Occasionally, people are surprised that there is a rule for cable trays and that it can greatly increase the minimum conductor sizes given in Tables 1 to 4. Once the trays are in place as originally designed, it’s far too easy to add cables, especially when the trays follow a convenient route to the end destinations of the added cables. Could it be that the rule for wiring in cable trays is sometimes taken less than seriously? We have all seen trays overloaded with cables, if not at the time of installation, then in the fullness of time. In my experience, a discussion of conductor numbers and ampacities in cable trays is frequently met with a snicker or knowing smile.