A short circuit in an industrial control panel can release a large amount of energy in a few milliseconds. Whether the panel contains that fault safely depends on its short-circuit current rating, or SCCR. This article explains what SCCR means, where the requirement comes from in North America, why one weak component can limit a whole panel, and what to watch for when you replace a component in an existing panel.
What SCCR means
The short-circuit current rating is the highest prospective fault current, at a stated voltage, that a component, an assembly or a complete panel can withstand without creating a fire or shock hazard, when it is protected by the overcurrent device specified for it. It is expressed in kiloamperes (kA, rms symmetrical) together with a voltage, for example "10 kA at 480 V".
SCCR is often confused with two other values. They answer different questions:
| Term | Applies to | What it tells you |
|---|---|---|
| Available fault current | A point in the installation | How much current the supply can deliver into a bolted fault at that point. It depends on the transformer, the conductors and the upstream system. |
| Interrupting rating (IR, AIC) | Fuses and circuit breakers | The highest fault current the protective device can safely interrupt at its rated voltage. |
| Short-circuit current rating (SCCR) | Components, assemblies, panels, machines | The highest fault current the equipment can withstand while the specified protective device clears the fault. |
The basic rule is simple: the SCCR of the equipment must be equal to or higher than the available fault current where it is installed, at the system voltage.
The marked SCCR must be at least the available fault current (example values).
Where the requirement comes from
- NEC Article 409 (industrial control panels) requires the panel to be marked with its SCCR and does not allow it to be installed where the available fault current is higher than that rating.
- NEC Article 670 (industrial machinery) carries the same idea to the machine nameplate.
- UL 508A contains a supplement (Supplement SB in recent editions) that gives the accepted methods for determining the SCCR of a control panel.
- Canada: the Canadian Electrical Code and CSA C22.2 No. 286 apply similar principles to control panels and assemblies.
Article and clause numbers move between editions, so always check the current edition adopted in your jurisdiction.
The weakest component sets the panel rating
Every component in the power circuit of a panel needs its own rating, including:
- disconnect switches and branch circuit protective devices;
- fuse holders;
- contactors, motor starters and overload relays;
- manual motor starters and motor protectors;
- terminal blocks, power distribution blocks and busbars;
- variable frequency drives and solid-state controllers.
In the basic method, the panel's SCCR is the lowest rating found among these power circuit components. A panel with a 65 kA main disconnect can still end up with a 5 kA panel rating because of a single drive or distribution block.
| Power circuit component | Example rating |
|---|---|
| Main disconnect switch | 65 kA |
| Branch circuit breakers | 25 kA |
| Contactor + overload relay (with the specified breaker) | 10 kA |
| Power distribution block | 10 kA |
| Variable frequency drive | 5 kA |
| Panel SCCR (lowest value) | 5 kA |
Illustrative values only, not the ratings of any specific product.
To raise the panel rating, the designer replaces the limiting component with one that has a higher rating, or uses a combination the manufacturer has documented at a higher value. The method also has rules for current-limiting fuses and breakers in a feeder circuit and for control transformers, which can sometimes raise the effective rating of downstream components. These adjustments apply only under the exact conditions described in the standard; check the current edition rather than applying them by rule of thumb.
Control circuits are treated differently from the power circuit when they are separated from it, for example by a control transformer. Here again, the standard describes which components count.
By the basic method, the lowest-rated power component sets the panel SCCR (example values).
Standard and high fault ratings
Product standards assign every motor controller a minimum short-circuit test level that depends on its size. This is often called the standard fault (or low fault) rating. It is modest, and on many industrial sites the available fault current is well above it.
Manufacturers therefore publish high fault ratings for many devices. A high fault rating is always tied to a specific protective device, typically stated in a form such as "X kA at Y V when protected by Class J fuses, maximum Z A" or "when protected by circuit breaker type ...". If the protective device on site does not match that description, the high fault rating does not apply.
Combination ratings
A combination rating applies to a group of devices evaluated together: for example a specific breaker or fuse, a contactor and an overload relay. The rating is valid only for the listed combination. Changing the breaker type, the fuse class, the maximum fuse size or even the contactor size can remove the basis for the rating.
Two North American constructions deserve a mention because they appear often on legacy equipment built around manual motor starters:
- Type E – a self-protected combination motor controller, usually a manual motor starter with a line-side adapter or terminal block that provides the required spacings.
- Type F – a Type E device combined with a contactor.
Both depend on the manufacturer's published combinations and voltage ratings, which may be slash ratings limited to solidly grounded wye systems. Our article on manual motor starters explains how these devices are applied.
Type 1 and Type 2 coordination
IEC 60947-4-1 adds a second question: what condition is the starter in after the fault? With Type 1 coordination, the starter must not endanger people or the installation, but it may need repair or replacement afterward. With Type 2 coordination, it must remain fit for further use; light welding of the contacts is accepted if the contacts can be separated easily.
Replacing a component in an existing panel
- Replace like with like: same manufacturer, same complete part number, including the suffixes.
- If the exact part is not available, check the manufacturer's SCCR and combination tables for the replacement, and keep the same protective device type, class and maximum size.
- Do not change the fuse class (for example from Class J to Class RK5) or the breaker type without checking the effect on the ratings.
- Read the panel nameplate and drawings: the marked SCCR is a commitment based on the original components.
- If the change affects the rating, have the panel builder or a qualified engineer review it and update the documentation.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Key takeaways
- SCCR is a withstand rating in kA at a voltage; interrupting rating belongs to fuses and breakers.
- The equipment's SCCR must meet or exceed the available fault current at the installation point.
- In the basic method, the lowest-rated power circuit component sets the panel SCCR.
- High fault and combination ratings are valid only with the protective device and components named by the manufacturer.
- When you replace a part, keep the same part number or verify the published ratings before installing.
Related parts at 3JIndustry
See our used circuit breakers, disconnect switches and contactors, and check the ratings against your panel documentation before ordering. Looking for a discontinued part? Send the full part number to sales@3jindustry.com or use our contact page.
