Current and Breaker Size Calculator from Active Power
Calculate the approximate line current of motors, heaters and other electrical loads from their active power.
Formula used
Three-phase: I = P / (√3 × U × cos φ × η)
Current by power · three-phase 400 V (cos φ 0.90, efficiency 0.90)
Formula results for cos φ 0.90 and efficiency 0.90 only. For motors, cos φ and efficiency vary with power; at small and medium ratings the nameplate current is higher than this value. Motor calculations and protection selection use the nameplate current.
| Active power (kW) | Line current (A) |
|---|---|
| 0.37 | 0.66 |
| 0.55 | 0.98 |
| 0.75 | 1.34 |
| 1.1 | 1.96 |
| 1.5 | 2.67 |
| 2.2 | 3.92 |
| 3 | 5.35 |
| 4 | 7.13 |
| 5.5 | 9.80 |
| 7.5 | 13.36 |
| 11 | 19.60 |
| 15 | 26.73 |
| 18.5 | 32.97 |
| 22 | 39.20 |
| 30 | 53.46 |
| 37 | 65.93 |
| 45 | 80.19 |
| 55 | 98.01 |
| 75 | 133.65 |
| 90 | 160.38 |
| 110 | 196.01 |
| 132 | 235.22 |
| 160 | 285.11 |
Current by power · single-phase 230 V (resistive load)
For resistive loads such as heaters, cos φ 1.00 and efficiency 1.00 are assumed. The last column is the next standard rated current; the protective device is selected separately together with cable capacity and tripping curve.
| Active power (kW) | Line current (A) | Next standard rated current (A) |
|---|---|---|
| 0.5 | 2.17 | 3 |
| 1 | 4.35 | 6 |
| 1.5 | 6.52 | 8 |
| 2 | 8.70 | 10 |
| 2.5 | 10.87 | 13 |
| 3 | 13.04 | 16 |
| 3.5 | 15.22 | 16 |
| 4.5 | 19.57 | 20 |
| 5.5 | 23.91 | 25 |
| 7.5 | 32.61 | 40 |
Example: how many amps does an 11 kW, 400 V three-phase load draw?
- Data: P = 11 kW, U = 400 V, cos φ = 0.90, η = 0.90.
- Calculation: I = 11,000 W / (√3 × 400 V × 0.90 × 0.90) = 19.60 A.
- The next standard rated current is 20 A. For motors, the nameplate current is decisive; starting current, tripping curve and cable capacity are assessed separately.
How is current calculated from active power?
Active power, operating voltage, power factor and efficiency are used together. Single-phase lines have no √3 factor; for direct current the power factor drops out of the equation.
How should the breaker rating be read?
The next standard rating is only a preliminary choice. Cable capacity, tripping curve, starting current, breaking capacity, short-circuit level and selectivity must be verified separately.