Load and motor current from power
Operating current of a three-phase or single-phase load from its power; for a motor, from the shaft power and efficiency, with the starting current. Used to select the cable, circuit breaker and starter.
3~: I = P / (√3 · U · η · cos φ)1~: I = P / (U · η · cos φ)I_st = K_i · IThe motor current is at rated shaft load; at part load it is lower, but not in proportion: the no-load current of an induction motor is 20–50 % of the rated one. The cable and protection are selected on the nameplate current; the starting current matters for the circuit breaker and for the voltage dip at starting. In the presets the efficiencies are the IE3 minimums of IEC 60034-30-1, cos φ and the starting current ratio are typical catalogue values.
Source: Three-phase power; motor efficiency classes per IEC 60034-30-1
Inputs
You can change a field's unit: the value is converted to the formula's units automatically.
Line-to-line voltage for three phases, phase voltage for a single phase.
Induction motors at rated load 0.80–0.90, lightly loaded 0.3–0.6; heaters 1.
Direct-on-line start of an induction motor: 5–8 (from the catalogue); soft starter 2–4; variable frequency drive 1–1.5; other loads 1.
Unit converter for this formulaPower · Voltage · Current · Apparent power · Fraction and percent
- kW1
- W1,000
- MW0.001
- PS (metric hp)1.35962
- kcal/h859.845
- hp1.34102
- BTU/h3,412.14
PS is the metric horsepower (735.5 W), hp the mechanical horsepower (745.7 W).
- P_in — Active power drawn–
- S — Apparent power–
- I_st — Starting current–
More in Power and current
Results are engineering estimates from standard formulas; for design decisions check them against the codes, project documents and specialists' calculations. The formulas carried over from the original set are unchanged, and their errors are described in the notes.