I'd advise against running any "3" wire such as 14/3 or 12/3 as well for much the same reason as 2pts has mentioned. I wired up some bedroom circuits with 14/3 on a single Siemens AFCI circuit breaker specially designed for two circuits. I then saw a "Note" in the "Electrical Code Simplified" book by PS Knight, that states an AFCI breaker "cannot be used with three wire cables, only two supply wire cables may be used." Mind you, the illustration in the book shows a Square D single pole Arc Fault breaker and there's a newer version of the book available with much more info on AFCI & GFCI requirements. I haven't had a chance to look at the latest revisions, but I reckon if Siemens produces double pole AFCI breakers designed for their panels and for use with 3 wire cable, it's more than likely safe and I haven't had a problem since installing mine. Not sure whether I could have used two 14/2 cables with this breaker instead of the 14/3, can't seem to find the installation instructions at the moment. Looks like the center neutral connection on the breaker could handle two neutrals. Anyhow, I digress, I think. I did say I regret doing this. Main reason is the size of the breaker itself, it's massive & uses up several spaces in the panel, and while you save a bit on wiring with the 3 conductor cable, I don't really like the idea of the single neutral wire carrying the return current for both hot leads feeding two separate circuits. This seems to go somewhat against another code requirement calling for adequate separation of wire runs to protect from overheating & fire. In 3 wire cable, they're all bundled together nice & tight in the outer sheathing.
How does that relate to wiring a GFCI protected circuit with 3 wire cable? While I didn't find the same Note I alluded to in the previous paragraph in my trusty "Simplified Code" book, it does say to use only 2 wire cable because the GFI circuit breaker will not work if wired with 3 wire cable. This does make sense since the principle behind a GFI is to compare incoming to outgoing current in the hot & neutral, then trip once the current mismatch reaches the design threshold which is in the 3 to 5 milliamp range. It's easy to see that with a 3 wire cable (2 hots & 1 neutral), it's pretty much guaranteed that there will be a current mismatch in fairly short order. Therefore any circuits in your home wired using 3 wire cable are poor candidates for a GFCI device. Having said that, the link 2pts provides in his post clearly shows a dandy Siemens 2 pole GFCI circuit breaker that's got the same form factor as the 2 pole AFCI breaker I installed. I can only surmise that one would use two separate 14/2 cables to wire up this device and the magic involved in comparing current flow happens inside the breaker.
In any case, my preference is to run two separate 15 amp circuits using 14/2 wire over a single 20 amp circuit wherever possible. The smaller guage 14 wire is easier to work with and you get a total of 30 amps with the redundancy of two separate circuits and additional flexibility as to the use of GFCI devices (breaker or receptacle, your choice). Of course for places like the garage, workshop, outdoor outlets where you know you'll be using devices & tools capable of drawing close to 20 amps per single device, 20 amp or larger circuit is the way to go. Just be prepared to pay a premium for 20 amp GFI devices & other hardware.
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