Proper box size for splicing 6/3 NM cable

According to my Black & Decker Advanced Home Wiring book, a 15 amp circuit can carry 1440 watts of electricity at 120 volts. A 20 amp circuit can carry 1920 watts at 120 volts or 3840 watts at 240 volts. It goes on like this up to 50 amps. It seems to me that this book is either understating the capacity of various circuits or circuit breaker manufacturers are overstating the amperage rating of their products. After all 120x15=1800 and 120x20=2400. If this is true, then a typical 15 amp circuit should be unable to run a 1500 watt electric space heater or hair dryer. What gives? Robert

While testing my outlets with a circuit tester, two of them indicated an "open ground" condition; the other outlets registered normal. The circuit is 20 amps with four duplex receptacles, one combination switch outlet, and one dimmer switch. These outlets are on the main floor of the house(dining room). House was built in 1959 and the electrical wires are black, white, and bare cooper. The "open-grounded" outlets are middle-of-the-run. I opened every up every single outlet box and switch plate and checked for proper grounding connections(to both box and receptacles, pigtailed)and every one of them were properly wired. I guess somewhere in the house is a ground wire that is not connected that these two particular outlets were tapped off from? Had a similar problem with another circuit; however I managed to find the culprit-an unconnected ground wire in a wall switch in an adjacent room! I am concerned that this "open ground" is a safety problem, although we have lived in this house for 13 years now. Unfortunately, the basement ceiling is newly finished, so short of tearing apart the sheetrock, not really sure how much more I can do to try to find this problem-already spent days to no avail! (Especially enjoyed crawling around in a hot attic!) Thanks for any suggestions, advice, and alternative solutions. Also, as far as the use of these particular outlets, one is used for a 13" TV, and the other one has a nite light plugged into it.