Calculate rated current, starting current, overload protection setting, back-up fuse and supply cable recommendation for three-phase and single-phase motors.
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Motor current from nameplate to installation
The rated current follows from shaft power, efficiency and power factor: In = P / (η × √3 × U × cos φ) for three-phase, without √3 for single-phase. This calculator derives everything needed for the installation: starting current, overload relay setting, back-up fuse and a cable recommendation.
Worked example: 4 kW three-phase motor
Nameplate: 4 kW, 400 V, η = 90 %, cos φ = 0.85. Electrical input: 4 / 0.9 = 4.44 kW. Rated current: In = 4444 W / (√3 × 400 V × 0.85) = 7.55 A. The overload relay is set exactly to this value. Direct-on-line starting draws about 7 × In ≈ 53 A for a short time – which is why the back-up fuse is chosen slow-acting with margin (here: 16 A gG).
Starting methods
DOL starting causes the highest inrush (≈ 7 × In). Star-delta reduces it to about one third (≈ 2.3 × In) but needs six conductors. Soft starters limit electronically to ≈ 3 × In; variable frequency drives start almost without overcurrent and add speed control.
Notes
The overload relay is always set to the nameplate current, never higher; with star-delta it sits in the phase and is set to In / √3 ≈ 0.58 × In. The cable recommendation is a conservative guide for copper – verify installation method, grouping, ambient temperature and voltage drop with our cable sizing calculator.
