Motor starter sizing equations
Motor power provides an estimated full-load current. Starter selection then uses the motor nameplate current, starting duty, utilization category, and overload curve.
Istart = IFL × KstartIe ≥ IFL × Kdutywhere:
- IFL
- Motor full-load current[A]
- Pout
- Motor shaft output power[W]
- VLL
- Three-phase line-to-line voltage[V]
- PF
- Motor power factor
- η
- Motor efficiency as a decimal
- Kstart
- Starting-current multiple for the selected starting method
- Ie
- Required contactor utilization-category current rating[A]
Use nameplate current and the manufacturer's tested Type 1 or Type 2 coordination tables for final contactor, overload relay, MPCB, breaker, or fuse selection.
How to select a motor starter
Start with motor nameplate current whenever available. Then select the contactor by AC-3 or AC-4 duty, choose an overload relay whose adjustable range contains the required setting, and coordinate the short-circuit protective device with the starter assembly.
- Confirm nameplate current and voltage.
- Identify AC-3 or AC-4 duty.
- Estimate starting current and acceleration time.
- Select overload range and trip class.
- Use a tested manufacturer coordination table.
Motor starter example
A 7.5 kW, 400 V motor at 0.85 power factor and 90% efficiency draws about 14.2 A by calculation. The actual nameplate current controls the final overload setting and may differ from the estimate.
IFL = 7500 / (√3 × 400 × 0.85 × 0.90)
IFL ≈ 14.2 A
AC-3 vs AC-4 contactor duty
AC-3 is the normal starting and stopping category for squirrel-cage motors that open after reaching speed. AC-4 covers inching, plugging, and reversing, which imposes much more severe making and breaking duty and requires the manufacturer's AC-4 ratings.
Overload and short-circuit coordination
The overload relay protects against sustained overload and phase-related heating; the fuse, breaker, or MPCB handles short-circuit duty within its rating. Final selection must check motor thermal limits, start duration, trip class, cable protection, breaking capacity, and Type 1 or Type 2 coordination.
Assumptions
- Balanced three-phase squirrel-cage motor
- Calculated current is an estimate when nameplate current is unavailable
- Starting-current multiples are planning references by starting method
Important Warnings
- Use the actual motor nameplate current for final overload adjustment and manufacturer motor-coordination tables for contactor and protective-device selection.
- AC-4, reversing, plugging, high cycling, high inertia, VFD output switching, and safety functions require application-specific review.