UL 508A Explained: Industrial Control Panels and the UL-Listed Computers Inside Them

UL 508A is the UL Standard for Safety for Industrial Control Panels: the US rulebook for how a control panel is designed, built, wired and marked, and the basis on which a listed panel shop applies a UL mark to the finished panel. It is a construction standard for the panel as a whole, not a certificate that a computer, PLC or power supply carries on its own. The parts inside the panel are judged by their own listings, so a computer fitted in a UL 508A panel needs the right listing from UL or another recognised laboratory.

That split matters to anyone building panels for North America. Machine builders, OEMs and system integrators increasingly put industrial computers inside the cabinet next to the PLC, drives and I/O, for HMI, data collection, vision and edge analytics. A panel that is otherwise compliant can fail inspection because one component in it has no suitable listing, or has a listing that does not cover the way it is used.

This guide explains what UL 508A covers and how it relates to UL 508, UL 61010-1, UL 62368-1, cULus and ETL marks; what UL Listed, Recognized and Classified mean; what an NRTL is; how short-circuit current rating works; what to check before specifying a computer for a listed panel; and how hazardous locations, enclosure types and markets outside North America change the picture.

Several UL standards and marks turn up on the same drawings, and they are easy to mix up. The table sets out what each one covers, who holds the certification and why it matters for a computer inside a control panel.

The pattern is simple once it is laid out. The panel shop certifies the assembly to UL 508A; every product inside it is certified by its maker to a product standard; the installer and inspector apply the National Electrical Code on site. A computer only becomes a problem when its listing is missing, covers a different use or does not match the build that was actually fitted.

UL 508A applies to industrial control panels intended for general industrial use and operating from 1,000 V or less. It includes motor control panels, PLC panels, machine control panels and similar assemblies of two or more power circuit or control circuit components, built in a shop and shipped to site. The current third edition dates from 2018 and has been revised several times since, so the edition and revision a project is built to belong in the panel documentation.

In practice the standard asks the panel shop to show five things: that each component is suitable and used within its ratings, that the panel is wired correctly, that spacings and protection are right, that the short-circuit current rating has been worked out, and that the panel is marked so an installer can use it safely.

UL 508A also has supplements for particular uses. Supplement SA covers panels for industrial machinery and lines them up with NFPA 79, and Supplement SB sets out how to work out the short-circuit current rating. Panels for other uses, such as HVAC equipment or elevators, follow further supplements or their own standards.

The names differ by one letter, but they apply to different things. UL 508 is a product standard for industrial control equipment: the contactor, overload relay, push button or motor starter a panel shop buys. UL 508A is a construction standard for the panel those parts go into. A UL 508A panel is often full of UL 508 components.

The changeover from UL 508 to the UL 60947 series is gradual, so a panel built today often contains devices listed to either. For a panel builder the question is the same either way: is each device listed or recognised for what it is doing in this panel?

UL’s marks are grouped by what the product is and how it will be used. The difference between them decides whether a part can go into a UL 508A panel as it is, or only within limits set out in its UL file.

UL Listed is the mark most panel components carry. The maker’s product has been tested against the relevant standard and is covered by ongoing factory follow-up, so a listed computer or power supply can be specified for the panel provided it is used within its ratings.

UL Recognized Component marks, shown as a reversed UR symbol, cover parts that will be built into something else. Each comes with conditions of acceptability, such as a maximum temperature, a required enclosure or a particular power source, which the panel builder has to meet. Recognized parts are welcome in UL 508A panels, but the conditions are part of the deal. Both marks have US, Canadian and combined versions, shown below.

UL Classified products have been evaluated only for certain properties or hazards. Most are construction materials or instruments for industrial and commercial use, and the mark says nothing about other risks.

An NRTL, or Nationally Recognized Testing Laboratory, is a private testing and certification body that the US Occupational Safety and Health Administration (OSHA) has recognised to certify products to specific safety standards. OSHA’s workplace rules require many types of electrical equipment to be approved for safety, and certification by an NRTL is how that approval is usually shown. The programme is set out in OSHA regulation 29 CFR 1910.7.

