Circuit protection
Circuit Breaker Sizing Calculator (MCB & MCCB)
Enter the load to get the design current, the next standard breaker rating to IEC 60898-1 / 60947-2, the recommended trip curve, and a cable-coordination check on the Ib ≤ In ≤ Iz rule. A design starting point — verify against the load, the cable, the prospective fault current, and local wiring rules.
How breaker sizing works
A protective device is sized to protect the cable, not just to carry the load. The governing rule in IEC 60364 is a chain of three currents:
where Ib is the design (load) current, In is the breaker rated current, and Iz is the corrected current-carrying capacity of the conductor. The breaker rating must be at least the load current, and must never exceed what the cable can safely carry. The design current comes from the load:
Standard ratings: MCB and MCCB
| Device | Standard | Typical rated currents (A) |
|---|---|---|
| MCB | IEC 60898-1 | 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 |
| MCCB | IEC 60947-2 | 160, 200, 250, 320, 400, 500, 630, 800, 1000, 1250, 1600 |
Choosing the trip curve
| Curve | Instantaneous trip | Typical use |
|---|---|---|
| B | 3–5 × In | Resistive and lighting loads, long cable runs, low inrush |
| C | 5–10 × In | General distribution, socket circuits, small motors — the common default |
| D | 10–20 × In | High-inrush loads: motors, transformers, capacitor banks |
A curve that trips too early nuisance-trips on inrush; one that trips too late may not clear a fault fast enough. On low fault levels or long cables, a lower curve (B) helps meet the required disconnection time, which must be verified with the earth-fault-loop impedance.
The breaker also has to break the fault
Rating and curve are not the whole selection. The breaker’s rated breaking capacity — Icn for an MCB, Icu / Ics for an MCCB — must exceed the prospective short-circuit current at its installation point. Determine the fault level from a short-circuit study before finalizing the device.
Frequently asked questions
How do I choose the breaker rating for a cable?
Apply Ib ≤ In ≤ Iz: the rating must be at least the design current and no more than the cable’s corrected current-carrying capacity. Increasing the breaker above the cable capacity to stop tripping is unsafe.
When do I need an MCCB instead of an MCB?
Generally above 125 A, or when you need higher breaking capacity, adjustable trip settings, or larger frame sizes.
B, C or D curve — which one?
B for resistive/lighting and long runs, C for general distribution and small motors, D for high-inrush loads such as motors and transformers.
Choose the breaker
With the rating, curve, and breaking-capacity target set, compare frames, poles, and trip units — or send the circuit data for a fast recommendation for your market.
This calculator provides an engineering estimate for guidance only. Final circuit design, protection coordination, breaking-capacity verification, and product selection must be carried out by a qualified person against the actual load, conductor data, prospective fault current, applicable standards, and the manufacturer’s current datasheet.