Ten electrical parameter calculator works out the current a motor or machine draws and sizes the breaker, contactor and cable. Three-phase current I = P / (√3 × U × cos φ × η): a 7.5 kW motor at 400 V, PF 0.85 and 90% efficiency draws 14.2 A — use a 20 A breaker, an 18 A contactor and 2.5 mm² copper cable.
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LiveFull-load currentfull-load current · 3-phase 400 V
- Apparent power
- 9.80 kVA
- Input power
- 8.33 kW
- Circuit breaker
- 20 A (motor-rated: up to 40 A)
- Contactor (AC-3)
- 18 A AC-3
- Copper cable
- 2.5 mm² Cu (≈ AWG 14)
- Voltage drop
- —
- Energy consumption
- 66.7 kWh
Cable size for copper, PVC insulation, in conduit (IEC method B2), 30 °C, three loaded conductors; derate for grouping, high temperature or long runs and follow local electrical code (IEC 60364 / NEC).
Motor current formulas
Three-phase: I = P × 1000 / (√3 × U × cos φ × η)
Single-phase: I = P × 1000 / (U × cos φ × η)
- P rated mechanical (shaft) power in kW (1 hp = 0.7457 kW)
- U supply voltage (phase-to-phase for three-phase)
- cos φ power factor, typically 0.80–0.90
- η motor efficiency, typically 0.85–0.96
The breaker is the next standard size above 125% of full-load current; motor-rated (inverse-time) breakers may be set up to 250% to ride through the starting current. The contactor is chosen by AC-3 rating ≥ full-load current, and the cable carries at least 125% of the current. Voltage drop is checked against 5%.
How to use the Electrical Parameter Calculator
- Wejdź power of the motor or machine (kW or hp).
- Wybierać three-phase or single-phase i voltage.
- Keep the typical power factor I efektywność or use nameplate values.
- Add cable length and operating hours for voltage drop and energy use.
Worked example
7.5 kW motor, 400 V three-phase, cos φ 0.85, η 0.90. Input power = 7.5 / 0.9 = 8.33 kW; apparent power = 8.33 / 0.85 = 9.80 kVA; I = 9,804 / (1.732 × 400) = 14.2 A. Breaker ≥ 1.25 × 14.2 = 17.7 A → 20 A; contactor 18 A; cable for 17.7 A → 2.5 mm² Cu. Eight hours of operation use about 66.7 kWh.
Typical full-load current of three-phase motors
cos φ 0.85, η 0.90.
| Silnik | 230 V | 380 V | 400 V | 480 V |
|---|---|---|---|---|
| 1.5 kW (2.0 hp) | 4.9 A | 3.0 A | 2.8 A | 2.4 A |
| 2.2 kW (3.0 hp) | 7.2 A | 4.4 A | 4.2 A | 3.5 A |
| 4 kW (5.4 hp) | 13.1 A | 7.9 A | 7.5 A | 6.3 A |
| 5.5 kW (7.4 hp) | 18.0 A | 10.9 A | 10.4 A | 8.6 A |
| 7.5 kW (10.1 hp) | 24.6 A | 14.9 A | 14.2 A | 11.8 A |
| 11 kW (14.8 hp) | 36.1 A | 21.8 A | 20.8 A | 17.3 A |
| 15 kW (20.1 hp) | 49.2 A | 29.8 A | 28.3 A | 23.6 A |
| 22 kW (29.5 hp) | 72.2 A | 43.7 A | 41.5 A | 34.6 A |
| 30 kW (40.2 hp) | 98.4 A | 59.6 A | 56.6 A | 47.2 A |
| 37 kW (49.6 hp) | 121.4 A | 73.5 A | 69.8 A | 58.2 A |
| 45 kW (60.3 hp) | 147.7 A | 89.4 A | 84.9 A | 70.8 A |
| 55 kW (73.8 hp) | 180.5 A | 109.2 A | 103.8 A | 86.5 A |
| 75 kW (100.6 hp) | 246.1 A | 149.0 A | 141.5 A | 117.9 A |
Frequently asked questions
How do I calculate the current of a three-phase motor?
I = P × 1000 / (√3 × U × cos φ × η). For an 11 kW motor at 400 V with cos φ 0.85 and 90% efficiency: 11,000 / (1.732 × 400 × 0.85 × 0.9) = 20.8 A.
What size breaker do I need for a motor?
At least 125% of the full-load current, rounded up to the next standard size. Motors with heavy starting loads may need a motor-protection or inverse-time breaker set up to 250% of full-load current.
What cable size do I need?
Size the copper cable for 125% of the full-load current using your local code tables, then check voltage drop over the cable length (keep it below 3–5%). Long runs often need a larger cable than current alone suggests.
What power supply does a press brake need?
Hydraulic press brakes of 40–300 t typically have 5.5–30 kW main motors plus controls; servo-electric press brakes draw less. Use the machine’s total connected power in this calculator to size the supply.
Reviewed by the HARSLE engineering team · Last updated October 2026. Results are engineering estimates; confirm critical values with your machine and tooling documentation.
Full-load current—
