Motor Starter Sizing Calculator
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ℹ 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.
NEC covers starters for induction motors. UL 489-4 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 (NEC)
NEC / UL 489
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 NEC table for I ≥ I_FL × 1.25
📝 Worked Calculation Example
Example: A 37 kW, 415V, 3-phase motor with 92% efficiency and 0.87 power factor drives a centrifugal pump. Full load current: I_FL = P/(√3×V×η×PF) = 37000/(1.732×415×0.92×0.87) ≈ 64.3 A. Since the motor exceeds the 7.5 kW DOL limit, a star-delta starter is selected: I_start = 2 × I_FL ≈ 128.7 A (versus 320–450 A if started DOL). The AC-3 contactor is sized at I_FL × 1.25 ≈ 80 A (select a 90 A contactor), and the overload relay is set at approximately 64 A, matching the motor nameplate FLC.