The National Fire Protection Association (NFPA) classifies diesel fuel as a Class II fuel. Class II fuels are not considered to be flammable liquids. They are, however, considered to be combustible liquids.
Flammable liquids have a flash point that does not exceed 37.8 degrees Celsius, according to the NFPA. The U.S. Department of Transportation (DOT) considers the upper limits of flammable liquid fuels to reach 60.6 degrees Celsius.
Combustible liquids have a flash point of 37.8 degrees Celsius or greater. At this flash point, the liquid fuel can evaporate to form a combustible concentration of gas.
Class II liquids include diesel fuel, paint thinner, camphor oil, white spirit, and kerosene. The NFPA considers their flash point to be equal to or greater than 37.8 degrees Celsius but less than 60 degrees Celsius.