210.4(B) requires simultaneous disconnection of the ungrounded conductors that supply stuff on a common yoke. A listed handle tie would take care of that.
I see frequent examples of a 15 A and a 20 A circuit sharing a common 12 gauge neutral in residential systems based on EMT or Flex wiring methods. In this area, 7-county Minneapolis / St. Paul, the age of installation is most commonly 1950 thru the mid-70's. In 1975, the local "all metal" requirement was relaxed to permit NM.
Perhaps the most common configuration was running a general lighting circuit with the laundry circuit.
The slight economic incentive to save on material used is still available in NM if one uses a 12/3 NM homerun with one of the ungrounded 12s on a 15 A breaker, and the rest of the 15 A circuit beyond the 12/3 on 14/2 NM.
#82331 - 11/05/0208:37 PMRe: different size breakers on multiwire branch
One word of caution would be that you might want to tag the fifteen amp leg of the 12/3 w Gnd in the panel to indicate that it supplies 14 gauge wiring. This is to prevent some follow on genius from supplying the 14 AWG from a replacement 20 amp breaker. -- Tom
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
#82332 - 11/08/0205:13 AMRe: different size breakers on multiwire branch
Just curious, but are there any instances where you would ever want or need to do this? I could maybe see it for a multi-wire that serves a single piece of utilization equipment where it is possible that the loads on each leg are far from equal, but I can't see why you would ever need or want it on a multi-wire for general purpose branch circuits.
Where this would apply to general purpose branch circuits as you suggest, we're talking the difference in price of one single #14 conductor versus the price of one single #12 conductor? Why even bother?
#82333 - 11/08/0207:14 AMRe: different size breakers on multiwire branch
It is not neccesarily the price of the materials that would drive this. I have done it because of the diffculty of running new circuits in an area. I have also used an existing wiring in remodel in this manner. For new construction I probably wouldn't bother.
#82334 - 11/08/0201:56 PMRe: different size breakers on multiwire branch