Star Delta Contactor Size Calculator
Calculate Main, Delta and Star contactor sizes for star-delta starters using motor power, voltage and standard industrial sizing methods for accurate MCC and panel design.
Determine main contactor rating, delta contactor size and star contactor capacity for industrial star-delta motor starters instantly.
๐ Free Tool ยท No Signup ยท Instant Results
Star Delta Contactor Size Calculator
Results shown are estimates. IEC ratings based on standard AC-3 motor duty sizes. Sizing is compliant with industrial control panel design parameters.
How to Use Star Delta Contactor Size Calculator
Ensure precise switchgear specification by following these straightforward star-delta starting sizing instructions:
- Step 1: Enter motor power directly from the motor nameplate.
- Step 2: Select kW or HP unit to apply the correct conversion factor.
- Step 3: Enter voltage (V) of the three-phase power supply.
- Step 4: Enter power factor and efficiency percentages.
- Step 5: Click calculate to process calculations instantly.
- Step 6: Review main, delta and star contactor sizing ratings.
- Step 7: Select nearest IEC standard size above calculated limits.
How to Calculate Star Delta Contactor Size
Designing electrical panels with a star delta configuration requires calculating motor FLC and then applying the starter current path factors.
Step 1: Calculate motor current:
Where: P = motor power in Watts (1 kW = 1000 W, 1 HP = 746 W), V = line-to-line voltage, PF = power factor, and ฮท = nominal motor efficiency (decimal).
Step 2: Determine main contactor:
The main contactor carries winding phase current (58% of FLC) and is typically adjusted by a safety factor (default 125%) to handle standard continuous load parameters.
Step 3: Determine delta contactor:
The delta winding contactor carries winding phase current (58% of FLC) and, like the main contactor, is adjusted by the safety factor for matching duty.
Step 4: Determine star contactor:
The star contactor is active only during starting transient under reduced line voltages, carrying exactly 1/3 (or 33%) of the motor's total current FLC.
Engineering Notes:
- Main contactor carries approximately 58% motor line current when at speed.
- Delta contactor carries approximately 58% motor line current.
- Star contactor carries approximately 33% current during start.
Star Delta Contactor Size Chart
Use this reference sizing chart to quickly identify recommended standard IEC ratings for common motor horsepower capacities operating under three-phase 415V supply voltages (efficiency 90%, power factor 0.85, safety factor 125%):
| Motor Power | Voltage | FLC (Amps) | Main Contactor (Min/Std) | Delta Contactor (Min/Std) | Star Contactor (Min/Std) |
|---|---|---|---|---|---|
| 3 HP | 415V | 4.1 A | 3.0 A (9A) | 3.0 A (9A) | 1.4 A (3A) |
| 5 HP | 415V | 6.8 A | 4.9 A (9A) | 4.9 A (9A) | 2.2 A (4A) |
| 7.5 HP | 415V | 10.2 A | 7.4 A (9A) | 7.4 A (9A) | 3.4 A (6A) |
| 10 HP | 415V | 13.6 A | 9.9 A (12A) | 9.9 A (12A) | 4.5 A (9A) |
| 15 HP | 415V | 20.4 A | 14.8 A (18A) | 14.8 A (18A) | 6.7 A (12A) |
| 20 HP | 415V | 27.1 A | 19.7 A (25A) | 19.7 A (25A) | 9.0 A (12A) |
| 25 HP | 415V | 33.9 A | 24.6 A (25A) | 24.6 A (25A) | 11.2 A (18A) |
| 30 HP | 415V | 40.7 A | 29.5 A (32A) | 29.5 A (32A) | 13.4 A (22A) |
| 50 HP | 415V | 67.8 A | 49.2 A (50A) | 49.2 A (50A) | 22.4 A (32A) |
| 75 HP | 415V | 101.7 A | 73.8 A (80A) | 73.8 A (80A) | 33.6 A (50A) |
Note: Main and Delta contactor sizes carry 58% of FLC and include a 125% safety factor. Star is sized at 33% of FLC. Values inside parentheses represent typical commercial AC-3 contactor ratings.
Star Delta Contactor Size FAQs
Main contactor carries approximately 58% motor current (winding phase current) during normal delta-speed operation as it is connected inside the motor phase winding loop.
Delta contactor carries approximately 58% motor current since it is connected inside the motor phase winding loop rather than in the main supply feed.
Star contactor carries approximately 33% current as it is only active during initial star-mode starting transients to provide reduced voltage.
Normally no. Main, delta and star contactors have different duties. However, in standard industrial starter panels, engineers sometimes select identical Main and Delta contactors (sized at 58% of FLC, or up to 100%) to simplify inventory and parts matching, though the Star contactor is always sized smaller.
IEC 60947-4-1 (Low-voltage Switchgear - Motor Starters), IEEE standard guides, and NEC Article 430 practices are commonly used to coordinate contactor ratings with overload and short-circuit breakers.
More Electrical & Panel Design Calculators
Star Delta Starter Component Calculator
Calculate ratings for MCCBs, overload relays, and timers alongside contactor specifications.
Open CalculatorCable Size Calculator
Determine standard copper and aluminum cable sizes for distribution and motor loads.
Open CalculatorMotor Current Calculator
Determine nominal induction motor full load current ratings for standard operating voltages.
Open CalculatorMCC Panel Load Calculator
Calculate load demands, busbar sizing, and feeder breaker protection parameters.
Open Calculator