GLOSSARY



Short-Circuit Current Rating (SCCR)

The Short Circuit Current Rating (SCCR) refers to the short-circuit current withstand capability of electrical systems and control panels according to North American safety standards. The SCCR value indicates the maximum short-circuit current that electrical equipment can safely withstand under specified conditions.

Relation to UL Requirements

NFPA 79 requires the use of a surge protection device (SPD) under certain conditions, particularly when safety-critical control components could be affected by power surges.

The requirements for such surge protection devices are specified in UL 1449. The surge protection device (SPD) can either be installed directly in the control cabinet or incorporated into the upstream electrical installation, provided that the applicable regulatory requirements are met.

SCCR – Short-Circuit Current Rating for the North American Market

For industrial control panels in the United States, the evaluation and labeling of short-circuit current rating (SCCR) is an important safety requirement. With the release of the 2005 edition of the National Electrical Code (NEC), corresponding requirements for industrial control panels were introduced in Article 409. Requirements for short-circuit current resistance also apply to the Canadian market, as specified by the applicable regulations and standards there.

The purpose of these regulations is to improve the electrical safety of machines and systems and to reduce hazards caused by short circuits, particularly those resulting from arcing.

The SCCR rating indicates the maximum prospective short-circuit current that a control cabinet can safely withstand under specified conditions. The specified short-circuit current rating must be at least equal to the short-circuit current available at the installation site. This is the only way to ensure that the electrical equipment is suitable for the fault currents expected at that location.

Relationship Between SCCR and UL 508A

The requirements for short-circuit current withstand capability of industrial control panels are specified in the National Electrical Code (NEC), Article 409. To ensure these requirements are taken into account in the design and evaluation of industrial control panels, Supplement SB was added to the UL 508A standard.

This appendix describes a method for determining the short-circuit current rating (SCCR) of a control cabinet. Among other factors, it takes into account the short-circuit current withstand capability of the components used as well as the protective devices in place.

For a control panel certified to UL 508A, the SCCR value is determined and marked in accordance with the applicable requirements. This makes it possible to verify whether the UL-certified control panel is suitable for the short-circuit current available at its future installation site. This is an important prerequisite for safe installation and regulatory approval in the United States.

How is the short-circuit current rating (SCCR) determined?

The operator of a machine or system is responsible for ensuring compliance with applicable occupational safety regulations. This includes ensuring that the electrical equipment has sufficient short-circuit withstand capacity.

Therefore, a separate SCCR (Short Circuit Current Rating) value must be determined for each relevant industrial control cabinet and labeled in accordance with applicable regulations. The short-circuit current rating of the individual control cabinets is, in turn, an important factor in evaluating the entire machine or system. As a general rule, the component with the lowest short-circuit current rating can limit the SCCR value of the entire system.

In addition to a thorough, individual inspection and evaluation of the respective control cabinet design, two other methods are available for determining the SCCR value:

  • Coordinated Selection – Coordinated Component Selection
    In Coordinated Selection, components from a single manufacturer that are designed to work together are used to build an industrial control cabinet. These components are intended for use together and are matched in terms of their electrical characteristics and short-circuit behavior.

    Manufacturers provide tested and documented component combinations for this purpose, whose short-circuit current rating (SCCR) has been verified under specified conditions. Special selection tables, product catalogs, and design software facilitate the design process and simplify the calculation of the SCCR value.

    A major disadvantage of this method is the limited flexibility in component selection. In order to use the documented short-circuit ratings, the combinations and operating conditions specified by the manufacturer must be adhered to. The use of alternative components may require a reassessment of the short-circuit current rating.

  • SCCR Calculation for Control Panel Construction in Accordance with UL 508A
    UL 508A, with its Supplement SB, provides a standardized procedure for determining the short-circuit current rating (SCCR) of industrial control cabinets. This allows manufacturers to determine the SCCR value as early as the design phase, without having to perform a time-consuming and costly individual short-circuit test for each control cabinet.

