Meet the Multi-Wire Branch Circuit (MWBC)
It's your friend if you do it right - but it's easy to do wrong. It has several very important requirements that you have to get right:
- Neutral must be pigtailed. This is so removing a device won't sever neutral for the other half of the MWBC.
- It's absolutely critical the MWBC be powered from opposite poles - and the #1 mistake there is tying it to a "double-stuff" duplex/twin type breaker (whose 2 circuits are actually on the same pole).
- It needs to be supplied from a single 2-pole breaker, for 2 reasons:
- to assure the two legs are powered from opposite poles.
- to provide common maintenance shutoff, so both/all legs of the MWBC are shut off together when someone is attempting to service part of the circuit.
- The wires need to be marked as being grouped, but that happens automatically with cable
Technically speaking, you only need common maintenance shutoff, not common trip as a 2-pole breaker also provides. However, we advise 2-pole breakers because you do common maintenance shutoff (only) with handle-ties, and properly listed handle-ties are often hard to find, and they can be installed wrong. It's hard to go wrong with a 2-pole breaker that obviously has factory-linked handle action.
As far as GFCI, you can protect the entire MWBC with a 2-pole GFCI breaker, which includes all 3 wires. Or, you can fit GFCI outlets that serve only themselves. Or, you can fit GFCI devices that protect their half of the circuit after it splits.
As far as AFCI, that has to be done at the breaker, and methods vary depending on brand and need for GFCI also. Consult with either the manufacturer of your service panel, or maker of breakers which are UL-classified for your panel. You must not use random brands of breaker, even if they seem to fit.