No, it wont overload the N in a branch.
With a MWBC, where the neutral is shared between 2 hots, the 2 hots MUST be from opposite phases(240v potential between the 2 hot wires) to avoid overloading the neutral wire. The current on the hot legs cancels each other out when its even. Thus the current carried by the neutral is only the difference between the 2 hot legs. For example, leg A is 14a and leg B is 12a, so the neutral carries 2a.
If instead the 2 hot legs are from the same phase(no potential between the 2 hot legs) then the neutral, using the same example, is now carrying 26a. Lets say this is #14 wire. The neutral wire is now overloaded by 11a.
Step up to #12 and say 18a on each leg(which if 240v potential would cancel out and neutral carries nothing), the neutral is now carrying 36a. For #12, thats 16a over ampacity and sure to start a fire from overheating and melting.
Could you please elaborate?
I see 2 hots sharing a neutral. If these were 15A circuits on standard 14/3 romex that means a potential for up to 30A through the 14ga neutral conductor before a breaker would trip.
Perhaps this is part of the reason why MWBCs breakers need to be tied together? So a PoCo can quickly identify any 240V circuits?
your math is correct.
And the handle tie requirement has nothing to do with making things easy for the PoCo. Rather code requires all hot legs of an MWBC to be disconnected at the same time so handle ties are required.
If this code wasnt adopted, then u could just use 2 single pole breakers and put them anywhere in the panel. Then when some unsuspecting handyman or homeowner comes along to shut off the power to work on say an outlet, only flipping one breaker off, the other end is still energized. in the example of a split wired dishwasher/disposal outlet, one half of the outlet would still be energized. A seasoned electrician would of course check for power before commencing work but others may not.
