⚙️ ELECTRICAL

Motor Starter Sizing Calculator (DOL / Star-Delta / VFD)

Size motor starters — DOL, star-delta, or VFD. Get FLC, starting current, contactor rating, OL relay and cable size. IS 8544 / IEC 60947.

📐 Standard: IS 8544 / IEC 60947
✅ Free to use
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ℹ️ About This Calculator

Motor starters control the starting and stopping of electric motors. The right starter type depends on motor size — DOL (Direct Online) for small motors up to 7.5 kW, Star-Delta for 7.5–75 kW, and VFD (Variable Frequency Drive) or soft starters for large motors or those requiring speed control. The starting current of a DOL-started motor is 5–7 times the full-load current — this governs the selection of contactors, fuses, and the generator capacity.

IS 8544 covers starters for induction motors. IEC 60947-4 covers low-voltage switchgear and controlgear — contactors and motor starters. For motors above 7.5 kW started DOL, the starting surge often causes significant voltage dip on the supply (>15%), which affects other connected equipment. Star-delta reduces the starting current to 1/3 of DOL but also reduces starting torque to 1/3 — only suitable for low-torque applications at start (fans, centrifugal pumps). For high-torque applications (compressors, conveyors), use VFD or soft starter.

📐 Motor Starter Sizing Formulas (IS 8544)

IS 8544 / IEC 60947

Full Load Current (FLC):
  I_FL = P / (√3 × V × η × PF)   [3-phase]
  I_FL = P / (V × η × PF)         [1-phase]

Starting Currents:
  DOL:        I_start = 5–7 × I_FL
  Star-Delta: I_start = I_FL × 2 (1/3 of DOL)
  VFD:        I_start = 1.0–1.5 × I_FL (programmable)

Contactor Sizing:
  AC-3 duty: rated ≥ I_FL × 1.25

Overload Relay Setting:
  Range: 0.90 × I_FL to 1.10 × I_FL

Cable: Select from IS 732 table for I ≥ I_FL × 1.25

❓ Frequently Asked Questions

What is the maximum motor size for DOL starting in India? +
IS 8544 and most DISCOMs permit DOL starting for motors up to 7.5 kW on LV (415V) supply. Some utilities in rural feeders limit DOL to 5 kW. Above 7.5 kW, reduced-voltage starting (star-delta, auto-transformer, soft starter, or VFD) is required to limit the starting current surge and voltage dip on the supply.
When should I use a VFD instead of star-delta? +
Use VFD when: (1) you need speed control — HVAC fans, pumps, compressors, (2) the load requires frequent starting/stopping, (3) the motor drives a high-inertia or high-torque load at start, (4) you want to save energy at part-load (pumps and fans follow affinity laws — 50% speed = 12.5% power). Star-delta is adequate for centrifugal pumps and fans where speed control is not needed and starts are infrequent.
What is the contactor duty cycle — AC-1, AC-2, AC-3? +
AC-1: non-inductive loads (resistive heating, lighting). AC-2: slip-ring induction motors. AC-3: squirrel cage induction motors — starting, switching off during running. AC-4: inching and plugging. For all standard motor starters, use AC-3 rated contactors. The AC-3 rating is lower than AC-1 for the same frame — a contactor rated 32A AC-1 may be rated only 22A AC-3.
How do I set the overload relay for a motor? +
Set the overload relay trip current to match the motor's full load current (FLC) from the nameplate. The adjustment range on most relays is ±10% of the selected FLC. For motors with service factor (SF) > 1.0, you can set the relay up to SF × FLC. For motors in hot environments or running continuously on duty, set at 0.9–0.95 × FLC for additional protection.
What fuse size should I use for motor protection? +
IS 13947 and IEC 60947-4 specify Type 1 and Type 2 coordination. For Type 2 (no damage to contactor after fault): use HRC fuses at 2–4× FLC for short-circuit protection (the contactor and overload relay handle overload). Example: 22 kW motor FLC = 42A → use 80A HRC fuses + 45A AC-3 contactor + 42A OL relay.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

⚙️ Motor Starter Sizing Calculator
Reference: IS 8544 / IEC 60947