How to Select the Right Circuit Breaker
Type B, C or D — when each is correct, how to match MCB rating to cable size, and why LED circuits and VFD loads need special consideration.
MCB trip curve types
An MCB has two protection mechanisms: thermal (slow, for sustained overcurrent) and magnetic (fast, for short circuits and large inrush). The trip curve type defines the magnetic threshold — the multiple of rated current that causes instantaneous (magnetic) trip.
Trips instantly at 3–5× rated current. For circuits with no significant inrush — domestic socket and lighting circuits, purely resistive loads.
Use for: Domestic sockets, lighting (non-LED drivers), domestic appliances, office equipment
Trips instantly at 5–10× rated current. For circuits with moderate inrush — LED drivers, fluorescent luminaires, small motors, control circuits with transformers.
Use for: Commercial lighting, LED driver circuits, small motors (pumps, fans), transformers, commercial sockets
Trips instantly at 10–20× rated current. For high inrush loads — large motors, welding equipment, transformers, X-ray machines.
Use for: Large motors, compressors, lift panels, welding equipment, hospital equipment
Similar to Type D but with different thermal characteristics. Less common but specified for some motor and transformer applications in Germany and Austria.
Use for: Motors, transformers (German/Austrian market preference)
Cable protection — MCB must be ≤ cable ampacity
The fundamental rule: the MCB rated current must not exceed the cable current-carrying capacity. The MCB protects the cable, not the load.
| Cable size | Max ampacity (in conduit) | Max MCB rating | Standard MCB used |
|---|---|---|---|
| 1.5 mm² | 16 A | 16 A | B10 or B16 |
| 2.5 mm² | 20 A | 20 A | B16 or B20 |
| 4 mm² | 25 A | 25 A | C20 or C25 |
| 6 mm² | 32 A | 32 A | C25 or C32 |
| 10 mm² | 40 A | 40 A | C32 or C40 |
| 16 mm² | 52 A | 50 A | C40 or C50 |
Method B2 (in conduit, in wall), 230V, ambient 30°C. Reduce ratings for bundled cables or higher temperatures.
Breaking capacity — why it matters
Breaking capacity (kA) is the maximum prospective short-circuit current the MCB can safely interrupt. If a short circuit exceeds the MCB's breaking capacity, the MCB can weld shut — causing a fire or explosion.
Prospective short-circuit current at the panel:
• Residential 25A/40A supply, close to grid: typically 3–6 kA → MCB 6 kA minimum
• Commercial 3-phase 63A–125A supply: typically 6–10 kA → MCB 10 kA required
• Industrial, close to transformer (<50m): can reach 25–50 kA → require industrial MCCB
⚠️ Always calculate prospective fault current
The network operator (DSO) can provide the maximum prospective short-circuit current at the supply point. A 6kA MCB in a commercial building with 10kA fault current is non-compliant and dangerous.
Special cases: LED drivers and variable speed drives
LED driver circuits
LED drivers have capacitive input filters that cause high inrush current (10–30× rated) for 1–2 ms at switch-on. This can cause Type B MCBs to trip on simultaneous switch-on of many circuits.
Solution: Use Type C MCBs for LED driver circuits. For very large luminaire groups (warehouse, stadium), use Type D or an inrush limiter.
Variable frequency drives (VFDs)
VFDs inject harmonic currents that can cause nuisance tripping of standard thermal MCBs. The true RMS current exceeds the fundamental current.
Solution: Use Type D MCBs sized to 125–150% of motor rated current. Consider motor protection relays for motors above 4 kW.
EV charger circuits
Type 2 EV chargers (7–22 kW) require a Type B RCD (not Type A) because they can generate smooth DC fault currents. The MCB must be Type C, sized to charger rated current.
Solution: C32 MCB + 30mA Type B RCD for 7kW single-phase charger. C16 + Type B RCD for 3-phase 11kW charger.
Protection devices chosen correctly for your project
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