Turkish
A panel builder once asked us to quote a 2000 A main incomer as a molded case circuit breaker. The price looked great on paper. The problem was that the same panel needed full selectivity with six downstream feeders, and the switchboard had to stay energized through a single feeder fault. No molded case breaker was going to give him that. He ended up with an air circuit breaker, and the panel worked.
That is the real question behind MCCB vs ACB. It is rarely “which one is better” — it is “at what point does the molded case stop being the right tool?” This guide answers that with the ratings that actually decide it: current range, short-time withstand (Icw), selectivity category, breaking capacity, and serviceability. It also gives you a decision framework for the 630–1600 A zone where both devices are technically available and the choice is genuinely hard.
If you only look at current rating, the boundary looks simple. In practice the decision splits into three zones, and the middle one is where most specification mistakes happen.
| Rating zone | Default choice | Why |
|---|---|---|
| Up to 630 A | MCCB | Compact, cost-effective, adequate breaking capacity for most feeder duty |
| 630–1600 A | Either — decide on selectivity, Icw and maintenance | Both technologies are available; protection requirements decide |
| Above 1600–2000 A | ACB | Molded case frames become impractical; ACB is the practical and usually the only choice |
Notice that the middle zone is wide. A 1000 A main incomer in a factory with tight selectivity requirements is an ACB job. A 1000 A feeder to a fixed, non-critical load with no coordination study behind it is often an MCCB job, and the money saved is real. The deciding factors are not on the nameplate’s first line — they are in the protection behavior, which is where the two devices diverge.
Both devices are covered by the same product standard, IEC 60947-2, for low-voltage switchgear. The differences are in construction, trip technology, and what the standard lets each one declare.
An MCCB encloses its contacts, arc chutes and operating mechanism inside a molded insulating housing. It is a sealed unit: if the trip unit fails, you replace the breaker. Most MCCBs are fixed-mounted, though plug-in and some draw-out versions exist.
An ACB uses an open metal frame. Contacts, arc chutes, the operating mechanism and the trip unit are separate modules that can be inspected, serviced and replaced. The dominant mounting form is draw-out, with three physical positions — connected, test ve isolated — which lets you rack a breaker out for maintenance without touching the busbars.
MCCBs are commonly supplied with thermal-magnetic trip units — a bimetal element for overload and a magnetic element for short circuits — with electronic trip units as an option on larger or more adjustable frames. Thermal-magnetic units are simple and need no external supply, but their settings are fixed or only narrowly adjustable.
ACBs are almost always supplied with microprocessor-based electronic trip units providing the full LSIG function set: long-time overload, short-time with delay, instantaneous, and ground fault. Metering, event logging and communication are standard options rather than exotica. If your specification calls for adjustable ground-fault pickup or energy monitoring at the incomer, you are describing an ACB.
This is the formal distinction, and it is the one that matters most for selectivity.
| Characteristic | Category A (typically MCCB) | Category B (typically ACB) |
|---|---|---|
| Intentional short-time delay | Not provided | Provided, commonly 0.05–0.5 s depending on the trip unit |
| Icw (short-time withstand) | Usually not declared | Declared, commonly 25–100 kA for 1 s |
| Selectivity method | Current limiting and energy-based discrimination | Time-graded coordination |
| Trip behavior under short circuit | Trips as fast as possible | Waits deliberately so downstream devices clear first |
| Typical duty | Feeders, sub-distribution, motor circuits | Main incomers, bus-ties, generator connections |
Category A devices are described by IEC 60947-2 as not specifically intended for selectivity under short-circuit conditions. That is not a criticism — a fast, current-limiting trip is exactly what you want at the end of a radial feeder. It is simply the wrong behavior at the top of a switchboard.
Rated short-time withstand current (Icw) is the fault current a breaker can carry in the closed position for a stated duration — typically 0.05 s, 0.1 s, 0.25 s, 0.5 s or 1 s — without damage. It is the rating that makes time-graded selectivity physically possible.
Here is the mechanism. An upstream ACB waiting 0.4 s for a downstream feeder breaker to clear its fault has 0.4 s of full short-circuit current flowing through its own contacts and busbars. Without enough withstand capability, the waiting device is damaged by the very fault it is politely ignoring. That is why Category B construction uses high contact spring pressure and heavy thermal mass: the breaker must not blow its contacts apart while it waits.
MCCBs take the opposite approach. Their contacts are designed to repel electromagnetically and interrupt extremely fast, limiting let-through energy instead of withstanding it. That is excellent for protecting the downstream circuit and poor for sitting at the top of a coordination chain.
Practical consequences to check on any project:
For the IEC minimum values that apply to Category B devices, consult the current edition of IEC 60947-2 and the manufacturer’s published data; the figures are frame-size dependent and should not be assumed from a competitor’s table.
On Icu and Ics, the two technologies are closer than most people expect. Modern MCCBs reach Icu values well above 50 kA at 415 V and, on some frames, considerably higher. Large ACBs occupy a similar band. Breaking capacity alone rarely forces you from one technology to the other in the overlap zone — it is Icw and selectivity that do.
