UL 508A Standard – 15 Common Pitfalls

Sign telling readers to expect delays represents delays caused in complying with the ul 508a standard during control panel inspections

As an industrial control panel manufacturer certified to apply the UL Listing Mark, Cates Control Systems applies the UL 508A standard to every electrical control panel we design and build. With over 40 years of experience in panel fabrication, we have seen a lot of different situations occur while helping customers ensure their panels pass UL inspection. If you are designing an industrial control panel to comply with the UL 508A standard or outsourcing the design to another company, this post offers some things to consider. Below are 15 of the most common issues we see that cause panels to fail UL 508A inspection. We hope these tips help you avoid delays in getting your panel UL listed!

15 Common Pitfalls in Complying with the UL 508A Standard

As the standard for industrial control panels, UL 508A is designed to ensure safety in manufacturing environments. The 15 issues outlined below are some of the most common problems we see that cause delays in getting a panel UL Listed. Some of these areas can be tricky, especially for panel designers new to UL 508A.

  1. Using UL Listed or Recognized components that are not allowed for use in an industrial control panel
    When selecting components, it’s critical to choose UL Listed or UL Recognized components with listings or recommendations indicating they are designed for use in industrial control panels. Components that are not rated for industrial use may not meet the requirements of the UL 508A standard, and therefore may not pass inspection.

  2. Using a UL Listed component without referencing the component manual for circuit protection, wiring, and installation rules
    UL standards are in place to ensure safety, and failing to follow guidelines in the component manual for circuit protection, wiring, and installation could compromise the safety of the panel.

    Additionally, not following a manufacturer’s recommendations can result in performance issues, component damage, voided warranties, noncompliance with electrical codes and regulations, and possible legal liability. For these reasons, panels with components used in a manner inconsistent with manufacturer recommendations will fail UL 508A inspection.

  3. Using components that are required to be supplied by Class 2, PELV, or SELV power supplies
    Using components in an industrial control panel that are required to be supplied by Class 2, PELV (Protective Extra-Low Voltage), or SELV (Safety Extra-Low Voltage) power supplies but are not, can create several situations that lead to UL inspection failure:
    • Since these components are designed to be connected to power supplies that limit the voltage to safe levels, a power supply that could inadvertently connect them to higher voltages violates basic safety standards and National Electrical Code (NEC) standards by significantly increasing the risk of electric shock or fire.
    • Per the PELV and SELV standards, the circuits must be separated from higher voltages to prevent safety hazards. Using an incorrect power supply violates the requirements for isolation and can cause safety issues such as short circuits or ground faults.
    • High voltages can damage components designed for low-voltage operations, impacting the performance and reliability of the control panel. UL inspectors can identify an improper power supply, leading to a failed inspection.

      Cates finds that Human-Machine Interfaces (HMIs) are common components that are required to be supplied by Class 2 or PELV circuits.

  4. Using improperly sized branch circuit protection
    The UL 508A standard requires that branch circuits within industrial control panels be provided with overcurrent protection. The devices must be sized appropriately to properly protect the connected equipment and conductors. Proper sizing requires devices to have sufficient ampere ratings to safely handle the circuit’s maximum current.

    Moreover, using oversized branch circuit protection can lead to injuries or property damage with resulting liability issues, component damage that is costly to repair or replace, or lost revenue due to unplanned downtime in the machinery or process. For these reasons, the panel will fail the UL 508A inspection.

  5. Using supplemental circuit breakers where branch circuit protection is required
    Each branch circuit supplying power to the control panel power loads must be provided with overcurrent protection according to the UL 508A standard. Branch circuit protective devices can include circuit breakers or other suitable devices.

    In addition to branch circuit protection, supplemental circuit breakers can be used to add protection for specific components. However, supplemental circuit breakers cannot be used without proper branch circuit protection in place. This situation will cause the panel to fail the UL inspection.

    A circuit breaker used as a Branch Circuit Protective Device (BCPD) must be UL 489 Listed, while the common UL Listing on a supplemental protective device is UL 1077.

  6. Not following branch feeder rules for distribution block sizing and wire sizing
    According to the feeder circuit rules in UL 508A, distribution blocks must be properly sized based on the maximum expected current. Undersized distribution blocks can overheat, leading to possible fire hazards and component failures. The wires used in branch circuits must also be properly sized based on the anticipated current. Wires that are too small for the current they are expected to carry can overheat, leading to short circuits or fire hazards. These safety and performance concerns can cause panels to fail UL 508A inspection.

