46 C.F.R. § 30.10-6a

The term Category A machinery space means any space and trunks and ducts to such a space that contains:

(a) Internal combustion machinery used for main propulsion;

(b) Internal combustion machinery used for purposes other than main propulsion where the total aggregate power is at least 500 brake horsepower;

(c) Internal combustion machinery that uses a fuel that has a flash point of less than 43.3°C (110°F); or

(d) One or more oil fired boilers or oil fuel units.