Motor Protection Calculator
Calculation of rated currents and motor protection device sizing.
READY
motor_protection_calculator.tool
READY
Motor Data
Results
Rated Current (In)
7.81 A
Starting Current
46.8 A (6× In)
Thermal Relay Setting
7.4 – 8.2 A
Main Fuse (gG)
20 A
Recommended Motor Protection Switch
6.3–10 A
Setting: 7.8 A
In = P / (√3 × U × η × cosφ) = 4×1000 / (√3 × 400 × 0.85 × 0.87) = 7.81 A
Motor Protection Switches
| Range | Min (A) | Max (A) | Suitable |
|---|---|---|---|
| 0.1–0.16 A | 0.1 | 0.16 | – |
| 0.16–0.25 A | 0.16 | 0.25 | – |
| 0.25–0.4 A | 0.25 | 0.4 | – |
| 0.4–0.63 A | 0.4 | 0.63 | – |
| 0.63–1 A | 0.63 | 1 | – |
| 1–1.6 A | 1 | 1.6 | – |
| 1.6–2.5 A | 1.6 | 2.5 | – |
| 2.5–4 A | 2.5 | 4 | – |
| 4–6.3 A | 4 | 6.3 | – |
| 6.3–10 A | 6.3 | 10 | ✓ |
| 10–16 A | 10 | 16 | – |
| 16–25 A | 16 | 25 | – |
| 25–32 A | 25 | 32 | – |
| 32–40 A | 32 | 40 | – |
| 40–50 A | 40 | 50 | – |
| 50–63 A | 50 | 63 | – |
| 63–80 A | 63 | 80 | – |
| 80–100 A | 80 | 100 | – |
| 100–125 A | 100 | 125 | – |
| 125–160 A | 125 | 160 | – |
| 160–200 A | 160 | 200 | – |
| 200–250 A | 200 | 250 | – |
| 250–320 A | 250 | 320 | – |
| 320–400 A | 320 | 400 | – |
| 400–500 A | 400 | 500 | – |
// HOW IT WORKS
Motor protection switches are set to the motor's rated current. The rated current follows from power, voltage and power factor.
Rated current (three-phase)
I = P / (√3 * U * cos φ * η)
Example
P = 4 kW, U = 400 V, cos φ = 0.85, η = 0.9 → I ≈ 4000/(1.732·400·0.85·0.9) ≈ 7.5 A.
Notes
- Setting is usually 1.0–1.15× rated current.
- The motor nameplate takes precedence over the calculation.