Star Delta Starter Component Calculator
Calculate contactor size, overload relay rating, MCCB size and timer selection required for a star delta motor starter panel quickly and accurately.
Star Delta Starter Component Calculator
Results shown are estimates matching standard industrial guidelines. Always consult specific motor nameplates, manufacturer component catalogs, and local electrical codes before panel wiring.
How to Use Star Delta Starter Component Calculator
Follow these quick, practical instructions to find the recommended size for contactors, thermal relays, main circuit breakers, and timers for a standard motor starter control panel:
- Step 1: Select motor power unit. Switch between kW (Kilowatts) and HP (Horsepower) using the segmented unit toggle.
- Step 2: Enter nominal power value. Input the rated output capacity of your induction motor directly from its nameplate.
- Step 3: Select supply operating voltage. Choose your exact three-phase operating voltage (400V, 415V, 440V, or 480V).
- Step 4: Input efficiency and power factor. Verify or adjust nameplate efficiency percentage (default 90%) and power factor (default 0.85 cos φ).
- Step 5: Click Calculate. Press the button to instantly render standard industrial hardware sizing recommendations.
- Step 6: Review recommended components. Use the outputs to source the Main, Delta, and Star contactors, thermal overload relay (OLR) range, MCCB rating, and starter transition timer duration.
How to Calculate Star Delta Starter Components
Star Delta starter component selection relies on evaluating the motor's nominal full load current (FLC), applying exact branch winding sizing ratios, and choosing standard industrial rating sizes.
Primary Star Delta Sizing Formulas
1. Motor Full Load Current (FLC):
Where: FLC = Full Load Current (Amps), Power = Power (Watts, note that 1 kW = 1000 Watts and 1 HP = 746 Watts), Voltage = Operating Winding Line Voltage (Volts), PF = Power Factor, and Efficiency = Nominal motor efficiency expressed as a decimal.
2. Main Contactor Sizing:
In Star Delta configuration, the Main contactor only carries winding phase current (FLC divided by 1.732) when at full speed, requiring about 58% rating. Select the next higher standard contactor size from: 9A, 12A, 18A, 25A, 32A, 40A, 50A, 65A, 80A, 95A, 115A, 150A, 185A, 225A, or 265A.
3. Delta Contactor Sizing:
In Star Delta configuration, the Delta contactor only carries winding phase current (FLC divided by 1.732) when at full speed, requiring about 58% rating. Select the next higher standard contactor size.
4. Star Contactor Sizing:
The Star contactor is active only during startup under reduced voltage and carries about 33% (1/3) of the nominal current load. Select the next higher standard rating size.
5. Overload Relay Range:
When installed directly in the phase winding branch under the main contactor, set the thermal overload relay at 58% of FLC. Choose a commercial range that encloses this setting.
6. MCCB Sizing:
Sizes the thermal-magnetic molded case circuit breaker to handle transient starting currents. Select the nearest standard rating size from: 16A, 20A, 32A, 40A, 63A, 80A, 100A, 125A, 160A, 200A, 250A, or 315A.
Complete Example Calculation: 30kW Motor
Let's calculate all component recommendations for a standard industrial three-phase induction motor using our star delta starter component calculator rules:
- Motor Power: 30 kW
- Motor Voltage: 415 V
- Power Factor (PF): 0.85
- Efficiency: 90% (0.90)
Step 1: Calculate Motor FLC:
FLC = 30000 / 549.91
FLC ≈ 54.55 A (Round to 55 A)
Step 2: Size the Main Contactor:
Next Higher Standard Contactor (9, 12, 18, 25, 32, 40, 50, 65, 80...) = 32 A
Step 3: Size the Delta Contactor:
Next Higher Standard Contactor = 32 A
Step 4: Size the Star Contactor:
Next Higher Standard Contactor = 25 A
Step 5: Determine Overload Relay (OLR) Range:
Suggested Commercial Range = 32–50 A (Enables setting dial adjustment)
Step 6: Size the MCCB Breaker:
Nearest Standard Rating (16, 20, 32, 40, 63, 80, 100...) = 80 A
Step 7: Sizing Starter Timer Recommendation:
For a 30kW motor rating, standard starting dynamics call for a transition delay of 8–12 seconds to let the motor reach about 80% full load speed before delta switching.
👉 30 kW Sizing Summary: For a 30 kW motor at 415V, use a 32A Main Contactor, 32A Delta Contactor, 25A Star Contactor, a 32–50A Thermal Relay, an 80A MCCB Breaker, and an 8–12 second transition timer.
International Standards for Star Delta Starter Component Selection
Electrical control panel design and switchgear selection for star delta starters are governed by prominent international standards to ensure safety, reliability, and proper coordination under starting currents.
IEC 60947-4-1 (International Electrotechnical Commission)
This is the primary international standard regulating low-voltage switchgear, magnetic contactors, and motor starters.
- Contactor Sizing Selection: Specifies that contactors must be rated based on the AC-3 utilization category (starting of squirrel-cage motors). The Main and Delta contactors are sized to carry the winding phase current, which is 58% of the motor's full load current (0.58 × FLC), and the Star contactor is sized for 33% of the motor's full load current (0.33 × FLC).
