Fuse/MCB Selection

Suggested rated current/factor (continuous load, starting current)

Calculation
Quick Introduction

Fuse/MCB selection must balance operational continuity and protective function. Undersized devices cause nuisance trips, oversized devices can reduce protection quality.

This calculator suggests a practical rated current \(I_n\) and characteristic (B/C/D or gG-oriented) from continuous load and startup behavior.

Selection schema (simplified):
1) \(I_{n,min}=k_B\cdot I_B\)
2) Check startup ratio \(I_{start}/I_n\)
3) Select curve: B (low), C (medium), D (high)
Formulas (MathJax)
\[I_{n,min} = k_B\,I_B\]
\[I_{start} = k_{start}\,I_B\]
\[r = \frac{I_{start}}{I_n}\]
\[\text{B: }r\lesssim3\text{ to }5,\quad \text{C: }r\lesssim5\text{ to }10,\quad \text{D: }r\lesssim10\text{ to }20\]
Symbol Legend
  • \(I_B\): operating/continuous current [A]
  • \(I_n\): rated current of protective device [A]
  • \(k_B\): continuous load factor (reserve)
  • \(k_{start}\): starting current factor
  • \(I_{start}\): starting current [A]
  • \(r\): starting-current to rated-current ratio


Examples
Example 1: \(I_B=18\,A\), \(k_B=1.25\) ⇒ \(I_{n,min}=22.5\,A\) → next standard \(25\,A\). With \(k_{start}=5\), \(I_{start}=90\,A\), ratio \(r=3.6\) → characteristic C recommended.
Example 2: \(I_B=12\,A\), \(k_B=1.2\), \(k_{start}=2\) ⇒ \(I_n=16\,A\), \(r=1.5\) → B is sufficient.
Detailed Description & Summary

Protective device selection is always a balance between availability and safety. The reserve factor addresses continuous operation, while startup consideration helps avoid nuisance tripping for motors and inrush-heavy loads.

This calculator provides a technical preselection. Final engineering still requires checks for cable sizing, installation conditions, selectivity, and applicable standards.

Summary
  • Rated-current suggestion from continuous load and reserve
  • Characteristic suggestion from startup behavior (B/C/D)
  • Validation mode for existing protective-device rating

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