The statement in 501.40 ('08) that a multiwire branch circuit (MWBC) cannot be used in a Class I, Division 1 (C1D1) location is intended to provide additional clarification and emphasis on the requirement for simultaneous opening of all ungrounded conductors in such locations. While the exception in 501.40 ('08) refers to the requirement for simultaneous disconnection in 210.4(B), it does not negate the specific prohibition against using MWBCs in C1D1 locations.The reason for this additional prohibition is due to the increased risk of fire and explosion in C1D1 locations. In such locations, there is a high concentration of flammable gases, vapors, or dust that can be easily ignited by a spark or other source of heat. A MWBC, by its nature, has multiple ungrounded conductors that can simultaneously carry current. If a fault occurs on one of these ungrounded conductors, it is possible for the other ungrounded conductors to continue to carry current, which could create a spark or other source of heat that could ignite the flammable atmosphere.By prohibiting the use of MWBCs in C1D1 locations, the NEC ensures that all ungrounded conductors are simultaneously disconnected in the event of a fault, reducing the risk of fire or explosion. Flag for review