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.