The NEC requires that cable trays must be supported by members at an interval specified by the cable tray manufacturer, but not more than 5 feet for horizontal runs to support the weight of the cables and other loads. The NEC has a requirement for ladder-type cable trays. Some outdoor cable tray installations may have to span anywhere from 20 to 40-feet to cross roads. Extra-long. us-trations without notice. The mechanical and electrical characteristics, tests, certifications, overall quality management, recommendations mentioned. This publication is intended as a practical guide for the proper and safe* installation of cable ladder systems, cable tray systems, channel support systems and associated supports. Whether you're designing a new.