A preliminary Ib ≤ In ≤ Iz breaker relationship check

What the helper actually checks

The relationship Ib ≤ In ≤ Iz compares three different currents. Ib is the expected design current. In is the protective device’s nominal rating. Iz is the corrected current-carrying capacity of the cable under the assessed installation conditions.

If design current is 20 A and corrected cable capacity is 32 A, a 20 A, 25 A or 32 A nominal rating may satisfy the simple inequality. Which device is appropriate still depends on factors the inequality does not address.

Why a passing rating is not a completed design

A breaker must be able to interrupt the prospective fault current at its installation point. Its time-current behavior must protect conductors and work with downstream and upstream devices. Some circuits need residual-current protection, motor starting accommodation or special load characteristics.

None of those questions can be answered from Ib and Iz alone. The helper intentionally avoids producing a breaking-capacity value or saying that a candidate is compliant.

Keep inputs traceable

Design current should come from the actual load assessment. Corrected cable capacity should come from a reviewed cable-sizing calculation using applicable installation data. A nominal rating series should represent devices genuinely under consideration rather than a generic list copied without context.

Pole count and curve labels are descriptive in this tool. They help record a concept but do not perform coordination or fault-protection calculations.

A useful sequence

1. Establish the circuit design current.
2. Determine corrected cable capacity using traceable data.
3. Identify nominal ratings that satisfy the preliminary relationship.
4. Complete short-circuit, earth-fault, breaking-capacity, selectivity and product-specific checks.

Used this way, the helper documents one transparent boundary instead of pretending that a single current comparison selects a complete protective device.