Is finished grade the stuff that's below the gravel, or is it the gravel layer itself?

[Ohwell]

Is this why we also dimension to to the top of concrete instead of FG for bus work and other stuff?

 

[waross]

top of concrete

Foundation, stub wall, floor slab?
Finished Grade: Floor level or ground level.

 

[bacon4life]

Substation yard rock has lots of gaps/texture, so it any measurement from the surface of the substation rock would be limited to no better than 0.5 inches. That is find for generals measurement like a 14' truck plus 4' of additional clearance. However, that would be insufficient for constructing a rigid pipe bus.

Substations tend to be slightly sloped to aid in drainage. The substation design often calls for the bus to be horizonal, which means the height of the bus over the gravel will change over the length of the substation.

Gravel often gets shifted over time by traffic, so there can be variations in grade level that develop over time.

 

[waross]

Components are generally specified at a specific elevation, rather than above finished grade.
If you try to install and align extensive buss works in a substation by reference to "Above Finished Grade" you will probably be embarrassed, disappointed and possibly fired.