Article 250 as mentioned in the other answer does state that low voltage is 50 Volts or less, but it is in reference to grounding and bonding.  The NEC does not define low voltage!  You don't have to take my word for it, refer to Mike Holt. He is a well known expert on the NEC

What are the current and voltage limits when determining low vs. high voltage wiring?
In Bill and Ted's Bogus Journey, Rufus presumes that his old teacher,  De Nomolos is dead....Why?