What does differ is behavior during the fault. MCCBs current-limit aggressively, cutting let-through energy (I²t) hard. ACBs generally do not current-limit; they let the full prospective current flow until the trip unit decides to act, which is the deliberate price of time-graded selectivity. If you are protecting cable thermal withstand or sensitive downstream equipment on a fast feeder, that difference favors the MCCB.
Because the two ratings interplay, read your Icu and Ics together rather than separately — a high Icu with a low Ics ratio means a breaker that survives one big fault but may not be serviceable afterwards. Our guide to kopma kapasitesi (Icu/Ics) covers that selection detail in full.
This is the zone where both devices are quoted against each other, and where a defensible decision needs more than a current rating. Work through these five questions in order.
| Question | Points to MCCB | Points to ACB |
|---|---|---|
| Do you need full time-graded selectivity with downstream devices? | Basic discrimination is acceptable | Downstream faults must not black out the board |
| Is a declared Icw required by the coordination study? | No delay required, or delay is very short | Delay of 0.2 s or longer at high fault current |
| Can you tolerate replacing the whole device after a failure? | Yes — a spare is cheap and downtime is acceptable | No — you need to service contacts or the trip unit in place |
| Do you need LSIG, metering or communication at this position? | Overload and short-circuit protection only | Ground fault, power monitoring, BMS integration |
| How much panel depth and budget do you have? | Compact footprint, lower upfront cost | Space available, lifecycle cost drives the decision |
If three or more answers fall in the right-hand column, specify an ACB. If the load is fixed, the fault study shows no need for delay, and a spare breaker sits on the shelf, an MCCB at 800 A or 1250 A is a sound engineering choice — not a compromise.
Upfront price favors the MCCB almost every time in the overlap zone. Lifecycle cost is less one-sided.
A conventional low-voltage board uses both, in a deliberate hierarchy. Understanding that hierarchy usually resolves the MCCB vs ACB question before any calculation.
| Position | Typical device | Job |
|---|---|---|
| Main incomer / transformer secondary | ACB | Carry high continuous current, ride through downstream faults, provide LSIG and metering |
| Bus-tie | ACB | Selective coupling between two board sections |
| Generator or large feeder | ACB or large-frame MCCB | Depends on rating and delay requirement |
| Sub-distribution feeders | MCCB | Fast, current-limiting fault clearance |
| Motor circuits | MCCB or MPCB with contactor | Overload and short-circuit protection sized for inrush |
| Final circuits | MCB | Fixed-curve branch protection |
If you are still weighing the bottom of that range, our comparison of MCB ve MCCB Karşılaştırması covers the boundary below 125 A, and the MCCB selection guide walks through sizing for feeders. For the top of the molded case range, the ISO-S7 1600 A ve SNS 800–1600 A frames are the practical ceiling of what a molded case device does well.
Add-on functions are worth confirming at the same time. Where a residual current function is needed on a molded case frame, it is usually delivered by an add-on module rather than the breaker itself — see residual current protection explained for how those modules are specified.
Sometimes. If the position needs only overload and short-circuit protection, no intentional delay, and replacement-on-failure is acceptable, a 1600 A molded case breaker can be the right call. If the coordination study requires the device to ride through a downstream fault for 0.2 s or more, you need the declared Icw of an ACB.
Most do not declare one, because Category A design relies on fast interruption rather than withstand. Some larger electronic-trip frames publish a limited Icw. Never assume it — check the datasheet for the exact frame and compare the stated duration as well as the value.
No. On small boards with a single protection level and no downstream coordination requirement, a large-frame MCCB is common and correct. The ACB becomes necessary when continuous current, fault level, or selectivity demands exceed what a molded case device can declare.
Molded case circuit breakers are covered by UL 489, while low-voltage power circuit breakers — the functional equivalent of an ACB — fall under UL 1066 and ANSI C37.13. The standards test differently from IEC 60947-2, so devices are not interchangeable on paper across jurisdictions.
Partial selectivity is achievable, usually through energy-based discrimination and manufacturer coordination tables that specify which frame combinations hold and up to what fault level. Full time-graded selectivity is a Category B function. If the project specification says “full selectivity,” plan for an ACB upstream.
The MCCB vs ACB decision is settled by what the device has to do during a fault, not by how much current it carries. Below 630 A, and in the 630–1600 A zone where no selectivity delay is required, a molded case breaker is smaller, cheaper and faster. Where the coordination study demands a short-time delay, where LSIG and metering are specified, or where the incomer must be serviced without killing the board, the air circuit breaker is the only device that genuinely does the job.
WZSMO builds molded case circuit breakers from 50 A to 1600 A with thermal-magnetic and electronic trip units, 690 V AC rating, and IEC 60947 compliance. If you are specifying the top of that range and want a second opinion on whether a molded case device will hold up in your coordination study, send us your load list, fault levels and one-line diagram and we will review it with you.
