically 9 A. Star contactor = 0.33 × 13.3 = 4.4 A → also a 9 A frame, since that is the smallest common size. Overload relay set to 7.7 A (relay in the winding). Fuse ≤ 1.75 × 13.3 = 23.3 A → fit 20 A time-delay. MCCB ≤ 2.5 × 13.3 = 33.3 A → fit 32 A. Making capacity to check against ≈ 10 × 13.3 = 133 A.
Example 2 — 220 HP, 415 V, η 95%, pf 0.85
220 HP = 164,054 W, so IFLC = 164,054 / (1.732 × 415 × 0.95 × 0.85) = 282.6 A. Main and delta = 0.58 × 282.6 = 164 A each → 185 A frames. Star = 0.33 × 282.6 = 93.3 A → 95 A frame. Overload set to 164 A. Fuse ≤ 494 A → 400 A; MCCB ≤ 706 A → 630 A. At this size the open-transition surge is substantial and the supply impedance matters, so a soft starter deserves serious consideration.
Example 3 — starting current comparison
Take the 7.5 kW motor with a locked-rotor current of 6 × FLC. Direct on line it would draw 6 × 13.3 = 79.9 A. Star-delta starting draws a third of that, 26.6 A — about 2 × FLC. That reduction can help limit the voltage dip during starting, but cable size and protective-device rating still have to be determined independently from the applicable code and the installation conditions. The trade is torque: 33% of the DOL figure, so the load must be light at start.
Example 4 — the overload relay mistake
Same 13.3 A motor, relay fitted in the winding but set to 13.3 A because the technician read the nameplate. The relay now only sees 7.7 A when the motor is at full load, so it reads about 58% of its setting — the motor could run at nearly 1.7 times full load indefinitely before the relay noticed. The panel looks protected, the motor is not. Correct setting: 7.7 A.
These results are preliminary design figures based on standard star-delta sizing practice. Confirm every rating against the manufacturer's AC-3 selection tables at your actual supply voltage, the motor nameplate, and the wiring rules that apply in your country, and have the design checked by a qualified electrical engineer before installation. This calculator does not cover cable sizing, discrimination between protective devices, short-circuit withstand or type-1/type-2 coordination.
Common Mistakes When Designing a Star-Delta Starter
1. Setting a winding-mounted overload relay to full line current. The relay carries 58% of line current in that position. Set it to 100% and the motor can run heavily overloaded without tripping.
2. Sizing the main and delta contactors at full line current "to be safe". Harmless electrically but expensive, and it fills the panel. Both carry winding current; 58% is the correct basis.
3. Selecting contactors from the AC-1 column. AC-1 is a resistive rating. Motor duty is AC-3, where the same physical contactor is rated far lower. This is a genuine safety error, not a paperwork one.
4. Wiring the delta contactor straight across instead of cross-connected. L1 must reach U1 through KM1 and W2 through KM3. Wire it the obvious way and the motor either runs backwards or shorts out on transition.
5. Omitting the mechanical interlock. The electrical interlock cannot help if a contact welds. Use both.
6. Using a star-delta starter on a heavy-start load. The available starting torque is only about a third of the DOL value, so the load must be able to accelerate on it. A loaded conveyor or crusher will stall in star and slam through the transition at near-DOL current.
7. Copying a timer setting from another machine. Run-up time depends on the motor and the load inertia. Commission it on the actual drive with a clamp meter.
8. Using a motor with only three leads brought out. Without both ends of each winding at the terminal box, star-delta is impossible — that motor needs a DOL, soft starter or VFD instead.
9. Treating fuses as overload protection. Fuses and breakers cover short circuits. Overload protection is the thermal relay's job, and neither substitutes for the other.
10. Ignoring starts per hour. Each star-delta start still heats the rotor. Frequent cycling needs a bigger motor or an electronic starter, not a tighter timer.
Frequently Asked Questions
Star-delta starter design, wiring and settings — answered
How do you calculate contactor size for a star-delta starter? +
Size the main and delta contactors at about 58% of the motor full load current, and the star contactor at about 33%. The 58% is 1/√3: in delta running both sit in series with a motor winding, so each carries winding current rather than line current. Round up to the next standard AC-3 rating at your supply voltage.
