MCCB Breaking Capacity: Icu vs Ics, Worked Example

Breaking capacity is the specification most buyers get wrong — usually by paying for more than they need, occasionally by buying less than the site requires. This guide explains Icu and Ics in plain language, shows where the numbers come from, and walks through a worked calculation you can reuse on any project.

What Icu and Ics Actually Mean

Both numbers describe a short circuit, but they answer different questions.

Icu (ultimate breaking capacity) is the highest prospective short-circuit current the breaker can interrupt under the standard test sequence — one heavy fault, then inspection. It answers: can this breaker clear the worst fault this board will ever see?

Ics (service breaking capacity) is the fault current the breaker can clear and then remain fit for continued service. It is usually written as a percentage of Icu — commonly 50%, 75% or 100%. It answers: can the breaker keep working after a fault?

A practical rule of thumb: design for faults above Ics to be rare events, and size Icu above the maximum prospective fault current at the installation point with real margin. The system designer calculates that current; the supplier must offer a matching level.

Where the Numbers Come From: C, S, M and H Types

Most manufacturers sell the same breaker family in several breaking-capacity levels. In the STM6 thermal-magnetic MCCB range, those levels are:

Code	Level	Icu at AC400V
C	Basic	from 5kA
S	Standard	mid range
M	Higher	mid-high range
H	High	up to 100kA

The full range spans roughly 5kA to 100kA at AC400V depending on frame size and type code, while the STM6E electronic MCCB covers 25kA to 100kA at AC400V with S, M and H options.

The point: the same 250A frame can be bought at several fault levels, with very different prices. Choosing the right level is the single biggest cost lever in MCCB procurement.

Worked Example: a 1000 kVA Transformer

A 1000 kVA transformer at 400V with 5% impedance produces a prospective fault current of roughly:

Isc = 1000 kVA / (1.732 × 400V × 0.05) ≈ 28.9 kA

That is the classic “infinite bus” estimate — it assumes an unlimited supply on the primary side and ignores cable impedance, so it overstates the real value at the board. A breaker spec for this installation might look like:

• Frame: 250A or 400A, depending on the feeder
• Trip rating: per the load (e.g. 200A or 250A)
• Icu: 36kA minimum — the next standard level above 28.9kA
• Ics: 75% or 100% of Icu, depending on how critical continuity is
• Poles: 3P for a standard three-phase feeder

Selecting the M type (36kA class) rather than the H type (100kA class) is the difference between a sensibly priced breaker and an over-engineered one. The margin between 28.9kA and 36kA is healthy because the real fault current will be lower once cable impedance is counted.

Icw: The Number That Makes Selectivity Possible

There is a third figure on electronic breakers: Icw, the short-time withstand current. It is written as a current for a duration, for example 10kA for 1 second. An upstream breaker with Icw holds the fault for a short window while the downstream breaker clears it — that is how selective coordination works.

The STM6E electronic range lists Icw from 10kA to 25kA for 1 second, which is exactly the parameter to check when two breakers must coordinate.

What to Check Before Ordering

• Ask for the exact-model Icu/Ics table at your system voltage (400V and 690V columns differ).
• Confirm the test sequence standard — IEC 60947-2 — and the report scope for the exact model you are buying.
• State the transformer kVA and cable length so the supplier can sanity-check the fault level.
• For new-energy projects, DC ratings are a different table entirely — see our solar and storage guide.

Send us your panel parameters — transformer size, system voltage and the feeder list — and we can compare candidate STM6 and STM6E models by frame, type code and breaking capacity. The final protection study stays with your panel designer; we match the data to their numbers.

Related reading: MCCB Selection Guide and MCCB vs MCB.