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 |
Frequently asked questions about breaker selection
Which breaker to install for 220V sockets?
Standard solution is a B16 or C16 breaker with 2.5 mm² (Cu) cable. For a dedicated line for a high-power appliance (water heater up to 4.5 kW, oven up to 3.5 kW) — C20 or C25 with 4 mm² cable. The breaker rating must not exceed the cable's ampacity.
What is the difference between B, C, and D characteristics of circuit breakers?
The difference is the instantaneous tripping threshold during short circuit. B trips at 3–5 times rated current — for residential circuits. C at 5–10 times — for circuits with air conditioners, pumps, compressors. D at 10–20 times — for welding machines and transformers.
How to ensure selectivity of breakers in a panel?
Minimum condition: the main breaker rating must be at least one or two steps higher than the branch breakers. Example: main C40, branch B16 and B25. For full selectivity: main — C or D, branch — B.
Why does a C16 breaker not protect a 1.5 mm² cable?
A 1.5 mm² cable in a wall conduit allows continuous current of 15 A. A C16 breaker at 1.13×In (≈18 A) may not trip within an hour. During this time, the cable operates in overload. The correct pair is a B10 breaker with 1.5 mm² cable.
What main breaker is needed for a private house with three-phase 15 kW connection?
With allocated power of 15 kW and three-phase 380V, the current per phase is ~23 A. Standard main breaker is C25 (3-pole). If allowed power is 20 kW — C32. The main breaker rating is often specified in the contract with the utility provider.
How to calculate breaker rating based on load power?
Formula: I = P / (U × cos φ). For single-phase 220V and cos φ = 1: I = P / 220. Example: water heater 2 kW → I = 9.1 A → breaker B10. For motors (cos φ = 0.8): same heater → I = 11.4 A → breaker C16. Select rating such that In ≥ I_load and In ≤ Iz cable.
Tools
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Cable cross-section calculation
Cable cross-section selection by power, current and installation method per the app's reference table.
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RCD or RCBO type, rating and differential current selection per IEC 60364.
House power calculator
Total load calculation by appliance list. Recommendations for incoming MCB, cable and connection type (1ph/3ph).
Assemble a complete panel with breakers online
In ElectroBoard, breakers are selected automatically — with coordination, selectivity, and IEC 60364 compliance checks.