MCCB Selection Guide: Frame Size, Breaking Capacity and Trip Curves Explained

Molded case circuit breakers (MCCBs) protect cables, equipment and people against overloads and short circuits. Choosing the right MCCB means matching five parameters — this guide explains each one in plain language.

1. Frame Size vs Rated Current

The frame size (also called shell rating) is the physical size of the breaker, e.g. 100A, 225A, 400A, 630A frame. The rated current (In) is the actual protection setting you need, e.g. a 225A frame can carry a 100A, 125A or 160A trip unit.

Choose the frame for the short-circuit level and future expansion; choose the trip rating for today’s load. A 400A frame with a 250A trip unit leaves room to grow.

2. Breaking Capacity: Icu and Ics

Icu (ultimate breaking capacity) — the maximum short-circuit current the breaker can interrupt once.
Ics (service breaking capacity) — what it can interrupt and still continue normal service, usually expressed as a percentage of Icu (e.g. 75% or 100%).
Icw — short-time withstand, relevant for selectivity with upstream breakers.

Example: an MCCB rated 36kA at 400VAC can safely clear a fault current up to 36,000A. The required breaking capacity depends on the transformer size and cable impedance at the installation point — your panel designer calculates this, but the supplier must offer the matching level: 25kA, 36kA, 50kA, 65kA or 100kA.

3. Trip Unit: Thermal-Magnetic vs Electronic

Thermal-magnetic (TM): bimetal for overload, magnetic coil for short circuit. Simple, robust, cost-effective — the standard choice for most installations.
Electronic (LSI): adjustable long-time (L), short-time (S) and instantaneous (I) protection with precise settings. Ideal for selectivity coordination and sensitive loads.

For motor protection, a motor-protection MCCB or MPCB with magnetic-only or adjustable magnetic trip avoids nuisance tripping on motor inrush.

4. Poles: 3P vs 4P

3P — three-phase circuits, the standard for industrial distribution.
4P — adds a protected or switched neutral; common in TN-S systems, generator changeover and where the neutral must be isolated.

5. Standards and Certifications

For international projects, look for:

IEC 60947-2 — the international standard for LV circuit breakers
CE marking — required for the European market
GB/T 14048.2 — Chinese national standard (equivalent)
UL 489 — North America (different system, not interchangeable)

Always request the type test certificate for the exact model and breaking capacity you buy — a valid IEC test report is your quality guarantee.

Worked Example

A 100kW motor at 400V draws roughly 190A. A designer might select:

Frame: 250A
Trip rating: 200A or 225A (thermal adjustable)
Breaking capacity: 36kA @ 400VAC (check fault level)
Poles: 3P
Accessories: shunt trip for remote tripping, auxiliary contacts for status indication

Need Help Choosing?

Send us your panel design parameters or existing part numbers — we will match an NYM series MCCB with the right frame, rating and breaking capacity, including cross-references for CHINT, ABB and Schneider models. Get a quotation →