Industrial Motor Starter & Protection Sizing Calculator

Calculates motor full load current (FLA), coordinates AC-3 contactor duty ratings (Main, Delta, Star), sets thermal overload relay calibration bands, and estimates backup MCCB / MPCB ratings for 3-phase induction motor feeders per IS/IEC 60947-4-1.

✔ 1. Formula Verified → ⚡ 2. Engineering Screening → 📐 3. Standards Referenced (IS/IEC 60947-4-1) → ⚖ 4. Engineer Review Required

⚙ Motor Nameplate & Starter Selection

Motor Rated Mechanical Power kW

Motor shaft output power rating (e.g. 45 kW = ~60 HP).

Operating Line Voltage (VL) Volts

Starter Technology Topology Type

Motor Full-Load Efficiency (η) Ratio

Energy efficiency class per IS 12615: Standard (IE2 ≈ 0.91), Premium (IE3 ≈ 0.93), Super Premium (IE4 ≈ 0.95).

Full-Load Power Factor (cos φ) cos φ

Motor rated full load power factor from manufacturer nameplate (typically 0.84–0.88 for 4-pole induction motors).

📊 Motor Sizing & Protection Sheet

Preliminary Screening

Motor Full-Load Current (FLA)
78.3 Amps
Suggested Backup MCCB/MPCB: 120 A (Type 2 Coordination Screening)

Main Contactor (AC-3)

45.3 A

Delta Contactor (AC-3)

45.3 A

Star Contactor (AC-3)

26.1 A

Overload Relay Range

38.5 – 52.1 A

Minimum Feeder Cable Ampacity

78.3 A (Continuous)

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Governing Engineering Standards & Calculation Model

Governing Standards & References

IS/IEC 60947-4-1: Low-voltage switchgear and controlgear — Contactors and motor-starters.
IS 12615: 2018: Line operated three-phase induction motors — Efficiency classes (IE code).
IS/IEC 60947-2: Low-voltage switchgear and controlgear — Circuit-breakers (MCCBs).

Engineering Equations & Derivations

I_FLA = (kW × 1000) / (√3 × V_L × η × cosφ)
Star-Delta Main & Delta Contactor = I_FLA / √3 ≈ 0.58 × I_FLA
Star-Delta Star Contactor = I_FLA / 3 ≈ 0.33 × I_FLA
DOL Main Contactor = 1.0 × I_FLA (AC-3 Duty)
Overload Relay Range = 0.85 to 1.15 × Contactor Current

Backup MCCB/MPCB sizing: Screened at $1.5\times I_{\text{FLA}}$ for Star-Delta and $2.0\times I_{\text{FLA}}$ for DOL to prevent nuisance tripping during inrush starting currents.

⚖ Statutory Notice & Engineering Disclaimer (Tier D Integration)

Contactor ratings and suggested MCCB amperages provide preliminary component sizing. Per IS/IEC 60947-4-1, complete feeder assemblies require Type 2 Coordination verification (where no damage or welding to contactors/relays occurs during short-circuit faults) based on manufacturer certified coordination tables (e.g. Schneider, ABB, L&T, Siemens). High-inertia loads (pumps, blowers, heavy crushers) require extended thermal trip class 20/30 overload relays and derated starting cycle checks.