Mercury Vapor Lt. over water?

[davedottcom]

Trying to find the code for the minimum height for a 175W Merc. Vapor light mounted on a boat dock, over a canal.
Customer wants it about 24" from water!

 

[ptonsparky]

Try Art 682 in the 2008 NEC. It is what I have handy.

 

[physis]

I'm by no means an expert with marina codes but I'm looking at 682.2(4) and it's saying the datum plane is 30" from the water line on a floating structure.

I'll be honest though, I have no idea what an electrical datum plane is nor could I find a definition.

The 2005 says :

Electrical Datum Plane. The electrical datum plane as used in this article is defined as follows:

I'm only seeing what the datum plane hights are, not what a datum plane is. Maybe someone could help me understand what these things actually are.

Edit: Errors A and B

 

[Rewire]

Article 682 uses a term new to the NEC: electrical datum plane. This term addresses four different locations, depending upon the situation. First, it is a horizontal plane 600 mm (2 ft) above the highest tide level under normal circumstances, in areas subject to tidal fluctuation to the water level. Second, it is a horizontal plane 600 mm (2 ft) above the highest water level for a body of water, not subject to tidal fluctuations, under normal circumstances. Third, it is a horizontal plane 600 mm (2 ft) above the prevailing high water mark, in areas subject to flooding or storm runoff. Fourth, for floating structures and landing stages (floating docks), it is the horizontal plane 750 mm (30 in.) above the water level of the floating structure or landing stage and a minimum 300 mm (12 in.) above the level of the deck. These structures or docks are restrained from horizontal movement, but are allowed to rise to the levels mentioned in the first and second conditions above and may fall with the water level.