Is the XO Jumper Sized Correctly?


Is the XO bonding jumper strap provided in certified (Listed) dry type core and coil transformers used in typical commercial installations adequately sized?


Dry type coil and core transformers typically found in commercial installations are certified by UL under the product category Power and General-Purpose, Transformers Dry Type (XQNX), located on page 452 of the 2012 UL White Book and also on UL’s Online Certification Directory at and enter XQNX at the category code search field.

Dry type transformers can have two bonding jumpers installed. One bonding jumper is for grounding the transformer steel core to the enclosure where the core is electrically isolated by the sound dampening pads that are typically made of rubber. This bonding jumper is covered by the requirements in UL 1561, the Standard for Safety for Dry Type General Purpose and Power Transformers 600 Volts or less, and must remain in place.

The other bonding jumper, that may or may not be supplied by the manufacturer, is installed between the XO terminal and the enclosure. This second bonding jumper is identified from the National Electrical Code as the “system bonding jumper.” UL 1561 does not contain requirements to install or size the system bond jumper between the XO terminal and the enclosure or equipment grounding bus. The evaluation of the transformer for Listing, therefore, does not include an evaluation of the size or adequacy for the system bonding jumper; it is really an item optionally provided by some manufacturers. For the same reason, if this bonding jumper were removed because the system grounding and bonding was done elsewhere as allowed by NEC 250.30, the UL Listing of the transformer is not affected. Since the size of the system bonding jumper is based on field-installed feeder conductors for the derived system, it would be difficult to determine the correct size of the system bonding jumper that is to be installed at the factory. If the inspector determines the supplied system bonding jumper does not meet the Code minimum based on the actual installed feeder conductors, then they can require it to be replaced with a suitable system bonding jumper meeting all the requirements from the NEC.

About Underwriters Laboratories: Underwriters Laboratories® (UL) is an independent product safety certification organization that has been testing products and writing Standards for Safety for over a century. UL evaluates more than 19,000 types of products, components, materials and systems annually with 20 billion UL Marks appearing on 66,000 manufacturers products each year. UL’s worldwide family of companies and network of service providers includes 68 laboratory, testing and certification facilities serving customers in 102 countries. UL is also the only National Certification Body (NCB) for PV in North America and an OSHA-accredited Nationally Recognized Testing Laboratory (NRTL). For more information, visit

About UL Solutions
UL Solutions helps create a better world by applying science to solve safety, security and sustainability challenges. We empower trust by enabling the safe adoption of innovative new products and technologies. Everyone at UL shares a passion to make the world a safer place. All of our work, from independent research and standards development, to testing and certification, to providing analytical and digital solutions, helps improve global well-being. Businesses, industries, governments, regulatory authorities and the public put their trust in us so they can make smarter decisions. To learn more, visit