The short answer
UL Standards for Controls
UL standards define the safety requirements that products and panels are tested against, and a UL listing is the evidence an inspector accepts that equipment is safe for its intended use. For control systems the central document is UL 508A, the standard for industrial control panels, under which a panel shop builds and labels panels using listed and recognized components: breakers to UL 489, fuses to UL 248, enclosures to UL 50, contactors and starters to UL 508 or UL 60947, and power supplies to UL 508 or UL 61010. A listed component may be used as intended; a recognized component may be used only in the conditions of its recognition; and the panel SCCR is calculated under UL 508A Supplement SB from those components.
Key points
- UL 508A is the standard a panel shop builds to; the panel label is the evidence the inspector accepts.
- Listed components are used as intended. Recognized components are used only under their conditions of acceptability.
- UL 489 breakers protect branch circuits; UL 1077 supplementary protectors do not.
- The SCCR of a panel comes from Supplement SB and the ratings of its components.
- A mark from another nationally recognized testing laboratory to a UL standard is equivalent; the standard is what matters.
What UL is
Underwriters Laboratories writes safety standards and tests products to them, and it is one of several nationally recognized testing laboratories that OSHA accepts for that purpose. The NEC requires that much electrical equipment be listed, and the inspector looks for the mark of a recognized laboratory on the equipment and on the panel. UL standards are also American National Standards and are increasingly harmonized with IEC documents, which is why a UL 60947 contactor and an IEC 60947 contactor are the same device with two certificates.
Listing, recognition, and the mark
- Listed
- The product has been tested to the standard for its category and may be installed as intended by anyone, under the NEC, with no further evaluation. A listed breaker, a listed enclosure, a listed drive.
- Recognized
- The component has been evaluated for use inside other equipment, under stated conditions of acceptability, and is not for field installation on its own. Relays, terminal blocks, and power supplies are often recognized; the panel builder must use them within their conditions.
- Classified
- Evaluated for specific properties only, such as a fire rating.
- The mark
- The UL mark, or the mark of another recognized laboratory such as CSA, ETL, or TUV, applied to the product or the panel, is the evidence. A panel with a UL 508A label was built by a shop UL has authorized under its program, from components used within their listings, to the standard.
The standards a panel depends on
| Standard | Covers | Note |
|---|---|---|
| UL 508A | Industrial control panels | The panel standard: construction, spacing, wiring, marking, and Supplement SB for the short-circuit current rating; the UL 508A page |
| UL 508 | Industrial control equipment | The legacy component standard for starters, contactors, relays, and controllers, being replaced by UL 60947 for switchgear and controlgear |
| UL 60947 series | Low-voltage switchgear and controlgear | The harmonized IEC-based standard for contactors, starters, and related devices |
| UL 489 | Molded-case circuit breakers and circuit breaker enclosures | Branch circuit breakers; the circuit breakers page |
| UL 1077 | Supplementary protectors | Not branch circuit protection; the circuit breakers page |
| UL 248 series | Low-voltage fuses | The fuse classes: 248-8 for Class J, 248-4 for Class CC, 248-12 for Class R; the fuses page |
| UL 50 and UL 50E | Enclosures for electrical equipment | Construction and environmental type ratings, the NEMA types; the enclosure selection page |
| UL 698A | Industrial control panels relating to hazardous (classified) locations | Panels with intrinsically safe circuits or purged and pressurized enclosures for classified areas |
| UL 61010-1 and UL 61010-2-201 | Safety of measurement, control, and laboratory equipment; control equipment | The listing basis for many controllers, power supplies, and instruments |
| UL 1449 | Surge protective devices | The type 1, 2, 3, and 4 SPD ratings used in panels and at services |
| UL 1012 and UL 1310 | Power units and Class 2 power units | Older power supply listings; Class 2 outputs that permit power-limited wiring |
| UL 508C and UL 61800-5-1 | Power conversion equipment; adjustable speed drives | Drive listings and their SCCR conditions |
| UL 1059 | Terminal blocks | Use groups and ratings; the terminal blocks page |
| UL 497 series | Protectors for communication circuits | Signal and data line surge protection listings |
How a panel shop uses them
- 1
Select listed components
Every component in the power and control circuits is listed or recognized to the applicable standard, and the panel shop keeps the certificates. A component with no listing may not be used in a listed panel, whatever its quality.
