MCB, MCCB or ACB: which breaker belongs where
Use an MCB for final circuits up to about 63 A, an MCCB for sub-distribution and motor feeders from roughly 63 A to 800 A, and an ACB at the main incomer above 800 to 1000 A. The three differ in current rating, breaking capacity and whether the trip setting can be adjusted — and it is breaking capacity, not current rating, that most buyers get wrong.
- MCB6 A–125 A · 6–10 kA breaking · fixed trip · IS/IEC 60898
- MCCB16 A–1600 A · 16–70 kA breaking · adjustable trip · IEC 60947-2
- ACB630 A–6300 A · 50 kA and above · fully adjustable, draw-out · IEC 60947-2
- Crossover≈63 A MCB to MCCB · ≈800–1000 A MCCB to ACB
What actually separates the three?
Everyone knows the current ratings overlap. What does not overlap is the other two properties, and those are what decide the selection.
Breaking capacity is the fault current the device can interrupt without destroying itself. It is quoted in kA. An MCB is typically 6 kA or 10 kA. An MCCB starts around 16 kA and runs to 70 kA on the larger frames. An ACB starts where the MCCB tops out.
Trip adjustability is whether you can set the thermal and magnetic trip points. An MCB cannot be adjusted at all — you get the curve the manufacturer built in. An MCCB on most frames lets you set the thermal trip somewhere between 0.7 and 1.0 of the rated current, and on the better frames the magnetic trip as well. An ACB gives you long-time, short-time, instantaneous and earth-fault settings independently.
| Property | MCB | MCCB | ACB |
|---|---|---|---|
| Typical rating | 6–125 A | 16–1600 A | 630–6300 A |
| Breaking capacity | 6–10 kA | 16–70 kA | 50–100 kA+ |
| Trip setting | Fixed | Adjustable thermal, often magnetic too | Fully adjustable, four parameters |
| Curves | B, C, D | Thermal-magnetic or microprocessor | Microprocessor release |
| Mounting | DIN rail | Fixed or plug-in | Draw-out, withdrawable |
| Maintainable | No, replace it | Contacts not usually serviceable | Yes, contacts replaceable |
| Where it belongs | House DB, lighting, sockets, small final circuits | Sub-distribution, motor feeders, MCC outgoings | PCC main incomer, bus coupler, large transformer feeder |
Which MCB curve for which load?
The curve letter is about how fast the magnetic element trips as a multiple of rated current. Getting this wrong is the most common cause of a breaker that trips for no visible reason.
- B curve — trips at 3 to 5 times rated current. Lighting, sockets, resistive loads. Any house circuit.
- C curve — 5 to 10 times. Small inductive loads: fluorescent banks, small pumps, fans, a fractional-horsepower motor.
- D curve — 10 to 20 times. Transformers, welding sets, larger motors with a hard inrush.
A householder in Bargarh who has put an air conditioner on a B curve MCB will be back at the counter inside a week. That is a C curve job.
Why does breaking capacity matter more than the ampere rating?
The ampere rating protects the cable in normal running. The breaking capacity is what stops the device exploding when a real fault happens. If the prospective fault current at the board exceeds what the breaker can interrupt, the breaker will not clear the fault — it will fail, and the fault carries on.
Fault level rises the closer you are to the transformer and the shorter and fatter the cable. A house 200 metres down a village line has a low fault level and 6 kA is genuinely enough. An LT panel bolted onto the bus of a 500 kVA transformer in a rice mill does not, and a 10 kA MCB there is a device pretending to be protection.
The single most common mistake in an industrial panel we are asked to quote against is a 10 kA MCB used on a motor feeder, chosen because it was on the drawing and cost a third of the MCCB. It works for months. Then a cable gets crushed or a winding shorts, and the MCB welds shut.
The second most common is the opposite: an ACB specified for a 400 A incomer because it sounded safer. A 400 A MCCB with a 50 kA frame does that job properly and leaves money on the table for the earthing, which is where it is usually needed more.
How do we pick one across the counter?
Three questions, in this order.
- What is the load, and is it a motor? A motor feeder goes to an MCCB or a dedicated motor protection circuit breaker almost regardless of size, because a fixed-trip device cannot ride through starting current without being oversized on the thermal side — which then leaves the cable unprotected.
- What is the fault level at that board? Transformer kVA, its impedance, and the length and size of the cable feeding the board. If you do not have it, we will work it out from the transformer rating and the run.
- Does anything downstream have to stay on? If the answer is yes, you need discrimination, and discrimination needs adjustable trips. That is an MCCB at minimum, and an ACB at the incomer if the plant cannot afford to lose everything for one downstream fault.
What does each cost, roughly?
Prices move with the market and with the make, so ring for today's rate — but the ratios are stable and worth knowing when you read a quotation. A 63 A four-pole MCB is the cheapest thing on the list. A 100 A MCCB of the same pole count is several times that. A 1000 A ACB is an order of magnitude above the MCCB again. When somebody quotes you an ACB where an MCCB belongs, the difference is not a rounding error.
The short version
- Final circuits, houses, shops, lighting and sockets — MCB, correct curve, 6 or 10 kA.
- Sub-distribution boards, motor feeders, MCC outgoings, anything 63 A to 800 A — MCCB, breaking capacity matched to the real fault level.
- PCC main incomer, bus coupler, anything above roughly 800 to 1000 A, or anywhere you need four-parameter discrimination — ACB.
Send us your single line diagram, or just the transformer rating and the list of loads, and we will mark up which device belongs on each feeder and quote against it.