Breaker Size Calculator
This breaker size calculator helps you determine the correct circuit breaker rating based on current, voltage, and safety margin. Use this electrical calculator for proper circuit breaker sizing in industrial and commercial applications.
For detailed guides and examples, see our Protection and Sizing Guide
Input Parameters
Calculation Results
⚠️ Professional Disclaimer: This calculator provides simplified breaker sizing for preliminary engineering only. Final protection device selection must follow applicable standards (IEC/NEC), manufacturer data, short-circuit studies, and coordination checks by a qualified engineer.
Why this result? (Optional)
Calculation InsightsHow to Calculate Breaker Size
Formula: Breaker Size (A) = Load Current (A) × Safety Factor
The calculated current is then matched to the next higher standard breaker rating (for example 15, 20, 30, 40, 50, 60, 80, 100, 125, 150, 175, 200, 225, 250, 300, 400, 600 A).
Use higher safety factors for continuous or motor loads to account for thermal loading, inrush, and future margin. Final selection must also consider short-circuit rating, coordination with upstream devices, and local code requirements.
Typical Examples
- Example 1: 32 A continuous panel load × 125% gives a 40 A breaker selection path.
- Example 2: 90 A feeder with safety factor can move to 125 A standard breaker rating.
- Example 3: Motor branch current with 150% factor provides initial breaker sizing before coordination study.
Guides for Breaker Selection
For deeper explanations and worked examples, see our power and protection guides:
Frequently Asked Questions
What is a breaker size calculator used for?
It is used to estimate a suitable breaker current rating from load current and safety factor during preliminary electrical design.
How do I calculate breaker size from load current?
First calculate the load current, then multiply by an appropriate safety factor (for example 125% for continuous load). Finally, select the next higher standard breaker size. This calculator automates those steps for you.
What is the standard breaker sizing formula?
The formula is Breaker Size (A) = Load Current (A) × Safety Factor, then round up to the next standard breaker rating.
Can you show a breaker size example?
Example: 80 A continuous load × 1.25 = 100 A, so select at least a 100 A standard breaker, then verify with coordination and code checks.
Why do continuous loads often use 125%?
Continuous loads heat the breaker for long periods. A 125% factor helps prevent nuisance tripping and keeps the breaker operating within its thermal limits, following typical code practice for continuous current.
When should I use 150% for motor loads?
Motors have inrush current that can be several times the running current. For many applications, using 150% as a starting point helps accommodate inrush while coordination and protection studies refine the exact rating.
Does voltage affect breaker size?
Breaker current rating (A) is based on load current, but its voltage rating (for example 230 V, 400 V) must be suitable for the system. This tool focuses on current sizing; voltage and breaking capacity must be checked separately.
Is this calculator enough for final design?
No. It is a preliminary sizing tool. Final designs must include short-circuit calculations, coordination studies, cable sizing, ambient conditions, and must comply with IEC/NEC or local codes.