Calculator Form
Formula Used
Three phase current: I = P × 1000 / (√3 × V × PF × Eff)
Single phase current: I = P × 1000 / (V × PF × Eff)
DC current: I = P × 1000 / V
Design current: Ib = Load current × Continuous factor × Demand factor
Target breaker: It = Ib × Spare capacity factor
Long-time pickup: Ir = Sensor rating × Long pickup percentage
Short-time pickup: Isd = Ir × Short-time multiplier
Instantaneous pickup: Ii = Ir × Instantaneous multiplier
Ground pickup: Ig = Sensor rating × Ground pickup percentage
How to Use This Calculator
Enter either the known load current or the load power. Select the system type. Add voltage, power factor, and efficiency when power is used.
Set the continuous load factor, demand factor, spare capacity, cable ampacity, and breaker rating. Leave manual breaker rating at zero to choose the next standard rating automatically.
Enter trip unit settings for long-time, short-time, instantaneous, and ground fault protection. Press the calculate button. Use CSV or PDF buttons to save the result.
Example Data Table
| Load | Power | Voltage | PF | Continuous Factor | Suggested Breaker |
|---|---|---|---|---|---|
| Pump motor | 25 kW | 415 V | 0.85 | 125% | 63 A |
| Panel feeder | 80 kW | 415 V | 0.90 | 125% | 160 A |
| Lighting board | 12 kW | 230 V | 0.95 | 125% | 80 A |
Manual Guide for Circuit Breaker Settings
Circuit breaker setting calculation is a practical protection task. It links load demand, cable limits, and fault protection. A breaker should carry normal current without nuisance trips. It should also open quickly during dangerous faults. This calculator gives a structured manual method for early design checks.
Load Current Review
The first step is finding the base load current. For a three phase load, the tool uses power, voltage, power factor, and efficiency. For a single phase load, it removes the square root three term. For direct current loads, voltage and power are enough. You may also enter a known current directly.
Design Margin
Real circuits often need margin. Continuous loads may need a higher setting. Demand factors can reduce the calculated load when allowed by design rules. Spare capacity helps future additions. These factors create the design current. The calculator then finds a target breaker value.
Trip Unit Settings
Modern breakers may include long-time, short-time, instantaneous, and ground fault controls. Long-time pickup protects conductors from overload. Short-time pickup supports selective coordination. Instantaneous pickup clears high faults fast. Ground fault pickup helps detect leakage or earth faults.
Coordination Checks
The selected breaker should not exceed the cable ampacity. Its interrupting capacity should exceed the available fault current. Motor circuits need careful inrush review. If the instantaneous setting is too low, the breaker may trip during starting. If it is too high, protection may become slow.
Engineering Use
This page is a calculation aid, not a final approval document. Always compare results with local codes, manufacturer curves, equipment labels, and a qualified engineer's study. Use the export buttons to keep a clear record of assumptions and settings.
FAQs
What is a circuit breaker setting?
It is the current or delay value used by a breaker trip unit. It decides when the breaker should trip during overloads, short circuits, or ground faults.
Why is continuous load factor included?
Continuous loads can heat conductors and breaker parts for long periods. A factor such as 125% gives extra allowance where design rules require it.
Can I enter a known load current?
Yes. Select the known current method and enter amperes directly. The calculator will skip the power based current formula.
What does long-time pickup mean?
Long-time pickup is the overload trip threshold. It is usually based on the sensor rating, rating plug, or breaker frame arrangement.
What is instantaneous pickup?
Instantaneous pickup is the fast trip level for high fault currents. It should be high enough to avoid inrush trips and low enough for protection.
Why check interrupting capacity?
The breaker must safely interrupt the available fault current. If the available fault current is higher, the selected breaker may be unsafe.
Can this replace a coordination study?
No. It is a manual screening calculator. Final settings need manufacturer curves, fault studies, cable data, code checks, and professional review.
Why export CSV and PDF?
Exports help save assumptions, settings, and review notes. They are useful for maintenance files, design records, and field verification sheets.