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Industrial Control Panel Design: UL 508A, NFPA 79 and SCCR Explained
An industrial control panel in the U.S. is designed to the National Electrical Code, usually to NFPA 79 when it serves a machine, and is often built and listed by a UL 508A shop. The most commonly missed requirement is the short-circuit current rating (SCCR): the panel's rating must equal or exceed the fault current available where it is installed.
Published · Last reviewed by Geek Engineering senior engineering review.
The standards and what each covers
UL 508A is a listing program: a panel carries the UL mark only when it is built by a shop that UL has certified to build to that standard. A design can follow UL 508A rules without being listed, but only a certified shop can apply the label.
| Standard | What it covers | Who usually requires it |
|---|---|---|
| NFPA 70 (National Electrical Code), Article 409 | Industrial control panels, including SCCR marking | The authority having jurisdiction (local inspector) |
| NFPA 70, Article 670 | Industrial machinery supply and nameplate data | The authority having jurisdiction |
| NFPA 79 | Electrical equipment of industrial machinery: wiring, colors, disconnects, emergency stop | Machine buyers, insurers and inspectors |
| UL 508A | Construction and listing of industrial control panels by a certified shop | Customers and inspectors who want a listed panel |
| NFPA 70E | Electrical safety in the workplace, including arc-flash labeling and work practices | The plant's safety program |
What a panel design package contains
- Electrical schematics with wire numbers and cross-references
- Panel layout drawing with component placement and wireway
- Bill of materials with manufacturer part numbers
- Terminal and I/O lists
- Heat-load calculation and enclosure cooling selection
- SCCR calculation and the documentation supporting it
- Nameplate data: voltage, full-load current, SCCR and diagram number
SCCR: the requirement most often missed
Short-circuit current rating is the maximum fault current the panel can withstand safely. It is set by the weakest component in the power circuit, unless current-limiting devices raise it. If the plant's available fault current is higher than the panel's SCCR, the panel is not suitable for that location, and the inspector can reject it.
Ask the plant's electrical engineer or utility for the available fault current at the point of installation before the panel is designed, not after it arrives.
Questions to ask a panel supplier
- Is the shop UL 508A certified, and will this panel carry the listing mark?
- What SCCR will the panel have, and how was it determined?
- Will you supply native drawing files as well as PDFs?
- How are design changes during the build controlled and recorded?
- What testing is done before shipment: point-to-point, insulation, functional?
Key takeaways
- Machine panels are designed to the NEC and usually NFPA 79.
- UL 508A listing requires a certified shop; design to the rules is not the same as listed.
- Get the available fault current before design so SCCR can be set correctly.
- Ask for native drawings and a defined pre-shipment test.
Related questions
Does every control panel have to be UL listed?
Not by federal law, but many inspectors, insurers and customers require listed equipment or a field evaluation. The authority having jurisdiction decides what is acceptable for a given installation, so confirm before you specify.
What is the difference between NFPA 79 and UL 508A?
NFPA 79 covers the electrical equipment of an industrial machine as a whole, including wiring practices and emergency stops. UL 508A covers how an industrial control panel is built and listed. A machine panel often has to satisfy both.
Can a panel be modified after it is UL listed?
Changes can affect the listing. Significant modifications are normally made by a UL 508A shop or evaluated in the field. Undocumented field changes are a common reason panels fail inspection.
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