Calculator Inputs
Enter corrected site conditions, steady load, and startup surge. The result appears above this form after submission.
Trip Curve Graph
The chart uses a log time scale. The shaded band marks the magnetic zone for the selected curve. This is a planning estimate, not a manufacturer-certified trip study.
Example Data Table
| Scenario | Curve | Breaker (A) | Load (A) | Surge × | Duration (s) | Ambient Factor | Cable Factor | Comment |
|---|---|---|---|---|---|---|---|---|
| Temporary lighting board | B | 16 | 9.5 | 1.2 | 0.05 | 1.00 | 0.95 | Low inrush and strong overload visibility. |
| Mixed site sockets | C | 32 | 24 | 3.5 | 0.35 | 0.95 | 0.90 | Balanced default for many construction boards. |
| Compressor feeder | D | 63 | 38 | 6.0 | 0.80 | 0.92 | 0.88 | Higher magnetic tolerance helps motor starting. |
| Control and instrument panel | Z | 10 | 4.5 | 1.1 | 0.03 | 1.00 | 0.98 | Sensitive protection for delicate electronic circuits. |
Formula Used
Effective Capacity = In × Ambient Factor × Cable Derating × (1 − Safety Margin)
Steady Multiple = Steady Load Current ÷ Effective Capacity
Peak Current = Steady Load Current × Surge Multiplier
Peak Multiple = Peak Current ÷ Effective Capacity
Apparent Power (kVA) = Voltage × Current ÷ 1000
Real Power (kW) = Voltage × Current × Power Factor ÷ 1000
This page uses log interpolation through common planning anchors for overload and magnetic regions. It then maps your steady and peak multiples onto the selected curve family.
Actual breakers vary by maker, frame size, temperature, and published standards. Use this result for planning and early comparison, then verify against the exact manufacturer trip curve before installation.
How to Use This Calculator
- Choose the breaker curve family that best matches the circuit type.
- Enter the rated breaker current and the circuit voltage.
- Add power factor and steady running current for the connected load.
- Enter startup surge multiplier and how long the surge lasts.
- Apply ambient and cable derating factors for site conditions.
- Add a safety margin to preserve spare capacity.
- Press the calculate button to show the result above the form.
- Review utilization, headroom, trip times, recommendations, and the plotted trip curve.
Frequently Asked Questions
1) What is a trip curve?
A trip curve shows how fast a breaker may open at different current multiples. It combines overload behavior and magnetic pickup behavior to help compare protection choices.
2) Why do B, C, D, K, and Z curves behave differently?
Each curve shifts the magnetic pickup region. Sensitive circuits often use lower pickup ranges, while motors and inductive loads usually need higher pickup tolerance during startup.
3) Is this calculator exact for procurement or compliance?
No. It is a planning estimator. Final design should use the exact breaker manufacturer curve, applicable standards, cable data, and a full coordination review.
4) What does the safety margin change?
Safety margin reduces usable capacity before comparison. It adds design headroom, helping you avoid circuits that look acceptable on paper but operate too close to the limit.
5) Why does surge duration matter?
A short surge may pass without tripping even inside a magnetic region. A longer surge gives the device more time to react, so nuisance tripping becomes more likely.
6) Why include ambient and cable derating?
Real construction sites are rarely laboratory conditions. Heat, grouping, and installation method can reduce practical carrying capacity, so these factors improve planning realism.
7) Should I increase breaker rating or change the curve?
Increase the rating when steady current is too high. Change the curve when steady load is acceptable but startup current still pushes the magnetic trip region.
8) Can I use this for temporary site distribution boards?
Yes, that is a common use. It helps compare construction loads, startup behavior, and headroom before you finalize board schedules and protection selections.