501.40

[jusme123]

501.40 ('08) states that a MWBC can not be used in C1D1 location, but than gives the exception (...exception: all ungrounded conductors must simultaneously open) The exception in 501.40 basically is the requirement for MWBC in 210.4(B) (states that MWBC must be simultaneously disconnected).

My question is why state that it can not be used when applied in 501.40, when 210.4(B) must be satisfied regardless to the location?

 

[don_resqcapt19]

210.4(B) was a new requirement for the 2008 code, the rule in 501 has been there for many code cycles. A proposal for the 2011 code was submitted to delete 501.40, but CMP 14 rejected it.

14-67 Log #329 NEC-P14 Final Action: Reject
(501.40)
_______________________________________________________________
Submitter: Alan Chech, Alan Chech Electrical Seminars
Recommendation: Delete section 501.40.
Substantiation: All multi-wire branch circuits are now required by 210.4(B) to be disconnected simultaneously.
Panel Meeting Action: Reject
Panel Statement: This multi-wire branch circuit requirement must be retained due to its importance. This is directed at prevention of fire and explosion as well as personnel safety, as addressed in 210.4. These two sections are not in conflict.
Number Eligible to Vote: 14
Ballot Results: Affirmative: 14

 

[jusme123]

Thanks Don!

 

[rbalex]

Don’s answer is correct, 501.40 and its predecessors (501.18 before 2002) has been around a long time. Except as provided in the Exception, MWBCs have generally been prohibited in both Classes I and II for a long time.

A little parsing is also valuable. With regard to 501.40 Exception, any and all disconnecting devices must open all ungrounded conductors of the circuit simultaneously. However, with the general rule in 210.4, only a single disconnecting means that opens all ungrounded conductors simultaneously at the point where the branch circuit originates is required. There is still a significant difference in the applications.