Circuit Breaker Size Calculator
Pick the next standard breaker size above your load.
Your numbers
Your results
Circuit Breaker Size Calculator by TheCalcTools
How the Circuit Breaker Size Calculator works
Enter the load, power factor, voltage and supply type.
The tool calculates the load current and applies the continuous-load margin.
It then picks the next standard rating from the common series.
The formula
Worked example
Using the default values shown in the calculator:
| Load (kW) | 20 |
|---|---|
| Power factor | 0.85 |
| Line voltage (V) | 400 |
| Supply | Three-phase |
| Continuous-load margin (%) | 25 % |
- Load current = 34.0 A.
- With a 25% margin the minimum is 42.5 A, so choose a 50 A breaker (47% headroom).
Result:
| Load current (A) | 34.0 |
|---|---|
| Minimum rating with margin (A) | 42.5 |
| Standard breaker rating (A) | 50 |
| Headroom above load | 47% |
Frequently asked questions
Does the breaker also protect the cable?
Yes. The cable’s current rating must be at least the breaker rating (or the design current if using adjustable settings). Check the cable table and the voltage drop.
What about short-circuit rating?
The breaker’s breaking capacity must exceed the prospective fault current at its location. This calculator does not check that.
Sources and further reading
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This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.