Re: Article 511
There are no heigth requirements. If you want them over a work bench or closer to the floor its up to you. The hazardeous Class-I Div-2 location is up to 18" above the floor so you may want to use standard non-hazardeous rated receptacles and install them above that location. The classification can be effected by the use of a ventilation system that meets certain requirements as long as it is approved by the enforcing agency. 511.3 details the classifications in commerical garages.
