Does UL List Skid Packs?

How do we know who has Listing (certification) for a product category in the White Book?

There are two ways to see a list of manufacturers who are certified (Listed) for a specific product category in the UL White Book.

The first is to use the 4 letter category code that appears at the end of the product category title [For example, Panelboards (QEUY)] and then go online to UL’s Online Certifications Directory at Once there, enter the category code at the category code search field and click search. That will give you a list of all the manufacturers that have been certified (Listed) for that product category. Then click on the links and view their certifications (Listings).

Another way would be to download the linked PDF version of the UL White Book at Once you’ve downloaded it, you can go to any of the product categories in the book and click on the title and if there are certifications (Listings) for that category, the link will take you to the results page in UL’s Online Certifications Directory displaying all the certifications (Listings) for that category.

Does UL have a certification (Listing) program for portable generators?

Yes, UL certifies (Lists) portable generators under the product category Engine Generators for Portable Use (FTCN), located on page 164 in the 2013 UL White Book and also online in UL’s Online Certifications Directory at and enter FTCN at the category code search field.  These products are evaluated for compliance with the Standard for Safety for Portable Engine- Generator Assemblies, UL 2201.

We are running into unlisted prefab skid pack buildings that are intended to be shipped to the oil and natural gas fields.  Does UL List (certify) these types of skid packs?  Can UL perform a Field Evaluation on these skid packs?

Yes and yes. UL certifies (Lists) un-enclosed pumping systems on skids or a steel mounting frame under the product category Packaged Pumping Systems (QCZJ) located on page 330 in the 2013 UL White Book and in UL’s Online Certifications Directory at and enter QCZJ at the category code search field.

Skid packs housed in a prefab building would be investigated under the product category Commercial and Industrial Prefabricated Buildings and Units (QRXA) located on page 360 in the 2013 UL White Book and in UL’s Online Certifications Directory at and enter QRXA at the category code search field. These prefab skid pack buildings would not be marked with a UL mark, they would be provided with a UL Certificate of Inspection.  The UL Certificate of Inspection is the only method provided by UL to identify prefabricated buildings and units inspected by UL under UL’s Building Inspection Certificate Services Program.

The prefabricated buildings and units for which UL issues Certificates are considered by UL to comply with the applicable requirements of the NEC at the time of inspection. The Certificate is only valid when accompanied by a completed UL Inspection Report. The UL Inspection Report identifies applicable plan drawings that indicate all the equipment included in the building at the time of manufacturing. UL does not know what the effect of a modification to the electrical system or equipment, or to the construction of a prefabricated building or unit, subsequent to the inspection, may have on the safety of the product or the continued validity of the Certificate unless the modifications have been specifically investigated by UL. Unless UL investigates a modified product, UL cannot indicate that the product continues to comply with the applicable requirements.
If the prefabricated building or unit is shipped in multiple sections or “knocked down,” the number and description of the sections required to complete the building or unit are included on a building’s nameplate. Instructions for completion of the building, including any wiring connections to be completed at the installation site, are also provided.

UL can also conduct a Field Evaluation on the skid pack building or skid packs if there is an uncertified skid pack building or skid pack in the field. For more information on a field evaluation, please contact UL’s Customer Service at 877-854-3577, prompt number 2 or online at

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