Employing the UL508A Standard for Control Panel Design

February 3, 2021

Control panel builders utilize several factors to create optimal functionality. Taking these considerations a step further to ensure safe operation, designers should adhere to widely accepted standards, notably Underwriters Laboratories (UL) UL508A.

UL508A provides guidance for panel design, including the approved method to determine short circuit current rating (SCCR). The SCCR tells you the amount of short circuit current the panel could be connected to and still be able to avoid creating shock, fire, and projectile hazards.

By complying to UL508A standards, designers increase confidence in their builds and most importantly, ensure greater safety.

Understanding UL508A

Setting the standard for control panel design, UL508A covers a number of guidelines in addition to SCCR calculation, including component selection and wiring methods.

“The 508A standard is essentially a handbook on how to put a panel together,” says Klaus Tum, a product director with Altech Corp. who sits on the UL508A Board. “The key thing is the diagram on how to construct a panel that shows the applicable standards for each component.”

The advantage of adherence to UL508A guidance lies with its widespread acceptance and conformity to the National Electrical Code (NEC). “Everyone knows what UL508A means: a high level of quality and safety when building your panel,” says Christy (Rosati) McElhinny, a field application engineer with EATON.

For panel designers, following UL508A guidance enables them to obtain a UL-approved label upon inspection by a UL field representative. “But not all industrial control panels are individually tested by UL,” said McElhinny. “UL 508A is a standard that allows manufacturers to safely design an industrial control panel and know it’s going to be safe without going through testing. It’s an efficient way to ensure safety on highly customized products.”

It is important that manufacturing locations maintain at least one UL508A-qualified manufacturer’s technical representative (MTR) on staff, who can effectively interpret and accurately apply the requirements of the UL508A industrial control panels standard. UL recently extended the deadline for the MTR requirement to August 1, 2021.

Advice on navigating UL508A

Panel builders must understand that UL508A compliance is not focused on individual panel components. “UL508A is the guide for the placement of components within a panel,” says Tum. “That’s the key thing that a lot of people get wrong in the first place.”

When it comes to selecting panel components that comply with the UL508A standard, UL provides two guidance documents; the UL Supplement SA provides requirements for components used in control panels, while Supplement SB provides direction on calculating overall assembly SCCR.

Other tips include using overcurrent protective devices with high interrupting ratings. “The short circuit current rating (SCCR) of your panel can never exceed the lowest interrupting rating in your panel, so to ensure a high SCCR, use overcurrent protective devices with a high interrupting rating,” says McElhinny.

When applied correctly, current-limiting devices, like fuses, used in the feeder circuits may be used to increase short circuit current ratings of the branch circuits. Also, utilizing branch and feeder components with high fault short circuit current ratings will help to increase the overall short circuit current rating of the panel.

Incorporating UL-Listed components in control panels

Designing and building control panels that conform with UL508A is easier with compliant components; RS offers a number of these through suppliers including EATON and Altech Corp, and these components feature the “UL-Listed” designation by passing rigorous safety testing.

By incorporating coordinated components with the ‘UL-Listed’ designation, panel builders are able to confidently achieve UL 508A compliance. “UL-Recognized” components may also be used, but they must be applied according to the strict conditions outlined by the component manufacturer.

EATON recommends several components to consider when designing a control panel:

Compact circuit protector (CCP2) disconnect switches offer an extremely compact design and high short circuit current rating.
Multi-wire terminals save panel space, mounting time, and wiring time.
CUBEFuse fuse systems offer a small footprint and high interrupting rating.

Additionally, Altech Corp provides a portfolio of components manufactured for control panels to meet UL508A standards, including:

Disconnect Switches rated for both UL98 and UL508
UL line of breakers with ring tongue feature
CP push-in series of terminal blocks that feature a modular system for time and space savings

Resources

It is critical that panel designers are knowledgeable and up to date with UL508A guidelines, especially in relation to SCCR compliance. In a survey conducted by EATON, more than 50% of OEMs expressed some degree of difficulty locating SCCR information for components needed to design an UL508A-compliant solution. Nearly half also stated that they had difficulty applying or interpreting the standard that determines panel SCCR.

There are a number of resources related to the UL508A standard offered by both Underwriter’s Laboratories and RS suppliers. Full UL508A documentation is available on UL’s website for purchase; additionally, UL offers a virtual training course on industrial panel design and UL508A adherence.

UL508A guidelines are updated every few years, with the most recent requirement changes in effect since July 1, 2020. A summary of those changes are found here, and a complimentary webinar outlining the most recent requirement updates made to 508A is also available.

EATON provides a guide to control panel design adhering to UL508A -compliant control panel design, as well as an FAQ page about all things related to UL508A.

Altech Corp includes diagrams in component data sheets to provide guidance on the right components for a panel build, especially for circuit protection. “It’s about showing the right components and right locations as not to overspec and not spend unnecessarily when building a panel,” says Tum.

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