Fault Current, Interrupting Rating (AIC) and SCCR Explained
Breakers, fuses and panels are marked with ratings for fault current. This guide explains what those ratings mean, what they must be compared with and how to check a panel.
What is fault current?
When a short circuit happens, the current is limited only by the impedance of the source and the wiring back to it. In a plant the current can be thousands of amps, far more than the normal load. It flows for a moment, until a breaker or fuse opens. The most that can flow at a point in the system is its available fault current.
Three ratings, three questions
| Rating | On what | Question it answers |
|---|---|---|
| Interrupting rating (often called AIC) | A breaker or fuse | What is the highest fault current this device can open safely? |
| Short-circuit current rating (SCCR) | A control panel or other equipment | What fault current can this assembly withstand without a hazard? |
| Available fault current | A point in the system | What fault current can the supply deliver here? |
The rule is simple to state: the interrupting rating of every breaker and fuse, and the SCCR of every panel, must be at least the available fault current where it is installed (NEC 110.9 and 110.10). A device that is under-rated can fail violently when it tries to clear a fault bigger than it can handle.
How to check a panel
- Find the available fault current at the point where the panel is supplied. The utility, or the facility's short-circuit study, provides it. The fault current calculator gives an upper estimate from a transformer's nameplate.
- Read the panel's marked SCCR. NEC 409.110 requires industrial control panels to be marked with a short-circuit current rating. A panel built to UL 508A is marked with it.
- Compare. The SCCR must be equal to or higher than the available fault current. If it is not, the panel cannot be used there as is.
- Check the individual devices. Each breaker and fuse needs an interrupting rating at least as high as the fault current at its own location.
- Find the weakest part. A panel's SCCR is limited by its weakest component or combination. Parts such as contactors, overload relays and terminal blocks have their own ratings, often tested only with specific fuses or breakers upstream.
If the rating is too low
Common solutions include using current-limiting fuses or breakers that reduce the let-through current, choosing components with higher ratings, adding impedance such as a line reactor, or moving the panel farther from the source. The right answer depends on the equipment, so use the manufacturers' tested combinations and the panel builder's instructions.
Worked example
A 112.5 kVA, 480 V transformer with 3.5% impedance can deliver about 3,866 A at its secondary. A panel fed from it, built with a marked SCCR of 5,000 A, is acceptable at that point, but one marked 3,000 A is not. If the panel were fed from a larger transformer with 30 kA available, it would need an SCCR of at least 30 kA.
Related topics
- Arc flash is a separate topic covered by NFPA 70E. Fault current is one input to an arc flash study.
- Labeling. The NEC requires available fault current to be marked on certain equipment, such as service equipment (110.24).
- Motor and breaker sizing. See the breaker and fuse sizing calculator and the motor circuit sizing guide.
This is general guidance for training and reference and is not a substitute for the electrical code, equipment instructions or a qualified engineer.