i have 2 single pole 15 amp 120v circuit breakers with a shared neutral. i would like to replace the 2 single pole breakers with a 15 amp double pole afci breaker. will the double pole breaker protect up to 15 amps on each leg or only 15 amps total for both legs?
A MWBC (shared neutral) is 1 circuit. It is not 2 circuits.
A double pole breaker is very similar to the two singles that you have now, except with two additional things:
- A handle tie, to force maintainers to shut off both sides of the MWBC (shared neutral) when they do repair work in the circuit. That way they don't get bit by the other half of the circuit.
- Common Trip to assure that if one side trips, the other side also trips. This happens via an internal mechanism in the factory-made 2-pole breaker, and would work even if you sawed off the handle tie. Common trip is only needed for things which contain both 120V and 240V loads, such as dryers, ranges or subpanels. You don't want the still-live phase to backfeed the tripped phase via the 240V load's resistance.
A MWBC which contains only 120V loads does not need common trip. Simple handle-ties will suffice between the breakers. (note that MWBCs are allowed to have 240V loads, but if so they need common trip.)
If you use a "handle tie" ($5) on two 1-pole breakers, you do not get common trip.
So all that to say... your AFCI breaker will perform the same as the two 1-pole breakers except for being AFCI of course... having a handle-tie (required), and having common trip (not required).