Every breaker and fuse in your facility carries two current numbers, and most people only ever look at one of them. The trip rating — 20 A, 400 A — is the one on the drawings. The interrupting rating is the one that decides whether the device safely stops a fault or fails while trying.
What is an AIC rating?
The interrupting rating — commonly called AIC, for ampere interrupting capacity — is the maximum fault current a protective device has been tested to interrupt at a stated voltage. A typical molded-case breaker might carry 10 kA, 22 kA, 42 kA, or 65 kA. Most current-limiting fuses are rated 200 kA or more, which is a large part of why they get specified.
The rating is voltage-dependent: the same breaker often carries a higher interrupting rating at 240 V than at 480 V. It is fixed by design and testing — there is no setting to adjust and no upgrade kit. If the available fault current at the breaker's line terminals exceeds its interrupting rating, the breaker is misapplied, full stop.
Equipment that doesn't clear faults — panelboards, switchboards, motor control centers, industrial control panels, disconnects — carries the companion number: a short-circuit current rating, or SCCR. It states how much fault current the assembly can withstand while an upstream device clears. A panelboard with a 10 kA SCCR fed from a system that can deliver 30 kA is just as misapplied as an under-rated breaker.
What does the NEC require?
Two short sections carry the whole obligation:
- NEC 110.9 requires equipment intended to interrupt fault current to have an interrupting rating at least equal to the current available at its line terminals.
- NEC 110.10 requires the circuit protection, component short-circuit ratings, and equipment characteristics to be selected so a fault is cleared without extensive damage to the electrical components of the circuit.
Neither section tells you what your available fault current is. That number comes from a calculation — the utility's contribution, every transformer between the utility and the bus, conductor impedances, and the contribution of running motors, which feed a fault for the first few cycles like small generators. This is exactly what a short circuit study computes at every bus, and it is why the study is the foundation the arc flash and coordination analyses are built on.
NEC 110.24 closes the loop at the service: service equipment in other-than-dwelling occupancies must be field-marked with the maximum available fault current and the date the calculation was performed, and the marking must be updated when system modifications change the number.
What happens when a device is applied above its rating?
A breaker asked to interrupt more current than it was tested for may not clear the fault. The contacts try to part, the arc doesn't extinguish, and the device can fail violently — which converts an ordinary downstream fault into an arcing fault inside the enclosure, with the energy of the full available fault current behind it. The same logic applies to an under-rated bus: the mechanical forces during a fault scale with the square of the current, and bracing designed for 10 kA is not a suggestion.
This is not a theoretical failure mode. It is the reason the comparison between calculated fault current and equipment ratings is the first table we check in every study.
Why does equipment become under-rated with no visible change?
Available fault current is set by what is upstream of you, and upstream changes without asking:
- Utility reinforcement. Utilities upgrade their distribution systems continuously. A stiffer source means more fault current at your service — and nobody sends a letter.
- Transformer replacement. A failed 500 kVA transformer replaced with a 750 kVA unit, or one with a lower impedance, raises fault current on everything downstream.
- Facility expansion. A new service or a larger transformer for an addition changes the numbers on gear that has been in place for decades.
Equipment that was correctly applied when it was installed can be under-rated today, looking exactly the way it always has. Older gear compounds the problem: interrupting ratings on legacy equipment are often lower than anything sold today, so the drift works against you from both directions. We wrote up a related modeling question — how VFD bypass operation changes calculated fault current — in this short circuit case study.
What are the options when equipment is over-dutied?
The study should point at the fix, not just the finding. In rough order of how often they get used:
- Replace the device with one carrying an adequate interrupting rating. Straightforward on a molded-case breaker; more involved when it's the whole panelboard's SCCR.
- Apply a tested series-rated combination under NEC 240.86, where an upstream current-limiting device is tested in combination with a lower-rated downstream breaker. Series ratings come with real conditions — the combination must be tested and marked (or engineered under a licensed PE's supervision for existing installations), and motor contribution connected between the devices is limited — so this is a calculation, not a catalog substitution.
- Reduce the available fault current, most commonly with current-limiting fuses upstream, occasionally with a higher-impedance transformer at replacement time.
Which one is right depends on the numbers, the age of the gear, and what else the facility needs from the same project — a fix that also lowers incident energy at a high-hazard bus is worth more than one that doesn't.
How do you find out where you stand?
Count on three things: a field-verified one-line, a calculated fault current at every bus, and a line-by-line comparison against the interrupting and withstand ratings of the installed equipment. That is the core deliverable of a short circuit analysis, and it reuses the same system model as an arc flash study, which is why running them together costs meaningfully less than running them apart.
If the study on file predates your last service upgrade, transformer change, or utility modification — or if there is no study on file — the honest answer is that nobody currently knows whether your equipment is properly rated.