You have to be careful with ratings. NEC and most of the products electricians frequently use all use RMS voltages- 600V insulation rating, for instance, is 600V RMS.
You can easily get into trouble with rectifier circuits and other things that are rated for peak voltage, though (like when the marketing department takes the product and misleadingly advertizes insulation rating), if you fail to consider than 480V RMS is 680V peak, 11.5kV RMS is 16.2kV peak, etc. More of an engineering issue than an electrician issue, but still worth having in the back of your head if you're trying to do anything fancy.
