On August 8, 1994, about 2220 Pacific daylight time, N20818, a Cessna 182, operated by the owner/pilot, impacted a powerline and was substantially damaged while executing a forced landing in Nampa, Idaho. The forced landing was precipitated by an in-flight loss of engine power during final approach. The private pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The business flight departed Mountain Home, Idaho, and was conducted under 14 CFR 91. According to the pilot, the fuel selector valve cockpit control was moved from the RIGHT tank position to what the pilot thought was the BOTH tank position during the downwind leg for landing at the Nampa Airport. The pilot stated that he did not visually check the control when he moved it, and that the airplane "... hit a sink rate" at the time he moved it. The pilot also stated that he then pulled the throttle to idle during the base leg, and then turned onto the final approach. He stated that his "... sink rate was excessive," so he pushed in the throttle to add power; there was no response from the engine. The pilot stated that he kept manipulating the throttle and "... hit the starter..." in an attempt to regain engine power. He further stated that he did not visually check the fuel selector valve control during this time. The engine did not respond, and the airplane struck a powerline and impacted a road short of the runway. The pilot egressed the airplane with no injuries. Several gallons of fuel were observed leaking out of both wing tanks by local authorities. The pilot stated that he then reentered the cockpit to turn off the fuel selector control valve. He stated that he rotated the control 180 degrees, but did not visually check if the control was pointed to the OFF position. According to the pilot, another person entered the airplane "a few minutes" after the pilot had rotated the control. That same person then approached the pilot and said "I turned the fuel off for you." The fuel selector was found in the OFF position by local authorities and FAA personnel after the airplane had been secured. As examination of the engine did not reveal any evidence of pre-impact mechanical malfunctions. According to the airplane flight manual, one of the engine failure emergency procedure checklist items is to move the fuel selector valve control to the BOTH position.