- Overload Relay Selection: Governs thermal and electronic overload relays, specifying Trip Class (Class 10, 20, or 30) depending on the motor starting duration in star configuration to prevent premature tripping during startup.
- Coordination Selection (Type 1 & Type 2): Defines safety boundaries during short-circuit faults between contactors, overload relay, and circuit breaker. Type 2 coordination is highly recommended in industrial applications; it mandates that after a short-circuit fault, no damage or welding of contactor contacts is allowed, and the starter must be immediately operational.
NEC Article 430 (National Electrical Code)
Governs the installation, protection, and sizing of motor circuits and controllers in North America.
- Magnetic Contactor/Controller Sizing (NEC 430.83): Mandates that motor magnetic controllers (starters) must have a horsepower rating not less than the horsepower rating of the motor. For star delta controllers, the rating applies to the combined starter assembly.
- Circuit Breaker Selection (NEC 430.52): Limits the maximum rating of inverse-time molded case circuit breakers (MCCB) protecting the starter to 250% of the motor's line FLC, although star delta starting reduces current inrush, allowing design engineers to optimize selection at 150%.
NEMA ICS 2 (National Electrical Manufacturers Association)
Establishes standard sizes and criteria for magnetic contactors and overload relays used in motor starting controllers.
- Magnetic Contactor Sizes: Classifies contactors into standard NEMA physical size classes (such as NEMA Size 1, 2, 3, etc.) representing current-carrying limits under AC-3 squirrel-cage starting categories.
- Thermal Overload Relays: Governs requirements for thermal overload heaters selection inside NEMA overload relays to protect motor branch circuit components.
🛡️ Design Note: Always verify coordination tables provided by switchgear manufacturers (such as ABB, Siemens, or Schneider Electric) to ensure that the contactors, overload relay, and MCCB selected are certified for Type 2 coordination under IEC 60947-4-1.
Star Delta Starter Component Size Chart
Use this reference table to find standard industrial sizing recommendations for three-phase induction motor starters operating at 415V under common load conditions (power factor 0.85, efficiency 90%):
| Motor Power (kW) | Motor Current (A) | MCCB Size | Main Contactor | Delta Contactor | Star Contactor | OLR Range |
|---|---|---|---|---|---|---|
| 5.5 kW | 10.0 A | 16 A | 9 A | 9 A | 6 A | 5.5–8.0 A |
| 7.5 kW | 13.6 A | 20 A | 9 A | 9 A | 6 A | 7.0–10.0 A |
| 11 kW | 20.0 A | 32 A | 12 A | 12 A | 9 A | 12.0–18.0 A |
| 15 kW | 27.3 A | 40 A | 18 A | 18 A | 12 A | 12.0–18.0 A |
| 22 kW | 40.0 A | 63 A | 25 A | 25 A | 18 A | 23.0–32.0 A |
| 30 kW | 54.6 A | 80 A | 32 A | 32 A | 25 A | 32.0–50.0 A |
| 37 kW | 67.3 A | 100 A | 40 A | 40 A | 25 A | 32.0–50.0 A |
| 45 kW | 81.8 A | 125 A | 50 A | 50 A | 32 A | 48.0–65.0 A |
| 55 kW | 100.0 A | 160 A | 65 A | 65 A | 40 A | 55.0–70.0 A |
| 75 kW | 136.4 A | 200 A | 80 A | 80 A | 50 A | 80.0–93.0 A |
| 90 kW | 163.7 A | 250 A | 95 A | 95 A | 65 A | 90.0–120.0 A |
| 110 kW | 200.0 A | 315 A | 115 A | 115 A | 80 A | 110.0–150.0 A |
* All table lookup figures are typical values calculated for induction motors operating on 415V supply voltages. Always check specific motor nameplate ratings for accurate protective sizing.
Star Delta Starter Component Frequently Asked Questions
In a standard star delta starter, both the Main contactor and the Delta contactor are connected inside the motor winding delta loop. Because they are inside the loop, the current through each contactor is the phase current (which is line FLC divided by 1.732, or approximately 58%), rather than the full line current. Sizing both at 58% of FLC provides safe, accurate performance.
The star contactor is active only during the starting transient, during which the motor's winding voltage and current are reduced. It carries about 33% of the motor's nominal full load line current, allowing the selection of a lighter and more economical standard rating contactor size.
A star delta starter reduces the starting current to about 33% (1/3) of the normal direct-on-line (DOL) starting current. This protects the local supply grid from heavy voltage drops and reduces mechanical stress on motor shafts, belts, and couplings during startup.
The overload relay is placed directly in series with the motor windings, so it only carries winding phase current rather than line current. Therefore, adjust the overload relay setting to approximately 58% of the motor's nameplate FLC line current to ensure accurate motor protection.
Star delta starting is typically recommended for three-phase induction motors with ratings above 5 HP to 10 HP (3.7 kW to 7.5 kW) where DOL starting currents would cause excessive line voltage drop or where mechanical torque spikes need to be dampened.