Calculation of circuit breaker (breaker)
Enter consumer power — the calculator will select the rating and characteristic (B/C/D), check MCB-cable coordination, and panel selectivity. Per IEC 60364 and IEC 60898.
⚙️ Load parameters
📊 Recommendation
MCB characteristics
| Characteristic | Trip range | Application |
|---|---|---|
| B | 3–5 In | Residential circuits, resistive loads |
| C | 5–10 In | Motors, mixed loads |
| D | 10–20 In | Transformers, heavy inrush currents |
How to select an MCB?
An MCB protects the cable from overload and short circuit. MCB calculation comes down to two steps: determine the line operating current and select a rating that does not exceed the cable's permissible current.
Main rule: I_load ≤ In of breaker ≤ Iz of cable. The breaker rating must be greater than the operating current but less than the cable's allowable current. Otherwise, the cable overheats before the breaker trips.
Characteristic (B, C, D) determines sensitivity to inrush currents. For residential networks, characteristic B is usually sufficient. For motors and compressors, C or D is needed to prevent nuisance tripping on startup.
Selectivity is when only the nearest breaker to the fault trips during a short circuit, while the upstream breaker remains on. For this, the downstream breaker rating must be lower than the upstream (by at least one step).
Current calculation formula for breaker selection
I = P / (U × cos φ) — for single-phase network
I = P / (√3 × U × cos φ) — for three-phase network
where: P — load power (W), U — voltage (220 or 380 V), cos φ — power factor (residential: 0.95, motors: 0.7–0.85).
Example: boiler 2000 W, 1ph 220V, cos φ = 1.0.
I = 2000 / (220 × 1.0) = 9.1 A → breaker B10 with cable 1.5 mm² (Iz = 15 A) ✓
Coordination rule: I_load ≤ In ≤ Iz of cable. The breaker protects the cable — therefore its rating must not exceed the cable's allowable current. If a B10 breaker is on a 1.5 mm² cable (Iz = 15 A) — all conditions are met.
Standard ratings of circuit breakers — reference table
| Rating (In), A | Typical load | Min. cable cross-section (Cu) | Characteristic |
|---|---|---|---|
| 6 A | Lighting (up to 1.3 kW) | 1.5 mm² | B |
| 10 A | Water heater up to 2 kW, single socket | 1.5 mm² | B |
| 16 A | Socket group (up to 3.5 kW) | 2.5 mm² | B or C |
| 20 A | Water heater up to 4.5 kW, oven | 2.5–4 mm² | C |
| 25 A | Air conditioner, hob | 4 mm² | C |
| 32 A | Electric cooker, instantaneous water heater | 6 mm² | C |
| 40 A | Incoming MCB (flat up to 8 kW) | 10 mm² | C |
| 63 A | Incoming MCB (house up to 14 kW) | 16–25 mm² | C or D |