It might help if you put the question in some context.  Why do you care about the definition, specifically.  

 

dpc,  Sorry for not including a context.  Area classification is greatly impacted by whether a space qualifies as a 'pit'.  The application is multi-fuel engine testing in a cell (i.e. cut-off room) with an inertia mass and bedplate located in the center.  Bringing services over to both the engine and dynamometer is usually accomplished via a trench or depressed floor (all but inertia mass area is say 24-inches below the bedplate and covered with open grating).  I'm now looking at OSHA [3138-01R] definition of confined space for guidance concerning this space because 'pit' is used in their space description.

In addition to testing ventilation, high and low scavenging is provided.  When Article 511 (Major Repair Garages is closest NEC match for this occupancy) is applied to this space, the question is how far above the lowest horizontal surface requires rated equipment and wiring methods?  The 18 in. classified volume is either entirely in the depressed floor or extends to 42 in. (18 +24).

Since you are bringing fuel to the engine, I wonder if Article 514 is a better match. NEC does appear to differentiate between a room and a pit within that room. Looks like a pit gets a Division 1 and the 18" deep volume above floor gets a Division 2.

You would have to take the worst-case fuel as the basis for the classification.  I'd suggest referring to the relevant NFPA 30A standard for fuel dispensing.  You may find a definition.

Gasoline vapors are heavier than air, IIRC, hence the concern regarding pits.  Diesel fuel would probably not be a concern.  

The local AHJ would get the final call, but with gasoline, the hazard is real, so I wouldn't want to try to wordsmith my way out of doing something that probably needs to be done.  



 

I had assumed that the fuels were heavier than air. If lighter-than-air fuels are in the mix, then you have to worry about the top of the room. If hydrogen is involved, then you are talking Group B fixtures/fittings. One can get equipment rated for Groups B, C and D, but a C/D rating is not sufficient for hydrogen (smaller molecule).