Star Delta Contactor Size Calculator
Size the main, delta and star contactors of a star-delta starter, plus the overload relay setting and cable current, using IEC 60947-4-1 practice or NEC Article 430 with Table 430.250 motor current.
Star Delta Contactor Size Calculator
How to Use Star Delta Contactor Size Calculator
- 1Choose the standard. IEC uses the calculated motor current. NEC uses the Table 430.250 current when you enter HP at 200, 208, 230, 460 or 575 V.
- 2Enter the motor. Power, line voltage, efficiency and power factor. The motor must be connected in delta at this line voltage (for example a 400 V Δ / 690 V Y motor on a 400 V supply).
- 3Read the three contactors. The calculator also gives the overload relay setting and the current the cables must carry.
How to Calculate Star Delta Contactor Size
A star-delta starter runs the motor in delta through two contactors, so each carries the winding (phase) current rather than the line current. In star, each winding sees 1/√3 of the line voltage, which cuts the starting line current and the starting torque to about one third of their direct-on-line values.
Main (KM1) = Delta (KM2) = 0.58 × FLC × margin
Star (KM3) = 0.33 × FLC
Overload relay in the winding circuit = 0.58 × FLC
P in watts (1 kW = 1000 W, 1 HP = 746 W), V = line voltage, PF = cos φ, η = efficiency as a decimal. 0.58 ≈ 1/√3.
| Contactor | Closed during | Current carried | Sizing basis (AC-3) |
|---|---|---|---|
| Main KM1 | Star and delta | Phase current, FLC / √3 | ≥ 0.58 × FLC |
| Delta KM2 | Delta (run) | Phase current, FLC / √3 | ≥ 0.58 × FLC |
| Star KM3 | Start only (seconds) | Star current, about 1/3 of the direct-on-line value | ≥ 0.33 × FLC for normal start times |
Main and delta are usually the same frame. For long run-up times (high-inertia fans, loaded starts) manufacturers select a larger star contactor; check their table. The margin input applies only to KM1 and KM2; leave it at 100% for the smallest compliant size.
IEC example: 30 kW, 400 V
KM1 and KM2: 0.58 × 56.60 × 1.25 = 41.04 A → 50 A (no margin: 32.83 A → 40 A)
KM3: 0.33 × 56.60 = 18.68 A → 25 A
Overload relay (after KM1): 0.58 × 56.60 = 32.83 A
NEC example: 50 HP, 460 V
KM1 and KM2: 0.58 × 65 × 1.25 = 47.1 A → 50 A AC-3 frame
KM3: 0.33 × 65 = 21.45 A → 25 A
Overload relay in the winding circuit: 0.58 × FLA (37.7 A if FLA = 65 A)
Conductors: line side ≥ 1.25 × 65 = 81.25 A; controller to motor ≥ 1.25 × 0.58 × 65 = 47.1 A (430.22(C))
Under the NEC, the starter as a whole must be horsepower-rated for the motor at 460 V (430.83). A pre-engineered NEMA wye-delta starter is rated by HP; when building from individual contactors, use the manufacturer's wye-delta table.
Star Delta Contactor Size Chart
IEC: 400 V motors (PF 0.85, efficiency 90%, 125% margin on KM1/KM2)
| Motor (kW) | FLC (A) | KM1 / KM2 min (A) | KM1 / KM2 | KM3 min (A) | KM3 | Overload setting (A) |
|---|---|---|---|---|---|---|
| 7.5 | 14.2 | 10.3 | 12 A | 4.7 | 9 A | 8.2 |
| 11 | 20.8 | 15.0 | 18 A | 6.8 | 9 A | 12.0 |
| 15 | 28.3 | 20.5 | 25 A | 9.3 | 12 A | 16.4 |
| 18.5 | 34.9 | 25.3 | 32 A | 11.5 | 12 A | 20.2 |
| 22 | 41.5 | 30.1 | 32 A | 13.7 | 18 A | 24.1 |
| 30 | 56.6 | 41.0 | 50 A | 18.7 | 25 A | 32.8 |
| 37 | 69.8 | 50.6 | 65 A | 23.0 | 25 A | 40.5 |
| 45 | 84.9 | 61.6 | 65 A | 28.0 | 32 A | 49.2 |
| 55 | 103.8 | 75.2 | 80 A | 34.2 | 40 A | 60.2 |
| 75 | 141.5 | 102.6 | 115 A | 46.7 | 50 A | 82.1 |
| 90 | 169.8 | 123.1 | 150 A | 56.0 | 65 A | 98.5 |
| 110 | 207.5 | 150.5 | 185 A | 68.5 | 80 A | 120.4 |
NEC: 460 V motors (Table 430.250 FLC, 125% margin on KM1/KM2)
| Motor (HP) | FLC (A) | KM1 / KM2 min (A) | KM1 / KM2 | KM3 min (A) | KM3 | 0.58 × FLC (A) |
|---|---|---|---|---|---|---|
| 10 | 14 | 10.15 | 12 A | 4.6 | 9 A | 8.1 |
| 20 | 27 | 19.6 | 25 A | 8.9 | 9 A | 15.7 |
