MCCB Selection Guide: Frame Size & Breaking Capacity

Molded case circuit breakers (MCCBs) protect circuits and equipment against overloads and short circuits. Additional protection functions and the overall system design determine the level of protection for people. 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. The STM6 thermal-magnetic MCCB spans 16A to 2000A frames, while the STM6E electronic MCCB adds fine current adjustment for larger feeders.

2. Breaking Capacity: Icu and Ics

Icu (ultimate breaking capacity) — the highest prospective short-circuit current covered by the specified ultimate test sequence; continued service is not assumed without the required checks.
Ics (service breaking capacity) — the current covered by the specified service test sequence, 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 Market Documentation

For international projects, look for:

IEC 60947-2 — the international standard for LV circuit breakers
CE marking — check the applicable EU legislation, declaration and technical file for the exact model and use
GB/T 14048.2 — Chinese national standard (equivalent)
UL 489 — North America (different system, not interchangeable)

Ask for the exact-model data sheet and available test evidence, then verify the model scope, ratings, report issuer and referenced standard. A standard named in a catalog is not proof that a certificate or report covers every model. For EU projects, the official CE guidance also distinguishes the manufacturer’s declaration and technical documentation from a universal “CE certificate.”

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 and we can compare candidate STM models by frame, rating and breaking capacity. The panel designer remains responsible for the final protection study and selection. Request a quotation