A basement electrical fire forced the evacuation of a large commercial building in South Boston Monday afternoon, but there were no injuries, a fire official said. Boston Fire Department spokesman Steve MacDonald said the fire in the eight-story building on Drydock Avenue in the Marine Industrial Park started shortly after 3 p.m. when an electrical cable in the basement began smoldering. He said a large section of the building was evacuated. Damage was contained to the basement and was estimated at $50,000, MacDonald said. It was unclear how the cable caught fire.