See the National Electrical Code ARTICLE 411 Lighting Systems Operating at 30 Volts or Less
This article covers lighting systems operating at 30 volts or less and their associated components.
Lighting Systems Operating at 30 Volts or Less. A lighting system consisting of an isolating power supply operating at 30 volts (42.4 volts peak) or less, under any load condition, with one or more secondary circuits, each limited to 25 amperes maximum, supplying luminaires (lighting fixtures) and associated equipment identified for the use.
411.3 Listing Required.
Lighting systems operating at 30 volts or less shall be listed for the purpose.
Joe Tedesco, NEC Consultant