Supplement SA and Supplement SB

One of the most significant developments in the history of UL 508A was the introduction of Supplement SA. First added to the standard during the development of the Second Edition of UL 508A, Supplement SA was created to provide guidance for the selection and application of components used in industrial control panels.

The supplement served as a comprehensive resource identifying categories of components that could be used in industrial control panels based on their existing UL certification. Supplement SA relied upon established UL product categories and certification requirements, allowing panel manufacturers to confidently select certified components without the need for additional investigation, evaluation, or testing of those components when used within the scope of their certification.

By consolidating this information into a single document, Supplement SA simplified component selection and provided a consistent methodology for determining the acceptability of components used in industrial control panels. Over time, it became one of the most frequently referenced resources within the UL 508A program and an invaluable tool for both panel manufacturers and component suppliers.

A major change occurred with the publication of the Third Edition of UL 508A in 2018. This edition became the first version of UL 508A to be recognized as an American National Standard (ANSI). As part of the ANSI accreditation process, certain content contained in Supplement SA could no longer remain within the standard itself. ANSI procedures do not permit American National Standards to contain terminology or requirements that are unique to a specific conformity assessment provider’s certification program, including requirements that mandate the use of certified components.

As a result, the content of Supplement SA was removed from the body of the standard and relocated to a separate UL publication. The material was initially published as Informative Appendix C to the Third Edition of UL 508A and later transitioned to a dedicated UL Solutions online resource. Today, Supplement SA is maintained by UL Solutions under the direction of the Principal Engineer responsible for UL 508A.

The document continues to be updated as technologies evolve, and new component categories enter the marketplace. Through these ongoing revisions, Supplement SA remains an essential tool that helps facilitate the safe and timely application of components in industrial control panels while supporting innovation within the industry.

Another landmark development in the evolution of UL 508A was the introduction of Supplement SB in 2005. Supplement SB established a standardized method for determining the Short-Circuit Current Rating (SCCR) of industrial control panels.

Prior to the publication of Supplement SB, determining the SCCR of a custom-built industrial control panel often required extensive testing or engineering judgment, resulting in inconsistent methods and ratings throughout the industry. At the same time, increased emphasis was placed on short-circuit ratings by electrical installation codes and inspection authorities. This focus ultimately culminated in the inclusion of industrial control panel SCCR requirements in Article 409 of the 2011 Edition of the National Electrical Code (NEC).

Supplement SB addressed this challenge by providing a practical engineering methodology for determining panel SCCR using a combination of:

• Tested combinations of components
• Current-limiting characteristics of protective devices
• Branch-circuit analysis
• Overcurrent protective device coordination
• Individual component short-circuit current ratings

The methodology provided a consistent and repeatable process that could be applied across a broad range of custom industrial control panel designs. This represented a major advancement for the industry, enabling manufacturers to determine control panel SCCR values without the need for extensive full-panel short-circuit testing in nearly all control panel designs and configurations.

Supplement SB’s methodology was largely developed in response to increasing concerns from electrical inspectors and users regarding available fault current and the need for consistent SCCR markings on custom-built control panels. Understanding this critical safety gap clarifies why Supplement SB became such a transformative addition to UL 508A, providing the industry with a uniform method for determining panel SCCR.

The addition of Supplement SB significantly influenced the design, documentation, and certification of industrial control panels. Today, it remains one of the most widely used and influential sections of UL 508A and continues to serve as the foundation for SCCR determination throughout the industrial control panel industry.