MCB and Cable Coordination: the Ib Less Than In Less Than Iz Rule

What is the Ib ≤ In ≤ Iz rule for MCBs? It is the overload-coordination requirement in IEC 60364-4-43 that ties a breaker's rated current to both the load it feeds and the cable it protects, so the breaker trips before the cable insulation is damaged. Get the ordering wrong — set In above the cable's capacity Iz — and the MCB stops protecting the cable at all; it only protects the connected load. This article covers the rule's three terms, the second condition on conventional tripping current, why the cable and not the appliance is the thing actually at risk, a worked sizing example, and the fire-risk consequence of an oversized breaker on undersized cable.

The Coordination Rule in One Line

IEC 60364-4-43 Clause 433.1 sets the condition every final circuit must satisfy for overload protection: the design current must not exceed the rated current of the protective device, and the rated device current must not exceed the cable's current-carrying capacity.

Three numbers, one inequality. Get any one of them out of order and the chain breaks — either the breaker trips on normal load (I_b > I_n), or it never trips before the cable overheats (I_n > I_z).

What Each Term Actually Means

Ib — Design Current

I_b is the current a circuit is expected to carry in normal service, calculated from the connected load, diversity factor, and duty cycle. For a socket-outlet ring it's an assumed diversified demand; for a fixed motor circuit it sits close to full-load current. I_b is a design number, not a measurement — pull it from the load schedule, not from a single clamp-meter reading.

In — Rated Current of the MCB

I_n is the nameplate rating stamped on the breaker — typically 6, 10, 16, 20, 25, 32, 40 or 63 A on standard MCB frames. It's fixed by the device, not adjustable on site, unlike an MCCB with an adjustable thermal-magnetic trip unit. Selecting I_n means picking the smallest catalog value at or above I_b, then checking that same value does not exceed I_z.

Iz — Cable Current-Carrying Capacity

I_z is not the cable's datasheet ampacity taken in isolation. It's that value after every installation correction factor has been applied: ambient temperature, grouping with other circuits, contact with thermal insulation, and installation method — clipped direct, in conduit, or buried. A 4 mm² cable rated 37 A in free air can drop to the mid-20s once grouped with several other circuits in a warm plant room.

See our guide on MCB derating for temperature and grouping for the correction-factor tables used to get from tabulated rating to I_z.

The Second Condition: I2 ≤ 1.45 × Iz

I_b ≤ I_n ≤ I_z alone doesn't fully close the loop. IEC 60364-4-43 adds a second check: the current that guarantees the device actually operates within its conventional time — I₂ — must not exceed 1.45 times I_z.

For fuses, I₂ varies by manufacturer and has to be read off a fusing curve. For MCBs to IEC 60898-1 it doesn't — the conventional tripping current is fixed by the product standard at 1.45 × I_n for every MCB, every curve, every manufacturer. That's what the conventional trip test in the standard verifies (1 hour up to 63 A, 2 hours above it).

Substitute that fixed relationship into the second condition: 1.45 × I_n ≤ 1.45 × I_z cancels down to I_n ≤ I_z — the first condition, already checked. What we see in the field: engineers trained on fuse boards sometimes go looking for I₂ as a separate lookup on MCB circuits and get thrown when it always clears alongside the first condition. It isn't a coincidence. For MCBs the two conditions collapse into one because the standard fixes the 1.45 factor. The second condition earns its keep on fuse-protected circuits, where I₂ is not a fixed multiple of I_n and has to be checked independently.

Why the Rule Protects the Cable, Not the Load

An MCB downstream of a socket, motor starter, or distribution board doesn't know what's plugged in. It only sees the current flowing through its own poles. Set I_n too high relative to I_z and the breaker will hold a current the connected appliance would never draw — but that the cable feeding it cannot sustain.

Overcurrent protection does two distinct jobs, sized by two different numbers. The load-facing side lives in I_b: pick a device that won't nuisance-trip on start-up current or normal duty. The cable-facing side lives in I_z: pick a device that trips before the conductor's insulation ages past its rated life. I_n ≤ I_z is entirely about the second job. This depends on the load's inrush characteristic too — a C or D curve keeps the breaker from tripping during motor start, but the thermal element still has to respect I_z on sustained current, regardless of curve. Curve letter changes the instantaneous trip threshold, not the thermal one — see MCB tripping curves B, C, D, K and Z explained.

Worked Example

Circuit: a fixed load with a calculated design current I_b of 28 A. The feeder is 4 mm² PVC-insulated cable, clipped direct, grouped with three other circuits at 30°C ambient. Table-rated capacity for that cable size and method is 37 A; applying a grouping factor of 0.82 for four circuits gives I_z = 37 × 0.82 ≈ 30.3 A.

Candidate breaker ratings on the shelf: 25 A and 32 A. 25 A fails immediately — I_n (25) is less than I_b (28), so the breaker would trip under normal load. 32 A passes the first check — I_b (28) ≤ I_n (32) — but fails the second: I_n (32) exceeds I_z (30.3). The 32 A breaker fails coordination even though it comfortably clears I_b.

The fix is not a bigger breaker — it's a bigger cable, or fewer circuits in the group. Moving to 6 mm² cable at the same installation method and grouping raises I_z past 32 A, and the 32 A breaker now satisfies I_b (28) ≤ I_n (32) ≤ I_z. The conventional tripping check follows automatically: I₂ = 1.45 × 32 = 46.4 A, and 1.45 × I_z now clears it too.

What Happens When an MCB Is Oversized for the Cable

Skip the check and the failure mode is silent for years, then sudden. A cable rated I_z = 20 A protected by a 32 A MCB will carry a sustained 28-30 A overload — inside the load's normal draw, never touching the breaker's trip threshold — indefinitely. The bimetal thermal element only responds meaningfully once current climbs well past I_n; a cable running 40-50% over its rated capacity for months degrades its insulation through cumulative thermal aging, well before the breaker registers a fault condition.

PVC and XLPE insulation embrittle and crack under sustained overtemperature. Once cracked, insulation resistance drops, and the failure path is a slow-developing short or an arcing fault inside a wall cavity or cable tray — exactly the ignition source cable overload protection exists to prevent. An oversized MCB on an undersized cable does not fail safe. It removes the one safeguard meant to catch this condition.

Not always obvious at inspection, either. The breaker looks correctly rated against the panel schedule, the load looks normal, and nothing trips. The mismatch only shows up when someone checks I_z against I_n directly, or after the cable has already been damaged.

Selecting Iz-Compatible Ratings in Practice

Work the sizing in this order, not in reverse: calculate I_b from the load schedule, size the cable and apply every derating factor to get I_z, then pick the smallest standard I_n that is at or above I_b and at or below I_z. If no standard MCB rating fits between I_b and I_z, upsize the cable rather than the breaker — a breaker rated above I_z is not a valid substitute for a larger conductor.

For a full walk-through covering curve selection, breaking capacity, and pole configuration alongside this cable check, see our MCB selection checklist. Standards references for both the device and the installation rule are covered in IEC 60898-1 vs IEC 60947-2 standards compared. Browse rated devices across breaking capacities and curves in our miniature circuit breakers collection, and see the full sizing framework in the MCB engineering guide.

Conclusion

I_b ≤ I_n ≤ I_z is not a formality for the panel schedule — it decides whether the MCB protects the cable or just rides along with it. For MCBs, the second condition on conventional tripping current collapses into the first because IEC 60898-1 fixes I₂ at 1.45 × I_n. That leaves one number to get right on every circuit: I_n has to sit between the load's design current and the cable's derated capacity, not above it. Skip that check and the breaker becomes decorative — the cable carries the risk instead.