A lot of folks seem to be willing to speak for UL, without bothering to ask UL first!
Here is what UL has to say, in part, about field modifications:
"UL does not know what the effect of a modification may have....unless the field modifications have been specifically investigated by UL. Unless UL investigates a modified product, UL cannot indicate whether such changes "void" the UL mark.
"The only exception....is when a product has specific replacement markings. For example, a switchboard may have specific grounding kits added in the field."
In other words, only UL can say a listing is "voided," and then only after an investigation.
I say, there is nothing wrong with making your own parts. Indeed, modifying "stock" components is an integral part of the trade. Yet, it would be irresponsible to make such changes without having some idea as to what the original requirements may have been.
