Short answer: an MCCB is chosen in three steps, in this order. First the frame size, which brackets the rated current you need. Then the breaking class letter, which selects the fault-current capability at the voltage you actually operate at. Then the trip unit, thermal-magnetic or electronic. The kA figure is not one number per breaker: it changes with voltage, so a class letter is a slot in the catalogue, not a promise. Two breakers with the same letter can carry different kA values at the same voltage, which is why the exact per-frame figure has to be read from the catalogue table rather than inferred from the name.
Step one: how to read the order code
An MCCB order code carries three pieces of information: the frame size, the breaking class letter and the pole configuration. Read it as a sentence, not a word:
- Frame size — the shell. On CHINT NM1 the frames run through the -125 / -250 / -400 / -630 / -1250 pattern; on NXM through 125 / 160 / 250 / 400 / 630 / 800 / 1000 / 1250 / 1600. The frame sets which rated currents are available in it.
- Breaking class letter — the fault-current capability band. NM1 uses C (basic) / S (standard) / H (higher) / R (current-limiting). NXM uses E / S / F / H. The letter is a family slot; the kA that sits behind it is read from the catalogue table for that frame at that voltage.
- Poles — 2P, 3P or 4P, sometimes written with the 3300 / 4300 style codes that also encode trip-unit configuration.
The three series at a glance
| Series | Frames and rated current | Breaking class letters | Operating voltage (Ue) | Trip unit options | Poles |
|---|---|---|---|---|---|
| NM1 | In 10–1250 A (frames -125 / -250 / -400 / -630 / -1250 and others) | C (basic) / S (standard) / H (higher) / R (current-limiting) | Up to AC 690 V, 50/60 Hz | Thermal-magnetic or electronic; fixed or plug-in | 2P, 3P, 4P |
| NXM | In 16–1600 A (frames 125 / 160 / 250 / 400 / 630 / 800 / 1000 / 1250 / 1600) | E / S / F / H | AC 220/230/240–690 V | Thermal-magnetic, instantaneous magnetic, or electronic; fixed or plug-in | 2P, 3P, 4P |
| NM8 / NM8N | In 16–1600 A (frames 125 / 250 / 400 / 630 / 800 / 1600) | Sample catalogue table graded by B / C / S / Q / H / R | Up to AC 690 V; AC and DC versions (NM8N-DC) | Electronic | 1P–4P |
The class letter is not the kA number
This is the single most useful thing to understand about MCCB selection. The class letter tells you where in the catalogue the device sits. The kA number tells you what it can interrupt, and that number is a function of both the frame and the voltage. The table below uses the code-level examples the manufacturer publishes, and states plainly what they are: examples, not a lookup table.
| Reference from the catalogue | Voltage | Breaking capacity (illustrative) | What it really means |
|---|---|---|---|
| NM1-125S / NM1-250S | As per catalogue table | ≈ 25 kA | Code-level example given for the S class |
| NM1-125H / NM1-250H | As per catalogue table | ≈ 50 kA | Code-level example given for the H class |
| NXM S-250H (nameplate example) | 400/415 V | Icu / Ics = 50 / 36 kA | Both ultimate and service breaking capacity published |
| NXM family (official figure) | 400 V | Up to 70 kA | Ceiling across the range, not a promise for every code |
| NXM family (official figure) | 690 V | Up to 50 kA | Same family, higher voltage, lower ceiling |
| NM8N (sample catalogue table) | AC 220/230/240 V and AC 440 V | 36 / 50 / 70 / 100 kA across the B / C / S / Q / H / R grades | Grades ascend within one frame; read the row you buy |
| NM8N (sample catalogue table) | AC 690 V | Up to 10 kA | Voltage dependence again, at the top of the range |
Two lessons follow. “Up to 70 kA at 400 V” describes the best code in the NXM range, not every NXM you can order; and the same family drops from that 70 kA ceiling to 50 kA at 690 V. Specify from the per-frame, per-voltage row, not from a headline figure.
