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The answer to that is a bit muddy, and depends on if you're speaking of wiring or gear. I don't think you'll find a clear cut answer.

IEEE has a more defined standard, but they classify low voltage as 600V and below, which ignores the NEC's separation of wiring that is <24 volts.

 
I don't think this came out of the NEC, but this worked for all my sample tests:

Low Voltage <1000V

Medium Voltage 1kV-100kV

High Voltage 100.1 kV-230 kV

Extra High Voltage >230 kV

 
If you have Wildi text books, it's mentioned in one of the distribution chapters

 
I have the IEEE values, just thought NEC had their own values as well..maybe im worng

 
NEC does have their own values, but they are not cut and dry, and they vary per article depending on how they're used.

 
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