First - read the "Electrical Primer for Brewers" http://www.homebrewtalk.com/f170/ele...rewers-145019/
In there you will find a way to determine the load or amperage needed to run a 120V element for whatever wattage. You did not mention the wattage element you expect to use.
For what it's worth, I recently built a RIMS tube and used a 4500watt/220volt element, but I'm running it at 120volts. This lowers the watt density per inch of element greatly reducing the likelihood of scorched wort. Using an element of this density at 120v amounts to about 1300 watts or about 12 amps. I would think that as long as you aren't running anything else like a vacuum cleaner, refrigerator, or hair dryer at the same time as the RIMS tube on a 20amp circuit, you should be fine. Make sure the system is grounded and that you're using a GFCI protected circuit. I'm not sure how much a pump draws, but I ran both my RIMS tube and my pump on the same 20amp circuit without any issue.
Second, look around this forum using the search utility. "RIMS" will yield a lot of results.