Ib, In and Iz: the three currents in a basic breaker check

Ib belongs to the load

Ib is the design current expected in normal service. It should come from a transparent load assessment: connected load, demand assumptions, operating profile and the applicable power relationship.

It is not automatically the sum of every nameplate, and it is not chosen by looking at the breaker you want to use.

In belongs to the protective device

In is the nominal current rating of the protective device. In a basic comparison it should not be below the design current, otherwise normal operation may be incompatible with the selected rating.

Nominal current is only one device characteristic. Curve, poles, breaking capacity, energy limitation, residual-current behavior and product-specific conditions remain separate.

Iz belongs to the installed cable

Iz is the cable’s corrected current-carrying capacity for the assessed installation. It is not the base table value before ambient, grouping or other relevant factors are applied.

That distinction matters. A cable listed at 40 A in one reference condition may have a lower corrected capacity in the proposed route.

The basic relationship

The preliminary inequality is:

Ib ≤ In ≤ Iz

Suppose design current is 20 A and corrected cable capacity is 28 A. A 25 A nominal rating satisfies the relation. A 16 A rating is below the design current; a 32 A rating is above the corrected cable capacity.

This example does not prove that the 25 A device protects the circuit correctly under faults or that its operating curve suits the load.

Avoid circular selection

A weak workflow chooses a breaker, then chooses a cable to match it, while quietly adjusting demand until the numbers fit. A better workflow keeps the sources visible:

1. Establish Ib from the load.
2. Establish candidate Iz from reviewed cable data and installation factors.
3. Compare available In ratings.
4. Complete device operating, fault and coordination checks.

If the later checks fail, revisit the design rather than relabelling the currents.

Why the symbols are useful

They create a compact audit trail. A reviewer can see whether a number came from the load, cable or breaker. ElectricalDyev’s helper stops at this narrow relationship on purpose; it does not infer breaking capacity or compliance from three current values.

What the inequality does not prove

Ib ≤ In ≤ Iz does not establish automatic disconnection, fault-loop performance, breaking capacity, selectivity, backup protection, RCD requirements or suitability for motor starting. It also does not validate the cable dataset or correction factors.

Use the inequality as one transparent checkpoint. Keep each current’s origin visible, then continue with the fault, device-characteristic and project-specific assessments required for a complete design.

Put the three numbers beside the result

Symbol	Meaning in this preliminary check	Where it comes from
Ib	Design load current	Load calculation
In	Nominal protective-device rating	Device data
Iz	Corrected cable capacity	Traceable cable data and applicable factors

Compare Ib, In and Iz. For example, 20 A ≤ 25 A ≤ 28 A satisfies this one relationship. It does not demonstrate breaking capacity, fault disconnection or coordination.

For cases where a breaker is too large or correction factors reduce Iz, continue with the Ib, In and Iz worked examples.