MCB & MCCB Sizing Calculator

A breaker is chosen for the cable, not for the load

This is the idea everything else follows from. The device protects the cable; the load is merely what decides the minimum. The rule that ties them together:

Ib ≤ In ≤ Iz

• Ib — design current. The breaker must be at least this, or it trips on normal load.
• In — the device rating.
• Iz — the cable's current capacity after derating, not the tabulated figure.

Miss the right-hand half and the cable can sit above its rating indefinitely with the breaker perfectly content. A cable protected by a breaker that is too large is not protected at all — and this is the single most common fault in a distribution design, because the left-hand condition alone always looks satisfied.

Note which Iz. Take the tabulated rating, apply the ambient, grouping and installation factors from the cable sizing calculator, and use that. A 70 mm² cable rated 174 A on the table might be 118 A on your tray, and 118 A is the number this rule uses.

The second condition, which only matters for fuses

I₂ ≤ 1.45 × Iz

I₂ is the current that guarantees the device operates within its conventional time — 1.45 × In for MCBs to IEC 60898, 1.30 × In for MCCBs to IEC 60947-2. Because those factors are at or below 1.45, any device satisfying In ≤ Iz satisfies this automatically.

It is worth knowing anyway, because it does not hold for fuses. A gG fuse can have an I₂ of 1.6 × In, so a fuse sized at Iz fails this check and must be taken down a size.

Trip curves

The failure people expect is a D curve on a lighting circuit nuisance-tripping. The failure that actually hurts is the opposite: a D-curve device at the end of a long run where the fault current never reaches 10 × In, so the magnetic element never operates and a short circuit is cleared only by the thermal element — seconds later, if at all.

That is why the minimum fault current at the far end of a circuit is worth calculating. It has to exceed the magnetic pickup shown above, with margin.

Breakers derate in a hot panel

A thermal element is a bimetallic strip, and it does not know why it is warm. Manufacturers calibrate at a reference ambient — 30 °C for MCBs, 40 °C for MCCBs — and inside a loaded enclosure the air is well above the room.

A 63 A MCB at 50 °C is a 55 A device. If the load is 58 A it will trip, intermittently, on hot afternoons — and it will test perfectly on the bench every time an electrician takes it out to check.

Motors are the exception

On a motor circuit the overload protection is the starter's relay, not the breaker. The upstream device is doing short-circuit duty only and is deliberately sized above the motor full load current so the starting inrush passes.

The In ≤ Iz limit then applies to the overload relay setting, not to the breaker. Size the whole starter — contactor, relay, breaker and cable — with the motor starter calculator, and prefer an MPCB, which packages an adjustable thermal element with a fixed magnetic element at about 13 × In.

Icu, Ics and Icn

• Icu — ultimate breaking capacity. The device interrupts this fault once, safely, and may be scrap afterwards.
• Ics — service breaking capacity. It interrupts and stays fit for service. Quoted as a percentage of Icu.
• Icn — the rated capacity for MCBs to IEC 60898, typically 6 kA or 10 kA.

Specifying on Icu alone accepts that after a serious fault the device is scrap and possibly no longer protecting anything. On incomers and anywhere downtime is expensive, ask for Ics = 100 % of Icu.

One economy worth knowing: cascading, where an upstream current-limiting device backs up a downstream one so the downstream device can be rated below the prospective fault at its position. It is valid only for combinations the manufacturer has tested and published — you cannot derive it, and you cannot mix brands.

Discrimination: which device should open

Sizing a breaker correctly for its own cable still leaves a question the calculation does not answer — when a fault occurs on a final circuit, does the final circuit breaker clear it, or does the incomer trip and take the whole board with it?

Getting that right is discrimination, and it needs a ratio in both dimensions. In the overload region a rule of thumb of about 1.6 between successive ratings usually holds: a 63 A upstream of a 32 A generally grades. In the short-circuit region it frequently does not, because two magnetic trips both operate in under 20 milliseconds and neither has time to wait for the other. Two devices whose curves cross at the prospective fault current will race, and the outcome is decided by manufacturing tolerance.

Which is why manufacturers publish discrimination tables for specific pairs of their own devices rather than expecting the ratio to be worked out. Where full discrimination genuinely matters, the tables are the only reliable source — and where it cannot be achieved with breakers alone, a fuse upstream or a short time-delay setting on an air circuit breaker is the usual remedy.

Overcurrent and earth fault are two separate duties

A breaker sized by everything on this page protects the cable against overload and short circuit. It does very little about an earth fault through a person, and nothing at all about a low-current fault to earth that never reaches the trip threshold.

Those need residual current protection, which measures the imbalance between line and neutral rather than the magnitude of either. A 30 mA device is the figure for additional protection against direct contact and is required on socket outlets and most final circuits under current practice; 100 mA and 300 mA devices exist for fire protection on distribution circuits, where the concern is a sustained earth leakage heating something rather than a person in the path.

Two practical points follow. Nuisance tripping on a 30 mA device is usually cumulative standing leakage rather than a fault — every filtered switch-mode supply leaks a small current to earth by design, and thirty of them on one RCD add up to a meaningful fraction of the threshold. Splitting the load across several devices is the fix, not raising the rating. And a residual device offers no overcurrent protection whatsoever unless it is an RCBO, so an RCD without a breaker in series is protecting people while leaving the cable entirely unguarded.