Breaker Size Calculator
Free breaker size calculator for preliminary MCB, MCCB, feeder, and load protection planning with formulas, sample data, warnings, and PDF reports.
Inputs
Forms are empty on load. Use sample data to preview the workflow or enter your own project values.
Learn more
Formula logic, use cases, assumptions, and limits for this tool.
How this tool helps
Use this for early breaker rating checks before final protective-device coordination. Final selection must verify cable ampacity, fault level, trip curve, selectivity, derating, and local code.
Common search intent
This page supports practical searches for breaker size calculator, free MEP calculator, design estimate, engineering planning, and construction coordination checks.
Assumptions and limits
This is a planning-level calculator. Final engineering decisions must be verified against authority requirements, project specifications, manufacturer data, installation conditions, and approved standards.
Keyword and design notes
Common searches around this task include breaker size calculator, MCB size calculator, MCCB sizing, circuit breaker calculator, and feeder breaker sizing. Use the result as an early protection size before checking cable ampacity, voltage drop, short-circuit duty, load type, trip curve, continuous load rules, and selectivity with upstream and downstream devices. For best results, compare the calculated breaker size against both the normal running current and the expected starting or inrush current. Motors, transformers, UPS systems, and nonlinear loads may need different protection settings than simple resistive loads.
Calculation methodology
Load current is calculated from kW, voltage, phase, and PF. Design current adds margin and rounds up to a standard breaker rating.
Inputs are treated as planning assumptions. The calculator shows warnings when entered values fall outside practical thresholds or selected standard-profile limits.
Version history
- v1.3.6-prelaunch: Added methodology, standard profile support, version notes, and direct PDF report download.
- v1.3.5: Calculator page added with sample data, warnings, and print-ready report layout.
Stage 2 roadmap: optional saved projects, dashboards, and shared calculation history without forcing login for free use.
Trust and verification
Every result includes assumptions, warning logic, and a downloadable report. Final engineering decisions still need qualified review, local authority approval, manufacturer data, and project specifications.
Worked example
Use the sample data button to load a realistic scenario, then download the PDF report to review inputs, results, methodology, warnings, and disclaimers in one place.
Input definitions and typical values
These notes explain what each input means and which values usually produce a more reliable planning result.
Code / standard profile
Selects the planning profile used for conservative defaults. Use the project authority or client standard where known; otherwise use the closest regional profile. Options: GCC project planning, NEC planning, IEC planning, BS 7671 planning.
Load kW
Load or power in kW. Use realistic running or design demand, not inflated connected load unless the tool asks for connected load.
Voltage V
System voltage. Common LV systems include 230 V single-phase and 400/415 V three-phase; use project nominal voltage.
Phase count
Electrical phase arrangement. Use single-phase for small loads and three-phase for larger balanced motors, panels, and feeders. Options: Three phase, Single phase.
Power factor
Power factor. 0.8-0.95 is common for many loads; motors and mixed loads should use measured or specified PF.
Sizing margin %
Design safety margin. 5-15% is common for planning; excessive margin can oversize equipment, cables, pumps, or tanks.
Practical verification checklist
- Confirm the selected standard profile is suitable for the project location and authority.
- Replace sample values with project drawings, schedules, equipment data, or measured site values.
- Review warnings, methodology, and PDF report before using the result outside early planning.
- Verify final decisions against official standards, manufacturer data, consultant specifications, and local approvals.