Select a standard MCB/MCCB breaker size from amps or kW, with optional IEC and NEC verification against cable ampacity (Iz), available fault current, ambient temperature, grouping, and altitude derating of the device.
Results are estimates. Breaker selection and installation must comply with the applicable standard, local amendments and the equipment manufacturer's data. Conductor ampacity must coordinate with the selected overcurrent device rating.
The multiplier is the whole decision. It is not a margin someone invented — each figure comes from a specific clause and a specific failure that clause exists to prevent.
Much of the confusion comes from the fact that 125% appears in motor work too — but for the conductor, not the breaker. NEC 430.22 sizes motor branch-circuit conductors at 125% of full load current; NEC 430.52 sizes the breaker at up to 250%. They are different components protected against different things. The breaker guards against short circuit and must let the motor start, while the overload relay — not the breaker — protects motor and cable against sustained overload. Size a motor breaker at 125% and it will trip on every start.
The most common shortcut in breaker calculators is to switch from MCB to MCCB above some current — 63 A, 100 A, whatever. That ignores the property that actually decides it. Miniature circuit breakers to IEC 60898 are commonly rated 6 to 10 kA breaking capacity and are available up to about 125 A. A moulded case breaker exists because it can interrupt far more.
So a 32 A circuit on a switchboard with 25 kA of available fault current needs an MCCB, while a 100 A circuit on a small installation with 6 kA might be perfectly served by an MCB. This tool asks for the available fault current before recommending a type, and says so plainly when you have not entered it — guessing from load current alone would be inventing an answer.
Two different corrections get mixed up constantly, so this tool separates them:
Applying the altitude factor to cable ampacity — as many quick calculators do — mixes a switchgear property into a conductor calculation and quietly understates the cable requirement.
The interrupting rating (Icu in IEC, AIC in NEC) is the largest prospective short-circuit current the breaker can clear without failing. NEC 110.9 requires it to equal or exceed the available fault current at that point. It has nothing to do with the breaker's current rating: a 32 A breaker on a 25 kA system still needs at least a 25 kA interrupting rating.
This tool asks for the rating of the breaker you actually intend to buy and compares it with the fault current you entered. Leave it blank and the check reports not checked rather than passed — a check with no data is not a pass.
Ib is the design current, In the rated current of the protective device, and Iz the cable's capacity after correction. The IEC rule is Ib ≤ In ≤ Iz.
Up to 250% of full load current on an inverse-time breaker (NEC 430.52). The 125% figure is the conductor rule (NEC 430.22), not the breaker rule.
No — mainly by breaking capacity. MCBs to IEC 60898 are commonly 6–10 kA and available to about 125 A, so a higher prospective fault current needs an MCCB regardless of how small the load is.
Thinner air cools the device less and quenches the arc less effectively, so switchgear rated current falls with altitude. Cable ampacity depends on ambient temperature, grouping and installation method instead.
The maximum prospective fault current the breaker can safely interrupt. It must equal or exceed the available fault current at the point of installation.
No. Discrimination between upstream and downstream devices, arc-flash calculation, earth-fault loop impedance and let-through energy all need a full protection study.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
Thanks for sharing an amazing tool.