The pilot stated that while in an extended traffic pattern to land at the conclusion of a cross country flight, the engine coughed and began windmilling. A forced landing was made during which the airplane contacted power lines. Post accident inspection revealed the fuel selector was found positioned on the left tank which contained 12 gallons of fuel. The right tank was severed by the power lines and found empty. A deputy who arrived at the scene just after the accident stated it did not appear as though any fuel had drained from the right tank. The pilot stated he took off on the left tank, switched to the right after approximately 1 hour, then switched back to the left approximately 8 miles from landing. The passenger sitting in the left front seat, next to the selector, stated the pilot only switched tanks once during the flight. The location of blood splatters and a puncture mark on the fuel selector suggest the selector was positioned on the right tank when the accident occurred.