Sizing a miniature circuit breaker is four decisions made in the right order: understand the load, pick the tripping curve, set the rated current, then check the breaking capacity. Work through them in sequence and the breaker practically selects itself; skip one and you inherit either nuisance tripping or a board that cannot clear a fault safely.
1. Start with the load, not the breaker
List what each circuit actually feeds. Lighting and socket outlets are mostly resistive with low switch-on inrush. HVAC units, pumps and machine tools draw several times their running current for a fraction of a second at start-up. LED drivers and switched-mode power supplies add short, sharp inrush peaks. The load profile — not habit, not what is in stock — is what the curve choice hangs on.
2. Pick the curve for the inrush
B curve (3–5 × In) suits long cable runs and resistive loads. C curve (5–10 × In) is the general-purpose answer for residential and commercial distribution — it rides through LED, fluorescent and small-motor inrush without tripping. D curve (10–20 × In) is reserved for transformers, large motors and welders with heavy starting currents. Our companion article explains each curve in detail.
3. Size the rated current (In)
Choose the next standard rating above the circuit's design current — 6, 10, 16, 20, 25, 32, 40, 50 or 63 A — while staying below the current-carrying capacity of the cable it protects. The breaker protects the cable first and the load second: a 32 A breaker on a cable rated for 20 A is a misapplication no curve can fix.
4. Check the breaking capacity (kA)
Confirm the breaker's rated breaking capacity (4.5, 6 or 10 kA) exceeds the prospective short-circuit current at the point of installation — highest at the main incomer, falling along the distribution. When in doubt, specify up: the cost step from 6 kA to 10 kA is small compared with a breaker that cannot clear a fault.
5. Poles — and when to ask us
Single-pole for single-phase final circuits, 1P+N where the neutral must be switched, three- and four-pole for three-phase loads and incomers. If you are scheduling a full board, send us the schedule: we will confirm curve, rating and breaking capacity for every way, matched to IEC 60898-1, before you order.