MCB Trip Curves Explained: What B, C and D Ratings Mean for Your Circuit
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MCB trip curves explained
The letter on a miniature circuit breaker (C32, D25 and similar) sets how much overcurrent trips the breaker instantly: a B curve trips at 3 to 5 times rated current, a C curve at 5 to 10 times, and a D curve at 10 to 20 times. The thermal part of the curve is the same for all three, so the letter is about inrush: lighting takes B, motors and general loads take C, and high-inrush loads such as transformers take D. The table below shows the C and D breakers we currently stock.
What the curve letter changes
A miniature breaker protects with two mechanisms. The thermal element trips on sustained overload, and its behavior depends only on the current rating: a 16 A breaker carries modest overload for minutes regardless of curve. The magnetic element trips instantly on short circuit, and this is where curves differ. Under IEC 60898-1, the standard covering breakers for household and similar installations, the magnetic trip must fall within fixed multiples of the rated current In: 3 to 5 times for B, 5 to 10 for C, and 10 to 20 for D.
The practical consequence shows up at switch-on rather than at fault time. A motor draws several times its rated current while spinning up, and a transformer draws a large inrush when energized. A B breaker sees that inrush as a fault and trips; a C or D breaker of the same rating rides through it and still clears a real short circuit.
Which curve for which load
- B curve: resistive loads with no inrush. Lighting circuits, heaters, general socket circuits in residential work.
- C curve: moderate inrush. Small motors, pumps, fans, air conditioners, and most mixed commercial and light-industrial circuits. This is the default in distribution boards.
- D curve: high inrush. Transformers, larger motors started across the line, welding equipment, and supplies where the upstream wiring can handle the delayed magnetic trip.
One caution on choosing D as a fix for nuisance trips: the higher magnetic threshold delays clearing on genuine faults. Moving to a D curve is a last step after checking the real cause, which is often an undersized rating or a load problem, not the curve.
Reading the markings on the breaker
The letter and the number sit together on the handle or face: C32 is a C curve with a 32 A rating, D6 is a D curve with a 6 A rating. Alongside them, the pole count (1P, 2P, 3P, 4P) shows how many conductors the breaker switches, and the breaking capacity figure (4.5 kA, 6 kA, 10 kA) states the fault current it can interrupt. All three belong on a replacement check: same rating, same curve, same or better breaking capacity, same poles.
C and D curve breakers in stock now
| Series | Poles | Curve | Rating | Listing |
|---|---|---|---|---|
| NXB-63 | 1P | C | 16 A | view listing |
| NXB-63 | 1P | C | 40 A | view listing |
| NXB-63 | 2P | C | 63 A | view listing |
| NXB-63 | 3P | C | 25 A | view listing |
| NXB-63 | 4P | C | 16 A | view listing |
| NXB-63 | 1P | D | 25 A | view listing |
| NXB-63 | 4P | D | 6 A | view listing |
| NXB-125 | 1P | C | 125 A | view listing |
| NXB-125 | 1P | D | 100 A | view listing |
| NXB-125 | 2P | D | 100 A | view listing |
| DZ47-60 | 1P | C | 10 A | view listing |
| DZ47-60 | 1P | D | 10 A | view listing |
| DZ47-60 | 2P | C | 32 A | view listing |
| DZ47-60 | 2P | C | 60 A | view listing |
| DZ47-60 | 3P | D | 40 A | view listing |
Listings verified October 2026. Our current listings carry C and D curves; if you need a B curve breaker, email the rating and poles and we will source it.
Common selection mistakes
- Matching the rating but not the curve. A D25 does not protect like a C25; swapping curves silently changes short-circuit behavior for everything downstream.
- Solving nuisance trips by stepping the rating up instead of checking the load. An oversized breaker leaves the cable as the weakest point.
- Ignoring breaking capacity. A breaker with a 4.5 kA rating in a panel where available fault current is higher can fail to clear a short circuit. Match or exceed the original figure.
- Using a single-pole breaker to switch a two-phase load. Both lines need interruption, which is what 2P and 1P+N formats are for.
Frequently asked questions
Can I replace a C16 breaker with a D16?
Electrically it fits the same rating, but the magnetic trip range moves from 5-10 times In to 10-20 times In, which changes how fast the breaker clears a short circuit. Match the curve of the original unless a qualified person has reviewed the circuit.
Why does my C curve breaker still trip when the motor starts?
Motor starting current can sit above the C curve's 10x ceiling for long enough to trip, especially with high-inertia loads. Check the motor's starting current against the curve; if it genuinely exceeds it, a D curve or a reduced-voltage starting method is the fix, not a larger rating.
Are NXB and DZ47 breakers interchangeable?
They occupy the same DIN-rail format and rating ranges, and NXB is the successor series to DZ47 in Chint's lineup. Match rating, curve, poles and breaking capacity, and confirm the busbar spacing when replacing inside an existing board.
What does 6 kA on the breaker mean?
It is the breaking capacity: the maximum fault current the breaker can safely interrupt. It must be at least as high as the prospective short-circuit current at the installation point.
Ordering breakers
Find your poles, curve and rating in the stock table above, or email the markings from your old breaker (letter, number, poles, kA) and we will match it before you pay. For contactors, drives and everything else in the panel, browse our MRO supplies catalog.