  7. Using improperly sized wire gauge
    UL 508A requires that the proper wire size be used in industrial control panels to meet both the current-carrying capacity (i.e., ampacity) requirements and to comply with UL and National Electrical Code (NEC) guidelines. Incorrectly sized wire gauge can cause panel inspection failure based on:
    • Overheating and Fire Risk: Improperly sized wire gauge can cause a panel to fail inspection due to increased risk of overheating and fire. UL 508A specifies the minimum required wire size based on the load.
    • Safety Issues: From a safety perspective, panels with incorrect wire sizes will fail inspection due to increased risk of electric shock, burns, and other injuries.
    • Compliance and Code Violations: The UL 508A standard is harmonized with the NEC. Using improperly sized wire gauge is a code violation, which will directly cause an inspection failure.

  8. Failing to ensure that components mounted on the outside of the enclosure have a UL Type environmental rating (UL ratings are different from NEMA or IP ratings)
    To pass a UL 508A inspection, any components mounted on the exterior of the enclosure are required to have a UL Type environmental rating. The UL Type rating reflects a component’s ability to endure specific environmental conditions encountered in an industrial setting such as corrosion, dirt, dust, and water.

    A UL Type rating is not the same as a NEMA (National Electrical Manufacturers Association) or IP (Ingress Protection) rating. NEMA and IP ratings also refer to the ability of a component to withstand environmental conditions; however, these ratings cannot be used interchangeably with UL ratings.

    Components bearing a UL Type rating have been rigorously tested and certified to meet strict UL standards to ensure they comply with specifications for performance and safety. Although NEMA ratings are similar to UL ratings, they do not always directly match UL standards in these two areas. IP ratings reflect the level of protection for water and solid objects. However, other safety considerations crucial for UL compliance such as hazard protection and durability are not included. For these reasons, NEMA or IP ratings are insufficient; components must have a UL Type rating for the panel to pass UL inspection.

    In the industry, UL Type-rated components are well-known and trusted for their compliance and safety. This widespread recognition helps streamline the approval process for the entire control panel during inspections because inspectors trust the validity of the UL standards.

  9. Failing to check Short-Circuit Current Rating (SCCR) on components, limiting the overall SCCR of the panel assembly
    To safely withstand and interrupt short-circuit currents without causing safety issues or damaging the panel, the UL 508A standard requires industrial control panels to have a sufficient SCCR. The overall SCCR of the panel is based on the SCCR of its components.

    Therefore, if the rating of any of the components is insufficient and is not confirmed before use, the overall SCCR of the panel will be limited as the SCCR for the panel is determined by the lowest SCCR value for any component or branch circuit.

    Placing fuses in the feeder circuit of your industrial control panel can help raise the SCCR rating of the assembly by limiting the peak let-through current.

  10. Failing to ensure components coordinate for SCCR
    To ensure that your control panel meets the UL 508A requirements for SCCR, component coordination must be thoroughly evaluated and documented. It must be demonstrated to the UL inspector that components were properly coordinated to ensure that the panel SCCR meets or exceeds the maximum available short circuit current. The NEC requires that the equipment owner or their representative calculate this for the point of installation.

    UL inspectors may assess component coordination by reviewing documentation, verifying component ratings, analyzing the coordination, checking panel labeling, and physically inspecting or testing the panel. Documentation should include details of the SCCR calculation process, including component SCCR ratings, SCCR calculations considering all configurations, test results, and any coordination studies.

    A manufacturer’s coordinated components that have been tested by UL can be found on UL’s website.

  11. Using circuit breakers to 100% capacity
    Using circuit breakers or fuses to 100% capacity in an industrial control panel can cause the panel to fail the UL 508A inspection for several reasons. UL 508A requires effective heat dissipation; circuit breakers operating at 100% capacity can raise the temperature beyond acceptable limits. In addition, UL 508A requires derating of components which involves operating them below maximum capacity to provide a safety margin.

    Consistently operating at 100% capacity also stresses the circuit breakers and other components, reducing the lifespan and increasing the possibility of component failure. Finally, UL 508A promotes the design of panels that can handle occasional increases in load. Circuit breakers at full capacity are incapable of handling unexpected surges, which could lead to circuit failures, electrical shocks, or fires.

    Most common circuit breakers must be limited to 80% current capacity. Manufacturers provide options for circuit breakers that are UL rated at 100%, but they are not as common or as readily available.

  12. Failing to include required UL markings in customer documentation (e.g., SCCR, FLA, Environmental rating, Load ratings, etc.)
    The UL 508A standard outlines specific UL markings that are required to be included in customer documentation. These markings provide customers with critical panel specifications and demonstrate compliance with UL 508A safety standards.

    Required markings for industrial control panels include the UL mark, SCCR, FLA, voltage, environmental rating, and load ratings. Additional markings and information may be required depending on the control panel’s specific design and features.

  13. Using the ECTB2 category of connectors on a UL 508A panel
    While ECTB2 connectors are designed for use in telecommunications and networking applications, they may not be suitable for use in industrial environments. Commonly used to connect various networking equipment with Ethernet cables, ECTB2 connectors may not adhere to the specific standards and ratings outlined in the UL 508A standard to ensure safety and proper performance.

