NFPA 30 (Flammable and Combustible Liquids Code) defines a combustible liquid as follows:
Any liquid that has a closed-cup flash point at or above 100°F (37.8°C), as determined by the test procedures and apparatus set forth in 1.7.4. Combustible liquids are classified as Class II or Class III as follows: (1) Class II Liquid — any liquid that has a flash point at or above 100°F (37.8°C) and below 140°F (60°C); (2) Class IIIA—any liquid that has a flash point at or above 140°F (60°C), but below 200°F (93°C); (3) Class IIIB — any liquid that has a flash point at or above 200°F (93°C).
API RP 505, the Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2, references NFPA 30 in the paragraphs that mention Class III liquids.
Consider reviewing API RP 2023. Bunker fuel applies to number 5 and number 6 fuel oil and other residuals. Bunker C is also known as 6 oil. You can find some property specifications in other API standards. API 2023 reflects flash points for number 6 AH > 150°F/ > 65.5°C > 407°C) > 100°C (closed cup); for CoS > 62°C; Alaska 83°C; I/W 115°C and another bunker fuel specification > 60°C min (closed cup). Check your actual fuel specifications to see if your bunker C is Class IIIA or Class IIIB or another combustible liquid classification.
