From the Desk of Lewis Bass

Welcome back to “Ask Lewis Bass,” our ongoing series where we answer real-world questions from engineers, panel builders, OEMs, and compliance teams working through the challenges of industrial electrical safety and equipment compliance. This week we’re tackling a question we receive surprisingly often from companies building custom industrial control panels.

Question:

We’re building a custom industrial control panel for one of our customers and several people on our team have mentioned UL 508A compliance. Some are saying we have to build the panel to UL 508A, while others think it’s only optional unless the customer specifically asks for it.

Can you explain when UL 508A actually applies, and is there any benefit to following it even if nobody specifically requires it?

– Controls Engineer Chris

Lewis Bass:

Hi Chris,

Great question. UL 508A is one of those standards that everyone has heard of, but there’s still a lot of confusion surrounding when it applies and why it matters.

The first thing to understand is that UL 508A is the Standard for Industrial Control Panels. It provides construction requirements that help ensure an industrial control panel has been designed with electrical safety in mind. It covers everything from component selection and conductor sizing to spacing requirements, overcurrent protection, short-circuit current ratings (SCCR), labeling, wiring methods, and much more.

Now, does every industrial control panel legally have to be built to UL 508A?

Not necessarily.

The answer depends on where the equipment is being installed, who has jurisdiction over the installation, and what your customer or specification requires.

For example, many machine builders construct panels following UL 508A because it represents an established industry benchmark and makes future inspections much smoother. Other customers specifically require a UL 508A Listed panel as part of their purchasing specifications.

On the other hand, some custom-built equipment is installed as part of a larger machine that ultimately undergoes a third-party field evaluation rather than being factory listed. In those situations, a panel that wasn’t manufactured under a UL 508A Listing program may still be acceptable, provided it can be shown to comply with applicable safety requirements during the field evaluation.

This is where people often mix up UL 508A compliance with UL Listing.

They’re related, but they’re not identical.

A panel may be designed using UL 508A construction principles without actually being manufactured under a UL 508A Listing program. Likewise, a UL Listed industrial control panel has been manufactured under an authorized UL panel shop program with ongoing factory inspections.

So why do so many engineers choose to follow UL 508A regardless?

Because it usually saves headaches later.

Building a panel around recognized construction practices tends to reduce surprises during commissioning, customer acceptance, and inspections. Items like SCCR calculations, proper conductor sizing, enclosure ratings, spacing, and component selection are all addressed before the equipment reaches the customer’s facility.

We’ve also found that when equipment eventually requires a field evaluation because it forms part of a larger custom machine, panels that closely follow UL 508A principles generally require fewer corrections than panels built without any recognized construction standard in mind.

Another point worth mentioning is that UL 508A doesn’t exist in isolation.

Many industrial machines also need to comply with NFPA 79, which governs the electrical standard for industrial machinery. While UL 508A focuses on the construction of the industrial control panel itself, NFPA 79 looks at the complete electrical system of the machine. The two documents often complement one another rather than compete.

So our recommendation is fairly straightforward.

If you’re designing industrial control panels on a regular basis, using UL 508A as your design foundation is almost always a smart engineering decision, even when a formal UL Listing isn’t specifically required. It promotes consistency, improves safety, and often makes downstream inspections significantly easier.

And if you’re ever unsure whether your equipment should be factory listed, field evaluated, or designed to satisfy a particular jurisdiction’s requirements, that’s exactly the kind of conversation we’re happy to have before construction begins. A short discussion early in the project can prevent expensive redesigns after the equipment has already been built.

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