    ECTB2 connectors can have the same appearance as common industrial-rated connectors, but will not have been tested to the same rigorous standards. Using connectors that are category PVVA or CYJV in place of ECTB2 connectors is recommended.

  14. Referencing an outdated SA table when the online UL version should always be referenced
    We commonly see the use of an outdated SA table when checking whether components meet UL 508A requirements for use in an industrial control panel.

    The current version of Supplement SA, “Supplement SA Specific Component Requirements for Industrial Control Panels” is housed on the UL Industrial Control Panel website. This version, dated November 15, 2022, supersedes the previous version dated March 30, 2022, and replaces what was Supplement SA in the Second Edition of UL 508A, and subsequently Appendix C in the Third Edition of UL 508A.

    Changes to Supplement SA can be made at any time, so it is important to always reference the latest version available online.

  15. Assuming that all UL inspectors interpret the requirements of the UL 508A standard in exactly the same way
    The final pitfall we see is companies checking with the UL inspector in their region about UL 508A requirements, but failing to check on how UL 508A requirements are interpreted by the UL inspector in the panel manufacturer’s region. Since the manufacturer is ultimately responsible for applying the UL Listing label, they must ensure all panels comply with UL 508A requirements according to the interpretation of the UL inspector in THEIR region.

What Kind of Delays Are We Talking About?

Non-compliance to UL 508A standards due to any of the 15 common pitfalls outlined will require correction. The extent of the correction can be small or large depending on the error made. Common corrections can include:

  • The entire redesign of a panel
  • Purchasing new components with a long lead time
  • Rewiring of a circuit
  • Reworking a panel that does not have the appropriate SCCR for the installation site.

Change orders can be costly to both the financial performance and schedule of your project.

Why Industrial Control Panel Designers Get Tripped Up

The requirements and guidelines for designing an industrial control panel are specified in the UL 508A standard, right? So, I can just read the standard and follow the guidelines and I won’t have any issues getting my panel UL Listed.


While that does seem like it should be the case, some areas of UL 508A can be tricky, especially for designers new to the standard.

The UL 508A Standard is Extensive

To correctly implement UL 508A and facilitate a smooth inspection, the design requirements must be fully understood. Since the standard is extensive this is a tall order for designers who are brand new to it. It takes time to absorb it all, and experience applying the specifications to panel design to learn the areas that can cause problems during inspections.

UL Has Unique Terminology

In addition to being a large document, UL has unique terminology. Designers must be well-versed in this terminology to fully understand the context of the requirements. Learning and interpreting UL terminology requires time and experience complying with the standard.

Assistance Interpreting the UL 508A Standard

Consider the UL 508A MTR Program

If your company’s engineering team is designing your control panel internally, consider having your designers complete the UL 508A Manufacturer’s Technical Representative (MTR) training. An addition to the UL 508A standard, the MTR program can help eliminate mistakes that can lead to delays in passing inspection.

Cates Can Help with UL 508A Standard Interpretation

If you have questions, please feel free to contact us. We want to help ensure your experience with UL 508A inspection is smooth and uneventful.

With over 40 years of experience in panel fabrication, Cates applies the UL 508A standard to every control panel we design and build. We work with customers daily on questions regarding UL 508A requirements. If your design relates to a section of UL 508A that is unclear, we can contact UL for clarification and input to ensure your panel will pass inspection.

Reducing Risk in Your Panels Before UL Inspection

Identify Risks Before Manufacturing

To facilitate a smooth inspection, Cates can also help by identifying possible design or safety risks in your panels before the manufacturing process even begins. We have the experience to help you avoid the common pitfalls that we often see during UL certification testing.

Confirm Interpretations with the UL Inspector

As a UL 508A control panel manufacturer, Cates is subject to UL inspections. Because of the number of UL Listings we have and the large number of panels we produce, we are subject to inspections more often than smaller panel builders. The UL field representative inspects at Cates monthly (small shops may be inspected quarterly). Through these regular inspections, we have developed a good relationship with UL.

When there is a question about a customer’s design or a section of the standard, we will involve UL through our field inspector to get clarification. We are very thorough and take the time to do this for customers instead of just choosing an interpretation and going with it. If you have a risk in your design, Cates will help you get the area clarified to reduce that risk.


Since the UL 508A standard is large and uses unique terminology, it can be challenging to fully understand the context of the requirements when designing industrial control panels. As a result, certain areas of the standard are commonly overlooked or misinterpreted, causing delays when UL inspects the panel before Listing it. Understanding and avoiding these common pitfalls can facilitate a smooth inspection and prevent the increased costs of a control panel redesign.

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