Why the kA figure changes with voltage
Breaking capacity is the current the device can interrupt and still remain safe. At a higher system voltage the arc that forms when the contacts open has more voltage behind it to sustain itself, so it is harder to extinguish and the device interrupts less. A breaker rated 50 kA at 400/415 V is therefore not a 50 kA device at 690 V. A submittal that lists a kA figure without a voltage is incomplete: state both.
Thermal-magnetic or electronic trip
A thermal-magnetic trip unit does the two basic jobs with two mechanisms: a bimetal strip for the overload (thermal) and a solenoid for the short circuit (magnetic). It is simple, it needs no auxiliary supply, and its behaviour does not change with firmware.
An electronic trip unit replaces the bimetal with current transformers and a processor. That buys adjustable settings, tighter tolerance, and often the option of earth-fault or communication features. It also adds a small amount of standing complexity: an electronic unit is a measurement product as well as a protection product.
The choice should follow the specification, not the fashion: electronic where adjustable settings are needed for coordination, thermal-magnetic where a fixed characteristic is all the job requires.
When to step up a frame
Stepping up a frame is not only about current. Three situations push you upward:
- Rated current sits at the top of a frame. A device running near its ceiling has no headroom for future load and no margin for derating in a hot enclosure.
- The required breaking capacity is not available in the frame you wanted. Sometimes the frame can carry the current but not the kA at that voltage, and the smaller frame with a higher class letter still falls short.
- Coordination demands it. Where discrimination between upstream and downstream devices must be proven, a larger frame often provides the trip settings the coordination study needs.
What we send before you pay
HAZAM is an independent trading company. We are not the manufacturer of these breakers and we are not an appointed distributor for any brand. Our job is to take your specification, put it against the catalogue, and send the documents that let you check our answer.
Before payment, on request, we send: the datasheet page for the exact ordering code, showing Icu and Ics at the voltage you are using; the certificate that covers that code; and the frame, class letter and pole configuration written out so there is no ambiguity between the name on the quote and the device in the box. Nothing is left to the class letter alone; if a figure is not published for the code, we say so rather than translate a family number into a per-code promise.
FAQ
Q: How do I choose an MCCB frame size?
A: Start from the rated current and pick the frame that brackets it with headroom, then confirm that frame offers the breaking capacity you need at your operating voltage.
Q: What do the breaking-class letters C, S, H and R mean on NM1?
A: They are capability bands: C basic, S standard, H higher breaking, R current-limiting. On NXM the equivalent letters are E, S, F and H. The letter places the device in the catalogue; the kA behind it must be read from that frame’s table at your voltage.
Q: Why does the same MCCB show different kA figures at different voltages?
A: Because a higher voltage sustains the arc for longer, the device interrupts less current. A 50 kA rating at 400/415 V is not a 50 kA rating at 690 V; state the voltage with the kA.
Q: Should I choose a thermal-magnetic or an electronic trip unit?
A: Electronic where the specification needs adjustable settings, tighter tolerance or earth-fault and communication features; thermal-magnetic where a fixed overload and short-circuit characteristic is all the job requires.
Q: When should I step up to a larger frame?
A: When rated current runs at the top of the frame with no headroom, when the required breaking capacity is not available in that frame at your voltage, or when coordination needs trip settings the smaller frame cannot provide.
Q: How do you confirm the exact rating before I pay?
A: The datasheet page for the exact ordering code, showing Icu and Ics at your operating voltage, plus the certificate covering that code. Ratings are confirmed per code, not per family or class letter.
Send the specification, get the code
Send: rated current · operating voltage and frequency · required breaking capacity (Icu) · poles · trip unit preference · quantity · destination country · the certificate your client needs. You get the datasheet page and the certificate for the exact ordering code, plus packing data, within 24 hours. Request a quotation.
Related: MCCB catalogue · MCB vs MCCB vs ACB · Cross-reference to Schneider, ABB and Siemens ranges · MCB catalogue · Genuine product verification · How to size a distribution board · Contact
CHINT is a trademark of its respective owner. HAZAM is an independent trading company; we are not the manufacturer and we are not an appointed distributor. Certificates and ratings always refer to the exact ordering code. Values above reflect manufacturer datasheets; the certificate for your code governs.