    A key advantage of this process is the high degree of flexibility it offers in control cabinet design. Manufacturers can combine different components to meet their customers’ technical requirements, provided that the applicable UL 508A specifications are followed.

    Important: Unlike Coordinated Selection, this method allows for the use of components with different short-circuit current ratings. When determining the SCCR value of the entire control cabinet, the component with the lowest short-circuit current rating is always the determining factor. Under certain conditions, however, suitable protective devices and tested component combinations can contribute to a higher rating.

    What happens if the short-circuit current rating is too low?

    If a component’s short-circuit current rating (SCCR) is lower than the short-circuit current available at the installation site, its use may still be possible under certain conditions. It is crucial that the electrical equipment be adequately protected against the effects of a short circuit through appropriate measures.

    One option is to use a suitable overcurrent protection device, such as a fuse or a circuit breaker. This device interrupts the circuit when currents exceed permissible limits and, under certain conditions, can help demonstrate a higher short-circuit current withstand capacity for the combination of components in question.

    It is essential to comply with the requirements of Annex SB of UL 508A. Not every overcurrent protection device automatically allows for an increase in the SCCR value. The decisive factors are the permissible combinations of protective devices, their documented characteristics, and the respective operating conditions.

    Short-Circuit Current Rating (SCCR) Labeling

    The National Electrical Code (NEC) requires that the short-circuit current rating (SCCR) of industrial control panels be permanently and clearly marked. This allows operators, qualified electricians, and testing agencies to determine the maximum short-circuit current for which a control panel is rated.

    The SCCR rating is typically marked on the control cabinet’s nameplate or in another permanently affixed, easily accessible location. It is essential that the specified value be clearly visible during installation and inspection.

    This labeling requirement applies regardless of UL certification. Even control cabinets without a UL label must meet the applicable requirements for specifying short-circuit current withstand capability.

    Regional Differences in SCCR Requirements in the United States

    This article explains the Short Circuit Current Rating (SCCR) and the associated requirements for industrial control cabinets as specified in the National Electrical Code (NEC).

    The NEC is a nationally recognized technical code for electrical installations in the United States. However, it becomes legally binding only when adopted by the regulations of the respective states or local authorities. Different editions of the NEC, as well as additional regional requirements, may apply.

    For German machinery and plant manufacturers, this means that the requirements for short-circuit current withstand capacity and the associated testing procedures may vary depending on the location of use. Therefore, it is important to familiarize yourself with the applicable regulations and the requirements of the relevant Authority Having Jurisdiction (AHJ) before exporting.

    Working with an experienced partner like Krayer can help ensure that regional specifics are taken into account early on, that SCCR requirements are properly implemented, and that delays in regulatory approval in the U.S. are avoided.

    • Conclusion: SCCR and Short-Circuit Current Rating of Control Cabinets


    The Short Circuit Current Rating (SCCR) refers to the short-circuit current withstand capability of electrical equipment and industrial control panels. For industrial control panels in the United States, the National Electrical Code (NEC), Article 409, requires the determination and marking of the SCCR value.

    To implement these requirements, UL 508A was supplemented with Appendix SB (Supplement SB). This allows for a standardized evaluation of short-circuit withstand capability without requiring each control cabinet to undergo a time-consuming short-circuit test individually.

    In what is known as "coordinated selection," coordinated and tested combinations of components are used. This method simplifies the SCCR calculation but limits flexibility in component selection.

    In contrast, control cabinet design in accordance with UL 508A offers greater design flexibility, as components with different short-circuit current ratings can be combined. The SCCR value is determined in accordance with the specifications in Appendix SB.

    To successfully export to North America, manufacturers must also take into account the applicable regional regulations and acceptance criteria. An early SCCR assessment that complies with standards contributes significantly to electrical safety and the smooth commissioning of the system.