Miniature Circuit Breaker (MCB) Sizing: kW-to-Amp Formula, Curve Codes & Cable Coordination
How to calculate MCB current ratings from single-phase and three-phase load power, read IEC 60898-1 B/C/D trip curves and kA breaking capacity codes, and coordinate with cable ampacity.
A miniature circuit breaker (MCB) combines thermal bimetal overload protection with electromagnetic short-circuit tripping. Under IEC / TS HD 60364-4-43, the primary purpose of an MCB is to prevent conductor insulation from exceeding its thermal limit (`Ib ≤ In ≤ Iz`).
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Calculating design current (Ib) from power
- Single-phase (230 V AC): `Ib = P / (230 × cos φ)`
- Three-phase balanced (400 V AC): `Ib = P / (√3 × 400 × cos φ) ≈ P / (693 × cos φ)`
Select the next standard MCB step (6, 10, 16, 20, 25, 32, 40, 50, 63 A) while ensuring In does not exceed the derated ampacity Iz from the cable current-carrying capacity guide.
| Application | Typical Power | Poles | Typical MCB | Minimum Copper PVC Cable |
|---|---|---|---|---|
| Lighting circuit | ≤ 2.0 kW | 1P (230 V) | B10 A | 1.5 mm² |
| Socket-outlet radial | ≤ 3.5 kW | 1P (230 V) | B16 A / C16 A | 2.5 mm² |
| Dedicated hob / heater | 5.5–7.0 kW | 1P / 2P | B32 A / C32 A | 6 mm² |
| 11 kW 3-phase EV / workshop | 11 kW (16 A) | 4P (400 V) | C20 A / C25 A | 4 mm² – 6 mm² |
Reading IEC 60898-1 markings (`C16`, `6000`, `3`)
- Curve + Rating (`C16`): Instantaneous magnetic trip window (B: 3–5 In, C: 5–10 In, D: 10–20 In) plus 16 A continuous thermal rating.
- Boxed number (`6000` / `10000`): Rated short-circuit breaking capacity Icn (6 kA residential/light commercial vs 10 kA main distribution near transformers).
- Squared `3`: Energy-limiting class 3 (lowest let-through I²t).
Frequently asked questions
Can I replace a tripping 16 A breaker with a 25 A breaker on 2.5 mm² wire?
No. Upgrading the breaker without upsizing the conductor violates `In ≤ Iz` and creates an overheating hazard at terminals and junction boxes. Check whether inrush requires a C-curve 16 A breaker or whether continuous load requires a larger cable.
Do B16 and C16 breakers have the same overload rating?
Yes. Both carry 16 A continuously under identical thermal curves; only their instantaneous magnetic trip threshold differs (48–80 A for B16 vs 80–160 A for C16).