- 2
Use them within their conditions
A recognized relay has conditions of acceptability, a maximum ambient, a mounting orientation, a spacing; the panel is built to them. A drive listed with specific upstream fuses is protected by those fuses.
- 3
Build to UL 508A
Spacings between live parts, wire sizes and colors, wireway fill, bonding, control transformer and power supply protection, and the marking requirements are in the standard, and the shop follows a procedure UL audits.
- 4
Calculate the SCCR
Under Supplement SB, from the component ratings and the published combinations, as the SCCR page describes, and mark it.
- 5
Label the panel
The UL 508A label with the shop file number, the SCCR, the voltage and current ratings, and the enclosure type. The label is what the inspector reads.
Reading a certificate
The UL online certification directory lists every listed and recognized product by category code, file number, and model, with the standard and, for recognized components, the conditions of acceptability. A panel shop looks up the components it uses; an engineer reviewing a submittal looks up what the vendor claims. A product that is not in the directory is not listed, whatever the catalog says.
Frequently asked questions
- Is a panel built by an electrician in the field a UL 508A panel?
- No. UL 508A panels are built by shops authorized under the UL program and labeled with the shop file number. A field-assembled panel is inspected by the AHJ as an installation and may be required to have a field evaluation by a laboratory before it is accepted. Many jurisdictions require the UL 508A label for any industrial control panel.
- Can I use a component with a CSA or ETL mark in a UL 508A panel?
- Yes, if it is certified to the applicable UL or harmonized standard by a recognized laboratory. The standard and the certification are what matter, not which laboratory issued them. The panel shop records the certificate.
- What is the difference between UL 508A and UL 698A?
- UL 508A is for industrial control panels in ordinary locations. UL 698A adds the requirements for panels that connect to intrinsically safe circuits in hazardous locations, or that are purged and pressurized to sit in one. A lift station panel with intrinsically safe barriers for the wet well floats may be built under 698A.
- Does a UL listing mean the equipment will work?
- It means the equipment met the safety requirements of the standard in the tests. Performance, accuracy, and suitability for the application are not part of a safety listing. A listed transmitter can still be the wrong transmitter for the job.
Related topics
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
- Short-Circuit Current Rating (SCCR)What a control panel’s short-circuit current rating means, how it is determined under UL 508A, why the NEC requires it on the label, and why the number is usually smaller than people expect.
- Circuit Breakers in Control PanelsMiniature, molded-case, and supplementary protectors: what each is listed to do, how a trip curve is read, where the NEC and UL 508A set the rules, and how breaker choice drives the short-circuit current rating of the whole panel.
- Fuses in Control PanelsFuse classes, time-delay and fast-acting types, current limitation and what it does for the panel short-circuit rating, semiconductor fuses for drives, control circuit and DC fusing, and the habits that make a blown fuse a diagnosis instead of a mystery.
- Enclosure SelectionChoosing the box: NEMA and UL 50 type ratings, materials for corrosive and outdoor sites, sizing for heat and wire bending space, and the details at the conduit entries and the door that decide whether the rating survives installation.
- NFPA Standards for ControlsThe National Fire Protection Association documents that reach into a control system: NFPA 70, the National Electrical Code, and the articles that govern panels, motors, and instrumentation; NFPA 70E for electrical safety at work; NFPA 79 for industrial machinery; NFPA 820 for wastewater plant hazardous area classification; and NFPA 110 where standby power meets the controls.
Direct contact
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