Recognition is granted standard by standard. An NRTL may be recognised for UL 62368-1 and UL 61010-1 but not for UL 121201, so the useful question is whether the laboratory behind a mark is recognised for the standard the product was tested to. OSHA publishes the current list and each laboratory’s scope.

UL is the best-known NRTL in North America, but not the only one. A product tested to UL 62368-1 by Intertek carries the ETL mark rather than the UL mark, and inspectors generally accept either for the same standard. Our explainers on ETL vs UL and the cULus mark go into the detail.

The list changes over time and each laboratory’s scope is specific, so check OSHA’s published list against the standard named on the product before relying on a mark you have not met before.

UL began in Chicago in 1894 as the Underwriters’ Electrical Bureau, testing electrical products for fire insurers after a run of serious fires blamed on early electrical wiring and equipment. Today its testing, inspection and certification work sits with UL Solutions, while the standards themselves are developed by UL Standards & Engagement.

For computers the key standard is UL 62368-1, the US version of IEC 62368-1. It covers audio/video, information and communication technology equipment and replaced UL 60950-1 for IT equipment and UL 60065 for audio and video equipment; UL 60950-1 was withdrawn in the US at the end of 2020. Unlike the older prescriptive standards, 62368-1 is built on hazard-based safety engineering (HBSE): identify each source of energy that could cause harm, classify it, and put safeguards between it and the people who use or service the equipment.

A computer that passes UL 62368-1 has been checked in normal operation, under foreseeable misuse and with single faults applied, which is why panel builders and inspectors treat it as a known quantity. Control equipment such as PLCs and HMIs is more often listed to UL 61010-2-201, the part of the UL 61010 series written for control systems. Either listing can be appropriate for a computer in a control panel; what matters is that the listing covers the product and its intended use.

Short-circuit current rating (SCCR) is the highest fault current, in amps at a stated voltage, that a panel can withstand safely if a short circuit occurs on its supply. Article 409 of the National Electrical Code requires an industrial control panel to be marked with its SCCR, and the installer must check that the marked figure is at least the fault current available where the panel is connected. A panel rated 5 kA connected to a supply that can deliver 35 kA is a violation, however well it is built.

UL 508A Supplement SB gives the method. In outline, the panel builder lists every component in the power circuits with its own short-circuit rating, applies any increase allowed by current-limiting fuses or breakers upstream, takes the lowest figure as the panel’s rating and checks it against the main protective device. Components without a marked rating are given conservative default values from the supplement’s tables, which is often what pulls a panel rating down.

The figures in the table are illustrative; real panels are calculated from the component markings and the supplement’s tables. The point for anyone specifying a computer is where it sits in that calculation. Most industrial computers run from a 24 V DC supply on the panel’s control circuit, so the computer itself is usually on the secondary side and outside the SCCR calculation. The industrial power supply that feeds it is in the calculation, and so is any computer powered directly from the mains. Choosing a listed DC supply with a marked short-circuit rating avoids a default value setting the whole panel’s rating. Our explainer on the computer power supply covers the basics.

In practice, yes. UL 508A expects the components in a listed panel to be listed or recognised for their use, and a computer is no exception. A computer without a suitable listing has to be evaluated separately, which costs time and money and may not be possible at all, so most panel builders treat a recognised listing as a condition of purchase.

Which listing depends on what the computer does and where it is mounted. The table covers the devices we most often see in North American control panels.

Two details catch people out. First, a listing covers a specific build. Neousys, for example, sells separate UL-certified versions of several computers, and on the POC-715-UL the PoE+ budget is 30 W in the UL-certified configuration, so the camera load has to be planned around that figure. Fitting an option that is outside the certified configuration can take a computer out of its listing. Second, a panel PC in the door is part of the enclosure: if the panel is a Type 4X enclosure, the panel PC’s front has to be rated to keep it that way.

