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What happens if I use the factory provided holes in the bus?

Posted By Underwriters Laboratories, Saturday, July 01, 2006
Updated: Monday, February 11, 2013

Question: Switchboard

Is it permitted to field drill a hole in the bus of a Listed switchboard, bolt on a wire connector and tap off the bus to feed another circuit? What happens if I use the factory provided holes in the bus?


Switchboards are Listed under the product category Deadfront Switchboards, (WEVZ), located on page 264 of the 2006 White Book or online at and enter the Category Code WEVZ. Switchboards are not Listed for drilling holes in the buswork in the field and tapping branch circuits off of the bus.

If the switchboard is modified after it leaves the factory, UL does not know if the product continues to comply with the applicable requirements, unless UL conducts a Field Evaluation on the modified construction. The only exception for a field modification authorized by UL is when the product has markings for specific equipment that may be added in the field.

Some of the considerations that would be of concern for the drilled holes, their filings and the choice of wire connectors would be the effect of these modifications on the electrical spacings, heating and the short-circuit current ratings of the equipment.

Tapping off of the holes that are provided on the bus is not permitted unless the hole is identified for a specific use. Provisions for terminals (i.e., holes in the bus) intended to be used as a tap are marked "Tap,” and the switchboard is marked to indicate the specific terminals or terminal kits intended to be field

If a switchboard has provision (i.e., holes in the bus) for the mounting of additional equipment (circuit breakers, fuse holders, contactors, meter sockets, switches, branch buses, through buses or the like), the switchboard is marked with the manufacturer’s name, or trademark of the manufacturer, or other descriptive marking by which the organization responsible for the equipment may be identified and the catalog designation or the equivalent of the equipment that may be field installed. In addition, instructions for the installation and wiring of such units are provided with the unit or switchboard, unless construction and orientation of the additional units make the installation method obvious.

If you would like more information regarding Field Evaluations, please call UL’s Customer Service at (877) 854-3577, and select prompt number 2, or on the web at www. For UL’s policy on Field Modifications of UL Listed products, see

Tags:  July-August 2006  UL Question Corner 

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