Why is star-delta starting current one third of DOL? +
In star each winding sees only V/√3, so winding current falls by √3; and in star the line current equals the winding current instead of being √3 times it. The two effects multiply to 1/3. Starting torque falls to a third as well, since torque follows the square of winding voltage.
How do I calculate motor full load current? +
For a three-phase motor, I = P / (√3 × V × η × cos φ), with P the rated mechanical output in watts. A 7.5 kW motor at 415 V with 98% efficiency and 0.8 power factor draws about 13.3 A. Use the nameplate current whenever it is available — it reflects the actual machine rather than assumed figures.
What should the overload relay be set to? +
In the usual layout the thermal relay sits between the main contactor and the motor, carrying winding current, so set it to 58% of full load current. If the relay is instead fitted in the incoming line ahead of the starter, it sees full line current and is set to 100%. A winding-mounted relay set at full line current leaves the motor effectively unprotected.
What size fuse and MCCB does a star-delta starter need? +
The 175% and 250% figures come from NEC-style motor branch-circuit sizing — they are not universal IEC values. Treat them as maximum calculated references: round to a standard rating without exceeding the ceiling, though some codes permit the next higher standard rating where the calculated value falls between sizes. For an IEC installation, take the device from your local code and the manufacturer's coordination tables instead. These give short-circuit and earth-fault protection for the branch circuit; overload protection remains the thermal relay's job.
How long should the star timer be set for? +
Long enough for the motor to reach roughly 80–90% of full speed in star — a few seconds for most pumps and fans, considerably longer for high-inertia loads. Set it by watching the actual run-up current with a clamp meter, not from a generic figure: too short and the changeover surges, too long and the motor sits in star producing only a third of its torque.
Why must the star and delta contactors be interlocked? +
If both closed together the windings would be shorted phase to phase — a dead short across the supply. Use the electrical interlock (each coil fed through the other contactor's NC auxiliary contact) and the mechanical interlock supplied with the contactor pair, because a welded contact ignores wiring logic.
Can any three-phase motor be used on a star-delta starter? +
No. The motor needs all six winding ends brought out to the terminal box, and it must be designed to run in delta at the supply voltage. A motor marked 400 V delta / 690 V star suits a 400 V star-delta starter; one marked 400 V star does not, since it would run permanently under-voltage.
What is the starting torque of a star-delta starter? +
Theoretically about one third of the direct-on-line starting torque, because torque falls with the square of winding voltage; practical figures of roughly 25–33% are quoted, depending on the motor, the supply and the load. That is why star-delta only suits loads that start light — unloaded pumps, fans, compressors with unloaders, machine tools — and not conveyors, crushers or loaded compressors.
What is open transition and why does it cause a current spike? +
A standard star-delta starter is open transition: star opens before delta closes, so the motor is briefly disconnected. It keeps turning and generates a decaying voltage that drifts out of phase with the supply, so when delta closes the inrush can exceed the DOL current. Keeping the gap short helps; where the surge matters, use a closed-transition or soft starter.
Star-delta starter or soft starter and VFD? +
Star-delta is cheap, robust and familiar, but gives fixed one-third torque, an open-transition surge and needs six motor leads. A soft starter gives an adjustable ramp with no transition surge; a VFD adds speed control and often repays its cost in energy on variable-flow pump and fan duties. New installations increasingly choose electronic starting unless cost is the deciding factor.
Why do star-delta starters need six motor leads? +
The starter reconfigures the windings, so both ends of each winding must be accessible at the terminal box: U1-U2, V1-V2 and W1-W2. A motor with its links bridged internally, offering only three terminals, cannot be started star-delta.
What making and breaking capacity do these contactors need? +
Utilisation category AC-3 defines the making and breaking duty for motor starting, and those values are multiples of the contactor's own rated operational current — not of the motor's full load current. Published AC-3 data is commonly of the order of ten times rated operational current for making and several times it for breaking, but the number that counts is the one in the manufacturer's table for your frame size at your supply voltage. AC-1 and AC-3 ratings are not interchangeable: a motor starter must be selected against the manufacturer's motor-duty (AC-3) rating.
Should the main contactor be 58% or 100% of FLC? +
Electrically 58% is correct — in delta running the main contactor carries winding current. Some panel builders still specify full line current as a margin, or so the same starter can be rewired direct-on-line later. The calculator above offers both bases so the output matches whichever convention your specification follows.
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