MCB trip curve and cable limit
Plot an MCB trip band against the cable it protects.
Reads AS/NZS 3008.1.2:2017 (NZ), AS/NZS 3008.1.1:2017 and 2025 (AU) and AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Check a breaker clears a long circuitA long lighting run with little fault current against its breaker and cable.
- See a D curve on a motor circuitHow long a D curve breaker can take at a modest fault, against the cable.
- Check a small cable near the boardWhether a high fault current outruns a small cable before the breaker trips.
- Compare a B and C curve breakerThe instant trip current of a Type B breaker on a socket-outlet circuit.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Breaker type
- Curve C
- Rating
- 20 A
- Fault current at the far end
- 300 A
- Fault current at the board
- 0 A
- Standard
- NZ · AS/NZS 3008.1.2
- Cable size
- 2.5 mm²
- Material
- Copper
- Insulation
- PVC 75 °C
Latest trip at the fault current
0.1s
- Cable withstand at the fault current
- 0.86s
- Instant trip, Table 8.1 basis
- 150A
- Instant trip, typical band
- 100 to 200A
- Conventional operating current
- 29A
- Largest Zs for 0.4 s
- 1.53Ω
This result is a design aid. Verify it against the current standard.
Calculation steps
- Conventional operating current29A
AS/NZS 3000:2018 Cl 2.5.3.1
- Mean tripping current, Type C150A
AS/NZS 3000:2018 Table 8.1
- Instantaneous trip at the mean, at most0.1s
AS/NZS 3000:2018 Table 8.1
- Largest Zs for 0.4 s1.53Ω
AS/NZS 3000:2018 Table 8.1
- Typical instantaneous band, low edge100A
- Typical instantaneous band, high edge200A
- k factor111x
AS/NZS 3008.1.2:2017 Table 52 p.121
- Cable withstand at the fault current0.86s
AS/NZS 3008.1.2:2017 Cl 5.3
- Latest trip at the fault currentResult0.1s
About this calculator
How this is calculated
A miniature circuit-breaker does not trip at one exact time for a current. Two breakers of the same type and rating trip at different times, so a maker draws a band: the earliest and the latest each can trip. This tool draws that band for a Type B, C or D breaker, draws the cable's limit beside it, and reads the latest trip time at the fault current you enter.
Three figures come from AS/NZS 3000. A breaker operates within its conventional time at 1.45 times its rating. Table 8.1 takes it as tripping instantly, within 0.1 s, at its mean tripping current: 4 times the rating for Type B, 7.5 for Type C and 12.5 for Type D. The largest loop impedance for a 0.4 s disconnection follows from that current. The rest of the band, the edges of the instant region and the thermal part, is a typical band. AmpSize does not hold AS/NZS 60898, and the result says so.
The cable's line is its adiabatic limit: the time it can carry a fault current before its insulation is damaged, from the k factor of AS/NZS 3008.1, through the fault rating tool. The circuit passes when the breaker's latest time is no longer than the cable's time at that current. Check it at the far end, where the fault current is least and the trip slowest, and give the fault current at the board too: there the current is highest, the cable's time shortest, and the worse of the two ends decides.
| Step | Table or clause | What it decides |
|---|---|---|
| Conventional operating current | AS/NZS 3000 Cl 2.5.3.1 | Where the band starts |
| Instant trip at the mean current | AS/NZS 3000 Table 8.1 and Para B4.5 | The current that trips it instantly, and the largest Zs |
| Instant region and thermal edges | A typical band, not a standard | The earliest and latest trip at each current |
| The cable's limit | AS/NZS 3008.1 Table 52 and Cl 5.3 | The time the cable withstands the fault |
| The check | The latest trip against the cable's time | Whether the breaker clears the fault first |
Worked examples
- A C20 lighting circuit in 2.5 mm² with a 300 A fault trips in 0.1 s at the latest, well inside the cable's 0.86 s. The Table 8.1 instant trip current is 150 A.
- A B20 circuit in 2.5 mm² with a 150 A fault trips in 0.1 s against the cable's 3.42 s. The Table 8.1 instant trip current is 80 A.
- A D32 motor circuit in 6 mm² with a 500 A fault can take up to 1.6 s on the slow edge of the band, just inside the cable's 1.77 s. Table 8.1 counts it as instant above 400 A.
- A long B16 circuit in 1.5 mm² with only 60 A of fault current fails: the breaker can take 27.74 s, longer than the cable's 7.7 s, and 60 A is under the Table 8.1 instant trip current of 64 A.
- The same C20 circuit with 3000 A of fault at the board fails at the board: the band shows 0.1 s there, and the cable lasts 0.01 s at 3000 A, though it takes 0.86 s at the far end's 300 A. The maker's let-through energy decides it.
- A C32 circuit in 4 mm² close to the board, 2000 A of fault fails on the band: it shows 0.1 s, and the cable lasts 0.05 s. The maker's let-through energy decides it.
Limits
The band is typical, not a maker's curve. A real breaker clears a large fault in much less than 0.1 s, and limits the energy it lets through; the maker publishes that energy as I²t. Where the band fails a cable at a high current, compare the maker's I²t with k²S² before you change the cable.
gG fuses are not drawn yet. Their curves are in IEC 60269, which AmpSize does not hold.
See also: Loop impedance, Fault rating and IDMT protection.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What is the difference between a B, C and D curve MCB?
How much current trips it instantly. AS/NZS 3000 Table 8.1 takes the mean as 4 times the rating for Type B, 7.5 for Type C and 12.5 for Type D. A higher type rides through motor and transformer starts.
Why is a trip curve drawn as a band?
Two breakers of the same type and rating trip at different times. The maker draws the earliest and the latest each can trip. The tool reads the latest time, so the check holds for the slowest breaker.
How do I know the breaker protects the cable in a fault?
The cable can carry a fault current for a time set by its k factor and size, (k × S ÷ I)². The breaker must trip within that time. The tool compares the two at the fault current you enter.
Is the band the maker's curve?
No. The operating points come from AS/NZS 3000, and the edges of the band are typical of the type. AmpSize does not hold AS/NZS 60898. Check a close result on the maker's curve.
What if the cable lasts less than 0.1 s?
Near the board the fault current is high and a small cable lasts only a few milliseconds. A breaker clears a large fault faster than 0.1 s. Compare the maker's let-through energy, I²t, with k²S².
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 2.5.3.1Operating points of circuit-breakers and fuses
- AS/NZS 3000:2018 Table 8.1Maximum earth fault loop impedance multipliers by MCB curve
- AS/NZS 3008.1.2:2017 Table 52Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.2:2017 Table 52, 53, 54, 55Short-circuit K values and temperature limits
- AS/NZS 3008.1.1:2017 Table 52Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.1:2017 Table 52, 53, 54, 55Short-circuit K values and temperature limits
- AS/NZS 3008.1.1:2025 Table 5.1Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.1:2025 Table 5.1, 5.2, 5.3, 5.4Short-circuit K values and temperature limits
Every clause and table, with how each was verified
Tables last checked on 2 Oct 2026.
Related tools
- Transformer inrush and its breakerCheck a transformer’s switch-on inrush against its breaker.AS/NZS 3000
- Earth fault loop impedance calculatorCheck earth fault loop impedance against the maximum Zs.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Cable fault rating calculatorCheck that a conductor can withstand the fault current.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
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