Our DIN rail PCs are the usual choice inside the cabinet, mounted on the same DIN rail as the PLC and I/O, while panel mount PCs fit a cut-out in the door and give operators an HMI without opening the panel. For data collection next to them, data acquisition and I/O modules and remote I/O carry their own listings in the same way.

A listing is a safety minimum, not proof that a computer will last in a control panel. Cabinets run hot: a closed steel enclosure with drives and power supplies inside often sits 10 to 15 °C above the room around it, and a panel in a hot plant or outdoors can reach the top of a commercial computer’s range on a summer afternoon. We size the computer to the temperature inside the cabinet, not to the room, which usually points to a fanless industrial computer rated to at least 60 °C and sometimes to wide temperature computers rated to 70 °C. The industrial temperature range explainer sets out the usual grades.

Lifecycle deserves more attention than it usually gets. A panel design is often repeated for years, and if the computer in it is discontinued the replacement needs its own listing check and an update to the drawings. Choosing a model with a long availability commitment, and planning a last-time buy when an end-of-life notice arrives, keeps the panel design stable. Our article on how long computers last covers the service-life side, and our guide to mounting DIN rail computers covers spacing and orientation for heat.

UL 698A covers industrial control panels that sit in an ordinary, unclassified location but have intrinsically safe circuits extending into a hazardous (classified) location, such as wiring to sensors in a solvent store or a refinery process area. It adds requirements for barriers, segregation, wiring and marking on top of UL 508A, and panel shops are listed for it separately.

Computers that are themselves installed in a hazardous area are a different case. In a Class I, Division 2 location, where flammable gas or vapour is present only under abnormal conditions, a computer typically needs nonincendive certification to UL 121201 or an equivalent, as well as its normal safety listing. Our explainers on Class I, Division 2 and intrinsically safe vs non-incendive protection set out the differences, and ATEX covers the European equivalent.

We supply intrinsically safe and hazardous area panel PCs and intrinsically safe computers for these locations. The certificate’s conditions of use, such as the temperature code and the permitted ambient range, belong in the panel documentation alongside the listing.

The enclosure is part of a UL 508A panel’s certification, and its type rating, defined in UL 50E and NEMA 250, tells the installer what environment it can go into. Anything cut into the door, including a panel PC or HMI, must keep that rating. The table gives the types most often met in industrial panels, with the IP rating usually considered closest. The two systems test differently, so a NEMA type is not a direct equivalent of an IP code.

For a panel PC in the door of a Type 4X panel, look for a front panel rated to Type 4X or at least IP66, stainless steel or a corrosion-resistant housing, and a gasket and mounting clamps that hold the seal. Our explainers on NEMA 4X and NEMA vs IP ratings go further, and our IP rating guide covers the IEC 60529 codes in full.

UL certification is not a legal requirement in itself: no US federal law requires the UL mark. What is required, through OSHA rules, the National Electrical Code and local inspectors, is equipment approved for safety, and an NRTL listing is how that is normally shown. A panel shipped to the US without listed components can be refused by the end customer or held up at inspection.

Outside North America other systems apply. The table summarises the main ones a panel builder exporting from the UK or Europe will meet.

The IECEE CB Scheme lets a product tested once by a participating laboratory use that report towards certification in more than 50 member countries, which saves repeating the same safety tests for each market. Electromagnetic compatibility is a separate requirement in every market; our explainer on EMC testing covers what is tested.

These systems carry UL certification in their manufacturers’ published specifications. As your system integrator we configure the computer for the panel, image the operating system and application, and test it before it ships, keeping to the certified build so the listing still applies. We supply the manufacturer’s certification details for your panel documentation; the listing itself always belongs to the manufacturer, so check the current certificate for the exact model and options you order.

Where a project needs a listing a model does not have, or a build outside the certified configuration, we say so at the quotation stage rather than after delivery. Many of the same computers also carry CE and UKCA marks, so one panel design can serve UK, European and North American customers.