Some commentary from an article in ECM, comments by MH, only a portion of herein:
"Per the Code, the grounding terminal for an IGR could terminate to the metal outlet box that contains it (Figure, point C). The NEC doesn't dictate where you terminate the grounding terminal for an IGR — just that you terminate to an effective fault current path. Nor does the NEC require each IGR to be on its own dedicated branch circuit.
However, the Code does require you to ground the metal enclosure. This is automatic with a regular receptacle; you must provide an additional grounding means when using an IGR (unless you bond the enclosure to the IGR). The Code does not permit you to use interlock, or standard, type MC cable when wiring an IGR because the outer interlock sheath is not recognized as an effective fault current path to ground metal enclosures. However, you can use AC cable with an insulated equipment grounding conductor, because type AC cable armor is listed for use as an equipment grounding conductor."