Motor full load current
Calculate the full load and starting current of a motor by power: voltage, efficiency, cos φ — and select a C-curve breaker.
Formulas
Full load current: I = P/(√3·U·η·cosφ) for 3-phase, I = P/(U·η·cosφ) for 1-phase. Starting current: Istart = (5–7)×In, up to 8×In for large motors.
Typical values (3-phase 400 V, η=0.85, cosφ=0.85)
| Power | Current | Breaker |
|---|---|---|
| 0.75 kW | 1.5 A | C6 |
| 1.5 kW | 3 A | C6 |
| 4 kW | 8 A | C16 |
| 7.5 kW | 15 A | C20 |
| 15 kW | 30 A | C40 |
| 30 kW | 60 A | C80 |
How to use
Enter power, voltage and circuit. For accuracy use the efficiency and cos φ from the motor nameplate. Select the breaker with C curve and 25% margin.
Related calculators
- Breaker selection — breaker-selector
- Cable size — cable-calculator
- Watts to amps — watts-amps
FAQ
What current does a 4 kW motor draw at 400 V?
I = 4000/(1.73×400×0.85×0.85) ≈ 8 A. Starting ≈ 50 A (6×FLA).
Why a C-curve breaker for motors?
B-curve trips at 3–5×In — the motor inrush (5–8×In) would cause nuisance trips. C-curve allows 5–10×In.
What breaker for a 15 kW motor?
I ≈ 30 A, inrush ≈ 180 A. C40: 180 A < 10×40 = 400 A — works.