None as long as the neutral remains connected. Another provision of the code requires MWBC neutrals to not be broken if a device is removed, so changing a receptacle or whatever shouldn't break the neutral.So what is the issue with neutral current if there's no potential?
One more provision, if the MWBC is wired to a single yoke (split wired receptacle), it requires a 2 pole breaker.It depends.
If there are only 120V loads and receptacles on the MWBC, two breakers with a factory handle tie will suffice.
If there are any hardwired 240V or 120/240V loads or receptacles (yeah, you can do that on a MWBC that also supports 120V loads)... then the breaker MUST be a 2-pole.
The reason for the latter requirement is that if one leg of the MWBC were to trip for good reason, the 240V load will die of course, however, current will leak through that load causing the tripped hot wire to "float" at 120V. Effectively it would put all 120V loads on the tripped leg in series with the 240V load. This would defeat the purpose of tripping the breaker.
That rule still stands, and has nothing to do with MWBCs. Here's where you'd use that.Prior editions of the code only required a common disconnect of the two legs if they landed on the same yoke in a residence. Commercial was different.
There's another reason. Requiring use of a factory handle-tie ensures that the MWBC is correctly phased. Factory handle-ties generally won't allow you to position 2 breakers on the same phase. unless you're doing some sort of deranged misuse such as using a 3-pole handle-tie in a split-phase panel.If one leg was turned off the neutral c a n carry current form the other leg. To enhance safety the code now wants to ensure both legs are off to eliminate any neutral current on the MWBC.
I have to remember this, thanks. Another scenerio that catches me once in a while (nothing related to MWBC) is also duplex receptacles with the tabs broken off, same circuit but the top is switched and the bottom is unswitched. With the switch off it's easy to assume the breaker has been turned off especially if you have walked back and forth to the panel like 30 times already in order to test things. But it's not off at the bottom until you tried to undo the conductors.Suppose I install a recep with both tabs broken off. I wire black/white to the top socket, and red/white-red to the bottom socket (/2/2 cable). Down at the panel, I land on a tandem
(1 space/pole 2 breakers). And that's fine because each leg has its own neutral. Definitely not an MWBC, right?
Later, master electrician comes by and needs to service the recep. Plugs in a loud radio, and flips breakers until the radio goes silent. Standard operating procedure, tried and true. See the problem? Master has turned off only half the yoke. Hence the handle-tie requirement: now, the above procedure will work.
The problem I see is complacency. A Master electrician carries a volt-tick or meter and checks again before working on it.Later, master electrician comes by and needs to service the recep. Plugs in a loud radio, and flips breakers until the radio goes silent. Standard operating procedure, tried and true. See the problem?