low voltage limits- regulatory
For the US this is mostly handled by UL standards.

There are a bunch of standards to read and unfortunately you have to purchase them.

Here is a few to get started:


UL840
UL796


Then it depends of your product, what is the intended use for which market and specific norms may apply, it is not the same for household, fab or laboratory equipment for instance.

If the voltage goes above 50VDC, then the standards are more restrictive, it depends if you have conductive or plastic casing, you might need double insulation.