You're on the right track! Let's break down the rules and options for MWBCs.

MWBCs and Breaker Requirements Pre-2008

As you stated, the NEC permitted using two single-pole breakers with a handle tie for MWBCs sharing a neutral. However, many electricians and inspectors still preferred a double-pole breaker for safety reasons, even before it became mandatory.

Post-2008 (and typically enforced today)

The NEC

requires a double-pole (common trip) breaker for MWBCs. The key is the "simultaneous disconnect" requirement. If one leg trips due to a fault, both legs must disconnect. This prevents the neutral from becoming overloaded if only one circuit is interrupted. The handle tie no longer meets this requirement.

MWBCs and AFCI Protection

Now, let's add AFCI into the mix. AFCI Requirement

If AFCI protection is required (either by code or by your preference), then the solution needs to be a double-pole AFCI breaker with common trip.

Why AFCI Double-Pole

Because of how AFCIs work, they must detect and react to both series and parallel arcs. This is more reliably done with a double-pole breaker that monitors both legs of the MWBC simultaneously. It also ensures the simultaneous disconnect mentioned earlier in case of an arcing fault.

AFCI/GFCI Combo

It's worth noting that you might also encounter AFCI/GFCI combo breakers, especially in areas where GFCI protection is also required (e.g., kitchens, bathrooms). These combo breakers will also be double-pole for MWBCs.

Summary For MWBCs today, you must use a double-pole (common trip) breaker. If AFCI protection is required or desired for an MWBC, you must use a double-pole AFCI breaker with common trip. You cannot use two single-pole AFCI breakers with a handle tie for MWBCs.

Important Considerations Local Codes

Always check your local electrical codes and with your local electrical inspector. They have the final say on what is required and permitted in your specific area. Local amendments can exist.

Breaker Availability

Double-pole AFCI breakers can sometimes be more difficult to find and can be more expensive than standard single-pole breakers. Plan ahead and ensure availability before starting your project.*

Neutral Connections: Proper neutral connections are critical for MWBCs. Ensure the neutral is properly sized and that there are no loose connections. Shared neutrals can be problematic if not installed correctly. Flag for review