Molded Case Circuit Breakers (MCCB) Selection Guide
When selecting a molded case circuit breaker (MCCB), compare IEC 60947-2 rated current (In), Icu/Ics breaking capacity, pole count and thermal-magnetic vs electronic trip units.
This guide explains what to look for when choosing products in the Molded Case Circuit Breakers category for main incoming panels, sub-distribution feeders and high-power outgoing circuits.
Rated current (In), frame size and breaking capacity (Icu / Ics)
Molded case circuit breakers (MCCBs) are manufactured to IEC/EN 60947-2 and operate above the current and fault-level limits of modular miniature circuit breakers (MCBs). Start by matching the continuous rated current (In) and the prospective short-circuit current (Ik) at the installation point. Icu (ultimate short-circuit breaking capacity) indicates the maximum fault current the breaker can safely interrupt, while Ics (service breaking capacity) defines the fault level after which the breaker remains fully operational. Main switchboards near transformers typically require high breaking capacity with Ics = 100% Icu.
Thermal-magnetic vs electronic trip units
Fixed or adjustable thermal-magnetic (TM) trip units (typically 0.7–1 × In thermal setting) provide reliable protection for standard cable and distribution feeders. Microprocessor-based electronic trip units (LSI / LSIG) are selected when time-current selectivity, fine overload/short-delay adjustment or ground-fault protection is required.
Molded case circuit breaker (MCCB) comparison table
| Selection Criterion | Sub-Distribution / Standard Feeder | Main Switchboard / Transformer Feeder | Motor & Generator Protection |
|---|---|---|---|
| Rated Current (In) | 16 A – 250 A | 250 A – 1600 A | Frame sized to full-load & starting duty |
| Breaking Capacity (Icu) | 18 kA – 36 kA | 50 kA – 100 kA (Ics = 100%) | 36 kA – 70 kA |
| Trip Unit Type | Fixed or adjustable thermal-magnetic (TM) | Electronic LSI / LSIG trip unit | Magnetic-only (M) or motor/generator electronic unit |
| Poles & Accessories | 3P / 4P, auxiliary & alarm contacts | 3P / 4P, motor operator, undervoltage release | 3P / 4P, rotary handle, shunt trip (MX) |
Frequently asked questions
What is the difference between Icu and Ics on an MCCB?
Icu is the maximum fault current the breaker can interrupt safely during the O-t-CO test sequence, whereas Ics is the service breaking capacity at which the breaker can interrupt the fault three times and continue normal service. Critical panels specify Ics = 100% of Icu.
When should a 4-pole (4P) MCCB be used instead of a 3-pole (3P) MCCB?
In TN-C systems the PEN conductor must never be switched, so 3P breakers are mandatory. A 4-pole MCCB (with neutral protection) is used in TT systems, dual-source transfer schemes and TN-S installations where neutral currents or harmonics require switching and protecting the neutral conductor.
What is the difference between a shunt trip (MX) and an undervoltage release (MN)?
A shunt trip coil (MX) opens the breaker remotely when energized by an external signal such as an emergency stop or fire alarm. An undervoltage release (MN) remains energized during normal operation and trips the breaker automatically if voltage drops below the threshold or fails completely.