NEC chapter 7, article 720 covers wiring of less than 50V. It's important to note that elsewhere in the NEC and other codes, "low voltage" is defined as <600V, <1000V, or <2000V, depending on the code and code cycle.
Your fire marshal might have passed 5V cabling duct taped to a wall, but it would be in violation of 720.11:
720.11 Mechanical Execution of Work
Circuits operating at less than 50 volts shall be installed in a neat and workmanlike manner. Cables shall be supported by the building structure in such a manner that the cable will not be damaged by normal building use.
Your fire marshal might have passed 5V cabling duct taped to a wall, but it would be in violation of 720.11: