I found more info on this at the ASHI website Notes regarding multiwire (shared-neutral) branch circuits 1. For a detailed article about how these circuits are wired, see the ASHI Technical Journal, Vol 2 No 1 Winter 1992 p. 27-30 In addition to the author, Neal Macneale III, Douglas Hansen and Dan Friedman edited and illustrated this material. 2. Use of linked double-pole or two-pole circuit breakers is recommended: Pending further research and development of authoritative citations, the following is the opinion of the author: Multiwire branch circuits should be protected by a double-pole common-internal trip circuit breaker, including the physical "trip tie" which bonds the two circuit breaker switches together. This is a safety measure which protects people working on the building wiring and which helps assure that the circuit is wired properly at the panel. Even if local building inspectors do not require this measure we recommend it as a safety item and as good construction practice. Explanation: a. Required by National Electrical Code for split receptacles, as per paragraph 210-4-b. b. Using a double-pole breaker assures that the two legs of the circuit will be forced (in most panels) to opposite legs of the 240-volt panel - a requirement when circuits use a shared neutral, such as in multiwire branch circuits. If individual 120V breakers are used, it's possible for a future modification or rearrangement of breakers in the panel to in advertently move one or both individual breakers so that they both end up on the same 120V leg of the panel - which is improper when a shared neutral is involved. (Improper because the shared unbalanced load could exceed the rating of the wire.) c. Using a double-pole assures that if one leg of the circuit is being turned off for electrical repair/modification work in the building, the other leg is forced off as well. Otherwise it is possible for the mechanic to be shocked while working on the circuit, since the neutral wire of the supposedly "dead" circuit could be carrying current from the sister "live" circuit. 3. Electrical code notes: 3.a. ASHI Member Frank Luciano spoke with Al Weiss, New York State building code authority (Building code support office at World Trade Center, New York City) regarding the requirement for linking or common-trip ties for these circuits. Mr. Weiss' opinion was that if he sees individual breakers in the panel on a multiwire circuit he will not call it out as an issue for failure to link the breakers together. The discussion did not review possible relocation of one of the breakers to the same phase or "leg" of the panel as the other. Mr. Weiss interpretation of the National Electric Code is that if, on a multiwire circuit, the two phases are wired to the same electrical receptacle (upper portion to one phase, lower to another phase, by breaking the tie on the receptacle sides) then a common-trip breaker should be used on that circuit. He also opined that if breakers were wired in parallel, rather than in series, as is done in some states, then common trip ties are not required. http://www.inspect-ny.com/electric/multiwir.htm