I'm sorry, but that answer was wrong, unless if it were a multiple choice question and 4X was not a choice, then it is just less wrong than any other choice would have been.
430.91 does NOT address "Motor Control Centers" by the way, it is referring to "Motor Controllers", as in individual motor controllers. There is NO SUCH THING as a NEMA / UL 6P Motor Control Center! There are a few companies who might make a NEMA 4X stainless steel MCC and Moeller used to make a plastic MCC (but only UL type 12), but none of them will have UL 4X, the testing cost would be astronomical and I don't even think UL-845 (the standard for MCCs) even addresses 4X testing! Also, the only difference between 4X and 6P is 6P's ability to be submerged, which was not addressed in the question. Whomever made that test question up should be subjected to a 6P test...
[This message has been edited by jraef (edited 07-26-2006).]