| 30 | 40 | 29.0 | 32 A | 13.2 | 18 A | 23.2 |
| 40 | 52 | 37.7 | 40 A | 17.2 | 18 A | 30.2 |
| 50 | 65 | 47.1 | 50 A | 21.4 | 25 A | 37.7 |
| 75 | 96 | 69.6 | 80 A | 31.7 | 32 A | 55.7 |
| 100 | 124 | 89.9 | 95 A | 40.9 | 50 A | 71.9 |
| 150 | 180 | 130.5 | 150 A | 59.4 | 65 A | 104.4 |
| 200 | 240 | 174.0 | 185 A | 79.2 | 80 A | 139.2 |
Frame steps follow a common AC-3 catalogue series and vary by manufacturer. In NEC applications confirm each contactor's UL horsepower rating, or use a listed wye-delta starter.
Common Star Delta Sizing Mistakes
- Motor not rated for delta at the supply voltage. A 230 V Δ / 400 V Y motor on a 400 V supply cannot be started star-delta; it needs a 400 V Δ / 690 V Y winding.
- Wrong overload setting for its position. A relay after KM1 (in the winding circuit) is set to 0.58 × FLC. A relay in the supply line is set to FLC. Swapping these gives no protection or nuisance trips.
- Six motor cables sized for line current. They carry phase current; size them for 0.58 × FLC (NEC 430.22(C) applies 58% of FLC to these conductors). The supply cable still carries full FLC.
- No KM2/KM3 interlock. Star and delta closing together is a phase-to-phase short circuit. Use mechanical and electrical interlocks and a short changeover delay.
- Changeover too early. Switching to delta well below full speed gives a current peak close to direct-on-line. Set the timer so the motor reaches near rated speed in star; if the load torque is too high for 1/3 starting torque, consider a soft starter or VFD.
- Undersized star contactor for long starts. The 0.33 × FLC rule assumes a normal start time; heavy loads need the manufacturer's long-start selection.
Star Delta Contactor Size FAQs
In delta, each contactor is in series with one motor winding, so it carries phase current. In a delta connection the phase current is the line current divided by √3, which is about 58% of the motor full-load current.
It only closes during starting, when the windings see 1/√3 of line voltage and the current is about one third of the direct-on-line value. It opens before the delta contactor closes, so manufacturers size it at about one third of FLC for normal start times.
If the relay is after the main contactor, in the winding circuit (the usual position), set it to 0.58 × motor FLC. If it is in the supply line before the starter, set it to the full FLC. Under the NEC use the nameplate FLA, with a maximum trip of 125% or 115% of it per 430.32(A)(1).
Starting line current and starting torque both fall to about one third of their direct-on-line values. A motor with a locked-rotor current of 7 × FLC therefore draws roughly 2.3 × FLC in star, plus a short peak at the changeover to delta.
IEC 60947-4-1 covers contactors and starters, including AC-3 ratings and Type 1/Type 2 coordination. In North America, NEC Article 430 sets motor current (430.6), conductor sizing including the 58% rule for wye-delta motors (430.22(C)), overload protection (430.32) and controller ratings (430.83).
Standards and References
The formulas and reference values on this page follow these standards. For design work, check results against the current edition that applies in your country and the equipment manufacturer's data.
- IEC 60947-4-1 – Low-voltage switchgear and controlgear: contactors and motor-starters (AC-3 utilisation category, Type 1 and Type 2 coordination). Published by IEC.
- IEC 60364-4-43 – Protection against overcurrent (IB ≤ In ≤ Iz), used for the cable current basis. Published by IEC.
- NFPA 70 (NEC) Article 430 – Motors and controllers: 430.6(A), 430.22(C), 430.32(A)(1), 430.83 and Table 430.250. Published by NFPA.
- NEMA ICS 2 – Industrial control: controllers, contactors and overload relays, including horsepower-rated starters. Published by NEMA.
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