What regulations/certifications apply for low voltage DC power supplies in US?

[Metatronic_Mods]

I was hoping someone could inform me regarding any regulations or required certifications for a low voltage (<=30V) DC power product to be sold in the US (or elsewhere for that matter)?

More importantly, how does one even become educated in such matters in general? I just discovered this book earlier, which seems like a good place to start, but I'm wondering what the typical pathway for an electronics engineer to learn about these topics is? Thanks

[Eka]

I'd ask Underwriters Laboratories (UL).

There are some Energy Star ratings for efficiency that must be observed for consumer and office devices.

There are also some regulations on power factor.

I just keep things simple for me by buying already made ones.

[drussell]

The main reason so many lower-power-consumption items use external wall-wart power supplies instead of internal power supplies is because in most places, below (approximately 48, 50 or 60 volts, depending on locale) volts going into to the actual device, you are in a class where all your safety certifications, etc. are then only needed on the wall-wart itself, so you just use properly certified pre-made units for a given market. 

Your mains electrical safety is all then handled by the wall-wart so you just have to contend with EMI radiation, etc. where applicable on your actual device.  As soon as you build the power supply into your device, you have to get all that stuff certified to the appropriate standards for each market you sell into.