A 100A. three phase four wire mainlug only sub panelboard is fed from a single phase service panelboard 120/240V 200A within the same electric room. There is a three pole 100A. breaker in the single phase service panelboard protecting the three phase panelboard. The three phase four wire panel is used as a single phase panel. I've never seen this set up before. It's obviously an incorrect installation. I don't think a licensed electrican would ever perform such work. We are in the process of trying to locate the General Contractor to get the name of his sub contractor that did the work. Even if the three phase panel is used as a single phase panel proper balancing of loads would be difficult. I located section 210.11(b) to address that violation. Perhaps section 110.3(b) would also be applicable, "Installation and Use" of labeled equipment. Would anyone happen to know any other code sections that would apply to this obvious violation?