How to Size an MCB for a Circuit

How do you size an MCB for a circuit? Correct sizing means finding the design current Ib the circuit actually draws, picking a rated current In that satisfies Ib ≤ In ≤ Iz against the cable's ampacity, then matching the tripping curve to the load's inrush and confirming the breaker's rated breaking capacity exceeds the prospective fault current at that point. Skip any one of these four checks and the breaker either nuisance-trips, lets the cable overheat under sustained overload, or fails to clear a short circuit safely. This guide walks through each step in order, then applies them to a worked example: a 3.5 kW single-phase load wired in 2.5 mm² cable.

Step 1: Find the Design Current (Ib)

Ib is the current the load draws in normal, continuous operation, not the nameplate maximum and not a guess. For a single-phase resistive or near-unity power factor load, Ib = P / V, where P is the load power in watts and V is the supply voltage (230 V nominal in most 50 Hz single-phase systems). For loads with a power factor below 1 — motors, ballasts, switch-mode supplies — divide by cosφ as well: Ib = P / (V × cosφ). Get this number wrong and every downstream step inherits the error.

What we see in the field: people size off the load's rated wattage printed on a nameplate, which already includes a manufacturer safety margin. That's fine as a starting point, but if the equipment has a documented running current, use that instead — nameplate wattage tends to overstate real draw on well-matched loads.

Step 2: Choose a Rated Current (In) So Ib ≤ In

MCBs come in a fixed series of standard ratings — 6, 10, 13, 16, 20, 25, 32, 40, 50, 63 A being the common ones up to 63 A, with select ranges (e.g. ABB S800) reaching 125 A. Pick the smallest standard In that is still ≥ Ib. Rounding down to save on breaker cost is the single most common cause of nuisance tripping on legitimate load. Rounding up excessively, on the other hand, defeats the point of overload protection: the breaker won't trip until well past what the load or cable can tolerate.

Step 3: Confirm Cable Ampacity (In ≤ Iz)

Iz is the cable's current-carrying capacity in the installation as built — insulation type, installation method, ambient temperature, and grouping with other circuits all change this number. A 2.5 mm² copper conductor is not one fixed ampacity; it is a range depending on whether it's clipped direct in free air, enclosed in conduit in a wall, or buried in insulation. In must never exceed Iz. If it does, the breaker will let the cable run hotter than its insulation rating before tripping, which is the exact failure mode overload protection exists to prevent.

Grouping with other cables and elevated ambient temperature both derate Iz below its base tabulated value — apply the correction factors from the cable's installation method table before comparing against In, not after. A cable that looks adequate at its base rating can fail the In ≤ Iz test once four other circuits are bundled alongside it in the same tray.

Step 4: Match the Tripping Curve to the Load

Once In is fixed, the curve decides how the breaker behaves on inrush, not on steady-state overload — the thermal element (bimetal, inverse-time) still handles sustained overload the same way regardless of curve. B curve trips instantaneously at 3-5× In: correct for resistive and lighting circuits with negligible inrush. C curve trips at 5-10× In and is the default for mixed general loads and small motors. D curve, 10-20× In, is reserved for transformers, welding sets, and heavy motor starting current where B or C would trip on every start. Some electricians default to C curve everywhere as a habit — that works until a transformer or a compressor with real starting current is on that circuit, and then it nuisance-trips on every energization.

See MCB tripping curves B, C, D, K, Z for the full multiple-of-In table across all five curves, and choosing the right tripping curve for load-by-load guidance.

Step 5: Verify Breaking Capacity Against Prospective Fault Current

The rated current and curve protect against overload and short-circuit magnitude on the load side. Breaking capacity (Icn per IEC 60898-1, or Icu/Ics per IEC 60947-2 on industrial breakers) is a separate check entirely: it's the maximum fault current the breaker can interrupt safely at that point in the installation. Common household/commercial values are 3, 4.5, 6, and 10 kA; industrial breakers to IEC 60947-2 quote 15-25 kA and above. If the prospective fault current at the point of installation exceeds the breaker's rated breaking capacity, the breaker can fail to clear the fault — welding contacts shut, or worse.

Fault current is highest close to the transformer and drops with distance and cable impedance downstream. Boards near the incoming supply or a dedicated transformer typically need the 10 kA class or higher; final circuits well downstream, with meters of cable impedance ahead of them, are often adequately covered at 6 kA. This is not a guess — it comes from a fault-level calculation or the supply authority's stated figure, not from habit.

Worked Example: 3.5 kW Single-Phase Circuit on 2.5 mm² Cable

Take a 3.5 kW single-phase load on a 230 V supply, near-unity power factor, wired in 2.5 mm² copper cable.

Step 1 — Ib: Ib = P / V = 3500 / 230 ≈ 15.2 A.

Step 2 — In: The nearest standard MCB rating above 15.2 A is 16 A. Ib ≤ In holds: 15.2 A ≤ 16 A.

Step 3 — Iz: 2.5 mm² copper, PVC-insulated, enclosed in conduit against a wall (a common reference installation method) typically tabulates in the high-teens to low-20s amps range before any derating for grouping or ambient temperature; clipped direct in free air pushes that figure higher. Assuming no grouping or elevated-ambient derating applies here, Iz sits comfortably above 16 A. In ≤ Iz holds. Combined with Step 2, the full chain is satisfied: 15.2 A ≤ 16 A ≤ Iz.

Step 4 — Curve: A general-purpose socket or appliance circuit with no unusual starting current is a C-curve candidate — it tolerates the modest inrush of small motors and switch-mode supplies without tripping on connection, while still offering tighter fault discrimination than D curve.

Step 5 — Breaking capacity: For a final circuit fed from a board with moderate distance from the transformer, a 6 kA breaker (the common commercial Icn class) covers typical prospective fault levels at that point. A board sitting directly at a transformer secondary, with minimal upstream impedance, would warrant checking the actual fault-level figure and likely stepping up to 10 kA.

Result: a 16 A, C-curve MCB with 6 kA breaking capacity sizes this circuit correctly, assuming the fault-current check confirms 6 kA covers the installation point. Change any one input — a 5.5 kW load, aluminum conductor, a longer conduit run bundled with three other circuits — and the answer shifts. Resize from Step 1 every time, not from memory of the last similar job.

For the underlying coordination logic in more depth, see the Ib ≤ In ≤ Iz coordination rule, and for breaking-capacity class selection see MCB breaking capacity classes. The full range of standard ratings, curves, and breaking-capacity tiers across brands is stocked in our miniature circuit breakers collection; for the complete reference on every MCB parameter covered here, see the MCB engineering guide.

Conclusion

Sizing an MCB is four checks in sequence, not one number picked from a chart: find Ib from the real load, choose In so Ib ≤ In, confirm In ≤ Iz against the cable as actually installed, pick the curve for the load's starting behavior, and verify breaking capacity against the fault current at that point. Any shortcut through this sequence shows up later as nuisance tripping, a cable running hot under load, or a breaker that can't clear a fault — none of which are cheap to diagnose after the panel is closed up.