Overload protection (§433) — the coordination rule Ib ≤ In ≤ Iz and I₂ ≤ 1.45 Iz

The cable-sizing-and-current-carrying-capacity guide covers how a cable cross-section is chosen based on normal operating current (§523). The §434 short-circuit protection guide covers how that same cable must withstand a short-circuit current for the duration of the disconnection time. §433 covers a third, separate topic that sits between the two: overload protection — protecting the cable against a sustained, moderate overcurrent that is not a short-circuit, but can still damage the cable over time.

The coordination rule: two conditions

§433 (based on IEC 60364-4-43) states that overload protection must satisfy two conditions simultaneously:

$$I_b \le I_n \le I_z \qquad \text{and} \qquad I_2 \le 1.45 , I_z$$

Symbol	Meaning
Ib	The design current of the circuit — the current the connected load actually draws in normal operation.
In	The rated (set) current of the protective device — circuit-breaker or fuse.
Iz	The current-carrying capacity of the cable under the applicable installation conditions (see the cable-sizing guide for the correction factors).
I₂	The current that actually causes the protective device to operate within the conventional time (the "effective operating current").

The first condition (Ib ≤ In ≤ Iz) is the best-known one: the protective device must be large enough to carry the normal operating current without nuisance-tripping, and small enough to operate before the cable is structurally overloaded.

Why the second condition is not redundant

The second condition (I₂ ≤ 1.45 Iz) may look automatically satisfied once the first condition holds — but that depends on the type of protective device:

Miniature circuit-breaker (MCB, IEC 60898-1/60947-2): the conventional operating current I₂ is, by definition, fixed at 1.45 × In. Once In ≤ Iz (condition 1), condition 2 is therefore automatically satisfied — no separate check is needed in practice for a circuit-breaker.
Fuse (IEC 60269): the conventional fusing current is higher, typically around 1.6 × In. For a fuse, condition 2 is not automatically satisfied by condition 1 alone — it must be checked separately, and in practice this can mean a slightly larger cable cross-section is needed than condition 1 alone would suggest.

Note: this is exactly why "the fuse matches the cable" does not automatically mean the same thing as "the circuit-breaker matches the cable" — the underlying coordination mechanism differs between the two types of protective device.

Difference from §434 (short-circuit protection)

§433 and §434 look similar — both concern protecting a cable via the same protective device — but they assess a fundamentally different time range and fault mechanism:

	§433 — overload	§434 — short-circuit
Fault current	Moderately elevated (e.g. 1.5-2× In)	Very high (often hundreds of amps to kA)
Time scale	Minutes to hours	Milliseconds up to a maximum of ~5 s
Assessment method	Coordination rule Ib/In/Iz + I₂	Adiabatic equation S = √(I²t)/k
Failure mechanism	Gradual thermal ageing of the insulation	Near-instantaneous thermal breakdown

One and the same circuit-breaker fulfils both functions in many final circuits — but that is a consequence of how circuit-breakers are built (thermal element for overload, magnetic element for short-circuit), not a guarantee that automatically applies to every type of protective device and every circuit.

Practical relevance

When sizing a final circuit, it is not enough to check only whether the circuit-breaker or fuse "matches" the expected load — both conditions of the coordination rule must be explicitly verified, particularly for fuses, where condition 2 does not automatically follow from condition 1. For specific exception cases (for example circuits for which the standard, under certain conditions, does not require separate overload protection), the full standard should be consulted.

Common mistakes

Checking only condition 1 (Ib ≤ In ≤ Iz) and assuming condition 2 (I₂ ≤ 1.45 Iz) is automatically satisfied — for fuses that assumption is incorrect.
Confusing §433 with §434 — the coordination rule of §433 assesses overload, not the thermal short-circuit withstand of §434; both checks are required, not just one.
Determining the current-carrying capacity Iz without applying the correct correction factors (ambient temperature, grouping, installation method) — see the cable-sizing guide — which tests the coordination rule against an overly optimistic Iz value.
Assuming a larger circuit-breaker is always "safer" — an oversized In relative to Iz breaks condition 1 and leaves the cable unprotected against overload.