Under the SPCC requirements (40 CFR 112), the 55-gallon minimum capacity also applies to oil-filled operating, manufacturing, or electrical equipment. Therefore, when determining if a facility meets the oil storage capacity threshold, an owner or operator must consider oil-filled operating equipment (gearboxes included) that can contain 55 gallons or more.
Specifically, EPA's position is that a primary containment system is the container or equipment in which oil is stored or used. Under SPCC rules, secondary containment is a requirement for all bulk storage facilities, large or small, manned or unmanned, and for facilities that use oil-filled equipment whenever it is practicable. The containment must at least provide for the capacity of the largest single tank (with sufficient freeboard for precipitation where applicable).
Also, in cases when secondary containment is not practicable, the owner or operator of a facility may deviate from the secondary containment requirement under 40 CFR 112.7(d). However, they must explain the rationale in their SPCC Plan, provide a contingency plan following the provisions of the oil removal contingency plans under 40 CFR 109, and otherwise comply with 40 CFR 112.7(d).
In short, the secondary containment provisions do apply to equipment containing more than 55 gallons.